SKELETAL SYSTEM

I. Skeletal System Organization

Bone is a biological marvel from Mother Nature; it makes vertebrates distinct from other life forms. Bone has the same strength as cast iron, but achieves this remarkable feat while remaining as light as wood. The front leg of a horse can withstand mechanical stress from heavy loads while this 1500-pound animal gallops at 40 miles per hour. The wing bone is able to keep birds aloft through entire migrations, sometimes over 10,000 miles without landing. The antlers of deer, used as weapons in territorial clashes with other deer, undergo tremendous impacts without a fracture – ready to fight another day. Without question, bone is the ultimate biomaterial. It is light, strong, adaptable to functional demands, and can repair itself.

Bones provide a rigid structure – the skeleton. Like an architectural framework, skeleton forms an internal structure that provides resistance to the force of gravity, move through space, and carry the physical body with grace and dignity. The large bones of the lower limbs support the trunk when standing. The fused bones of the cranium surround the brain to make it less vulnerable to injury. Vertebrae surround and protect the spinal cord and bones of the rib cage help protect the heart and lungs of the thorax. Bones work together with muscles as simple mechanical lever systems to produce body movement.

Humans are vertebrates (with spine or backbone) and they rely on a sturdy internal skeletal frame centered on a prominent spine. Skeletal system makes a perfect combination of form and function: the S-shaped spine keeps the body upright and supports the head, while the pelvis balances the upper body over the feet. The male and female skeletons are similar in nature. However, the female frame is usually lighter and smaller than the male frame, and includes a wider pelvis for childbirth.

Human skeleton consists of bones, cartilage, ligaments and tendons that accounts for about 20% of the total body weight. Bones, muscles and joints work together to constitute the skeleto-muscular system. Joints or articulations provide flexibility and movement. The ends of bone are covered by cartilage to reduce mechanical friction or grinding of bone joints. The joint is surrounded by a protective capsule called synovium, which produces synovial fluid, a clear substance that lubricates and nourishes the cartilage and bone joints. Bone joints are vulnerable to injury and degeneration with aging.

Bones, muscles, and joints are integral part of the ‘Skeleto-Muscular System’. Problems with any one part of this system can affect the other components. Thus, weakness of the muscles can lead to loss of bone and joint damage, while degeneration of the joints leads to changes in the underlying bone. Skeleto-muscular system is adapted to provide adequate strength and mobility to resist factures upon substantial impact, or during vigorous physical activity. Shape and structure of the bone are equally important as its mass in providing such strength. Bone mass and its architecture or shape is influenced by the mechanical forces on the skeleton. Genetics and life style factors play a critical role in the structural outcome of a skeleton. Bone mass and architecture continuously change throughout life in response to mechanical stress and function.

The Bone

The primary tissue of bone, “osseous tissue”, is a relatively hard and light-weight composite material with mineral crystals bound to protein. This provides both strength and resilience so that the skeleton can absorb impact without fracture. A structure made only of mineral would be more brittle and break more easily, while a structure made only of protein would be soft and bend too easily. The mineral phase of bone consists of small crystals containing calcium and phosphate, called hydroxyapatite. This mineral is bound in an orderly manner to a matrix that is made up largely of a single protein, collagen.

Collagen is made by bone cells and assembled as long thin rods containing 3 inter-twined protein chains. These chains gather into larger fibers that are strengthened by various chemical bonds. Other proteins in bone can help to strengthen the collagen matrix even more and regulate its ability to bind mineral. Small changes to the bone shape of the bone can affect the cells inside bone (the osteocytes) to produce chemical signals. This phenomenon allows the skeleton to respond to changes during mechanical loading. Other types of tissue found in bones include marrow, cartilage, nerves, and blood vessels. While bone is essentially brittle, it also demonstrates a significant degree of elasticity mainly due to the presence of collagen. All bones consist of several living cells embedded in the mineralized organic matrix.

The Muscles

Bones don't work alone – they need help from the muscles and joints. Muscles pull on the joints to facilitate movement. Every kind of movement involves at least one muscle. The human body has more than 650 muscles that make up 50% of the body weight. Muscular activity is critical for normal bone functions. When the muscle is deteriorated, bone mass and strength are also rapidly lost.

Human body has three different kinds of muscle: i) skeletal (voluntary), ii) smooth (involuntary), and iii) cardiac, involved in specific functions.

i) Skeletal (voluntary) muscles hold the bones together, give shape and move the body. These muscles work only after a voluntary decision to move them. This muscle is also called striated, because when magnified it appears to have stripes or bands. These muscles can contract (shorten or tighten) quickly and strongly; but get tired easily, therefore, have to rest between workouts. Skeletal muscles attach to the bone, mostly in the legs, arms, abdomen, chest, neck, and face. Most of the muscles in the body are voluntary; involved in regular functions like walking, eating, writing and playing.

ii) Smooth (involuntary) muscles are also made of fibers, but this type of muscle looks smooth, not striated. These muscles are under the automatic (involuntary) control of the nervous system; and continue to work even during sleep. Smooth muscles take longer to contract than skeletal muscles; however, they are not tired easily and can stay contracted for a long time. Examples of smooth muscles include the walls of stomach and intestines, which help break up food and move it through the digestive system; the walls of blood vessels, which pump the blood and maintain blood pressure.

iii) Cardiac muscles are also an involuntary type found in the heart. The walls of the heart chambers are composed almost entirely of muscle fibers. Its rhythmic, powerful contractions pump blood out of the heart as it beats.

Muscles work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. When the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor; and the triceps, at the back of the upper arm, is an extensor.

Muscle and bone flow seamlessly together and work in tandem at every intersection of the body. The biceps brachii muscle helps to flex both the arm and the forearm and also acts to turn the palm upward (“supination”) when the forearm is flexed. The pronator teres muscle allows turning the palm downward (“pronation”). The intercostal muscles, located between the ribs, are essential for breathing. Their contractions raise and lower the rib cage, providing room for the lungs as they expand and contract with each breath. The quadratus lumborum muscles, wide bands of muscles that connect the lower back to the hip, help to bend the back from side-to-side and to force air out of the lungs. The biceps femoris, one of the hamstring muscles, is visible on the back of the thigh. It assists in extending the thigh and in flexing and rotation the leg. Pulled hamstring muscles are common in athletes who perform quick starts and stops. Skeleto-Muscular system provides several bio-mechanical levers that facilitate many complex body movements. It can move the hip in a 360° arc known as “circumduction”.

The Joints

Many bones in the skeleton are connected by joints, which provide flexibility and movement. These joints are shock absorbers to combat mechanical stress. Bone joints are vulnerable to injury and degeneration with aging. Joints or Articulations are the areas where bones meet, which are classified by the range of allowable movement. The ends of bone are covered by cartilage, which allows for easy movement of the two bones. The joint is surrounded by a capsule that protects and supports it. The joint capsule is lined with a type of tissue called synovium, which produces synovial fluid, a clear substance that lubricates and nourishes the cartilage and bones inside the joint capsule.

Joints facilitate different bone movements. For example, hinge joints (elbows, knees, fingers, and toes) allow the bones to swing in two directions. Ball and socket joints (hip and shoulder) allow some rotation, as well as movement back and forth, and from side-to-side. Pivot joints facilitate a left to right movement similar to the course your head takes as it swivels on your spine.

Knee Joint is the largest and most complex joint in the body – as well as the weakest and most vulnerable to injury. Primarily a hinge joint, it is also capable of some rotational movement. The knee is formed where the rounded end of the femur (thigh bone) meets the flattened end of the tibia (shinbone of the leg). The third bone of the knee, the patella (kneecap), is embedded within the tendon of the powerful quadriceps femoris muscle of the thigh. The kneecap protects the knee and increase leverage of the quadriceps muscle.

Hip Joint is one of the strongest and most stable joints in the body. The hip joint (a ball and socket joint), is formed where the ball at the head of the femur fits in the socket of the hip bone. This flexible joint structure allows for rotation, as well as movement forward, backward, and from side-to-side. Held in place by 5 ligaments, as well as tough connective tissue deep in the joint, the hip joint is often called upon to withstand 400 lbs (180 kg) of force in everyday activities.

Elbow Joint  is formed by three bones, three filaments, and fourteen muscles. The elbow joint permits flexion, extension, and rotation of the forearm.

Ligament and Tendons

Ligaments connect bone to bone and tendons connect muscles to bone; with a purpose to transmit biomechanical forces. The fibers of both ligament and tendon are made of collagen. Normal healthy tendons are composed of parallel arrays of collagen fibers closely packed together. These collagens are held together with other proteins, particularly with proteoglycan in compressed regions of the tendon. Ligaments and tendons play a significant role in the biomechanics of the skeleto-muscular system. The elastic properties have endowed a unique ability for tendons to act as springs. Tendons can passively modulate forces during locomotion, and provide additional stability to bone joints with no active work. It also allows tendons to store and recover energy at high efficiency.

For example, during a human stride, the Achilles tendon stretches as the ankle joint dorsi-flexes. During the last portion of the stride, as the foot plantar-flexes (pointing the toes down), the stored elastic energy is released. Furthermore, because the tendon stretches, the muscle is able to function with less or even no change in length, allowing the muscle to generate greater force. Tendons and muscles work together and can only exert a pulling force.

Mechanical properties of ligaments and tendons increase from early childhood to young adulthood; however, further aging process affects their stiffness. One study found that stiffness of the ligaments decrease by 2-3 folds among the elderly versus younger adult knees. Immobilization of a joint for long periods of time is detrimental for joint structure and function, including decreased range of motion for the joint. The affects of both ligaments and tendons can be severe. Immobilization for 9 weeks can led to a 69% decrease in ultimate load and an 82% decrease in energy to failure. Upon remobilization, the mechanical properties of the ligament are gained back first, followed by the structural properties. Exercise and increased load on tendons and ligaments is believed to alter their structural makeup and lead to increased mechanical properties

Tendon length varies from person to person, and is practically the sensitive factor where muscle size is concerned. For example, should all other relevant biological factors be equal, an individual with a shorter tendon and a longer biceps muscle will have greater potential for muscle mass than a person with a longer tendon and a shorter muscle. Accordingly, successful bodybuilders will generally have shorter tendons. Conversely, athletes from sports such as running or swimming, have longer than average Achilles tendon and a shorter calf muscle. Tendon length is determined by genetic predisposition, and has not been shown to either increase or decrease in response to environment, unlike muscles which can be shortened by trauma, use imbalances and a lack of recovery and stretching.

The Cartilage

Cartilage is a dense connective tissue that allows bones to slide over one another at all moveable joints with ease; thereby, reduces friction, and prevents damage. Cartilage is found in many areas in the body including the articular surface of the bones, the rib cage, the bronchial tubes and the inter-vertebral discs. Its mechanical properties are somewhat between bone and tendon. Any breakdown of cartilage in the bone joints results in bone damage. Weakened cartilage in the spine can lead to a slipped or crushed vertebral disc. This breakdown of cartilage leads to a severe bone disease – osteoarthritis.

Cartilage is made of specialized bone cells called chondrocytes that produce large amounts of extra-cellular matrix composed of collagen fibers, proteoglycan, and elastin fibers. Based on the differences in the relative amounts of these 3 matrix chemicals, cartilage is classified in 3 types, i) hyaline cartilage, ii) elastic cartilage, and iii) fibro-cartilage.

i) Hyaline cartilage is the most abundant type that covers the end of bone(s) to form a smooth joint surface. Most of the skeleton of a fetus is laid down in cartilage before replaced by the bone. Hyaline cartilage in the adult is found in the nose, parts of the respiratory tract, the larynx and between the ribs and the sternum.

ii) Elastic cartilage contains large amounts of elastic fibers (elastin) scattered throughout the matrix. It is important to prevent the collapse of tubular structures. Elastic cartilage is found in the pinna of the ear, auditory tubes and in the epiglottis (in the throat).

iii) Fibro-cartilage is characterized by a dense network of type-I collagen. It is a white, tough material that provides high tensile strength and support; with properties similar to those of tendons. Fibro-cartilage is present in areas most subject to frequent stress like inter-vertebral discs, and the attachments of certain tendons and ligaments.

Cartilage is one of the few tissues in the body that does not have its own blood supply. Therefore, chondrocytes get their nutrients by diffusion. Thus, compared to other connective tissues, cartilage grows and repairs more slowly. Cartilage contains a lot of water, which decreases with age. About 85% of cartilage is water in young people. Cartilage in older people is about 70% water.

Bones change in size, shape, and position by modeling and remodeling processes. In modeling, a bone is formed at one site and broken down in a different site, which changes its shape and position. In remodeling, the cellular activity of bone removal and replacement occurs at the same site. While remodeling is the predominant process during early adulthood – the bone forming years; modeling, however, continues throughout life, in response to weakening of the bone. As a result, most of the adult skeleton is replaced about every 7 years.

Bone is dynamic living tissue under continuous reorganization; however, the normal bone maintains equilibrium between the old bone being dissolved and the new bone being laid down. This process is called – bone turnover. Bone turnover ensures the mechanical integrity of the skeleton throughout life and plays an important role in calcium homeostasis. The processes of bone building and breakdown in response to internal and external signals are carried out by specialized cells that build or break down bone. The cells that form bone are osteoblasts and those breaks down the bone are osteoclasts.

Calcium is one of the abundant minerals in the human body, and perhaps the one with most direct impact on bone health. More than 99% of total body calcium is stored in the bones and teeth, providing vital support to the skeleton. A constant level of calcium is required in body fluids and tissues to perform vital body functions. The blood, the heart, the muscular system, the nervous system, the hormonal system, the kidneys, and the gastrointestinal system are all affected by calcium and demand a specific calcium balance. It should be noted that the physiological levels of calcium can be maintained only through diet and supplements, because the human body can not produce this bone mineral by itself.

Calcium absorption in the gastrointestinal tract involves two specific cellular pathways: i) trans-cellular, an active transport that requires magnesium and vitamin-D; and ii) para-cellular, passive process that requires acidification of the calcium salts in the stomach. Factors that influence calcium absorption include age and health status of the individual, vitamin-D availability, calcium levels in food consumption, type and amount of fiber in the diet. It should be noted that all calcium ingested is not absorbed into the body. Calcium absorption begins in the stomach with its acid (hydrochloric acid), which dissolves, ionizes and facilitates mineral assimilation in the gut. However, secretion of stomach acid decreases gradually with age; up to 40% of post-menopausal women may be severely deficient in this natural stomach acid, with a predisposed risk of poor calcium absorption.

Several hormones affect calcium mineralization in the bone. The parathyroid hormone (PTH) transports calcium from the bones into the bloodstream. It also signals the kidneys to conserve calcium and other minerals from the urine. Additionally, PTH signals the kidneys to produce calcitrol, which is formed from vitamin-D that regulates the small intestine to absorb more calcium. The thyroid gland secretes calcitonin, which increases bone mineralization, and decreases the rate of bone breakdown.

Calcium homeostasis is a closely regulated process that maintains the levels of Ca2+ ions in the body within a tight normal range. During this process, calcium is removed from the bone and released into the blood circulation to maintain Ca2+ at 2.5 mmol/L (normal level). The typical calcium content of the adult human body is 1 kg, virtually all is found in the skeleton; the amount in body fluids and cells of the soft tissues accounts for the remaining for 1% calcium reserve.

Bone is a remarkable organ that serves structural function; provides mobility, support, and protection for the body; and a reservoir function, as the storehouse for essential minerals. Though delicate in appearance, the bones in our skeletal system are ounce for ounce, stronger than “mild” steel. Skeletal bone framework is adapted to provide adequate strength and mobility to resist factures upon substantial impact, or during vigorous physical activity. Shape and structure of the bone are equally important as its mass in providing such strength.

Mineral Bank: The primary function of the skeletal system is to operate the deposits and withdrawals of minerals. The dynamic process of withdrawal and storage of calcium is a multifunctional characteristic, which is critical for several vital functions, especially for the heart to beat, the nerve cell to flash an electric impulse, the muscle to twitch, the bowel to move and a kidney to filter.

Blood Production: Bone marrow is the body’s designated manufacturing site for blood components. Bone marrow constitutes 4% of total body weight in adults. Bone marrow generates red blood cells (RBC) that oxygenate the tissues, white blood cells (WBC) that provide immune barrier against foreign bodies, and platelets that facilitate blood clotting.

Protection: Skeletal system protects the vital organs of the body. It provides a sturdy ‘cage of bones’that are designed to house the delicate internal organs and fragile body tissues. The fused bones of the cranium enclose the brain and make it less vulnerable to injury; the vertebrae surround and protect the spinal cord; and bones of the rib cage efficiently shield the heart and lungs.

Acid-base balance: Bone contains large stores of buffer, in the form of mineral salts to effectively balance the pH changes that occur in the body. Acid-base balance (or titration) in the body has a significant metabolic outcome on the bone turnover, especially on the rates of bone resorption and calcium mobilization.

Detoxification: Skeletal system is designed to scavenge several toxic chemicals, in particular, the heavy metal contaminants (i.e. lead, mercury) in body fluids and effectively dispose these artifacts via the circulatory route that runs through the skeletal matrix. Bone is also the major sink for vanadium heavy metal that enters the body.

Sound Management: The human auditory system is designed to detect several aspects of sounds, including pitch, loudness, and direction. Sound waves are acquired by the external ear and channeled through the ear canal to the eardrum. Three bones in the body, called as the auditory ossicles, help to convert the sound waves (vibrations in air) to mechanical (hydraulic) vibrations in tissues and fluid-filled chambers. Even the smallest vibration of the eardrum results in a significant amplification in the fluid chamber. This allows us to hear even the faintest of whispers

Movement is a basic function of the skeleto-muscular system, which manifests into postures and motions. This function is dependent on several muscles that attach to the bones through tendons, ligaments and cartilage. Muscle contraction, a familiar phenomenon that flexes the ‘biceps’ or tightens the 'abs', is the quintessential force that enables mobility. Human evolution freed upper limbs from the burden of bearing body weight during locomotion; this enabled us to grasp and manipulate objects with precision.

Shape or figure of a human body is defined exclusively by the structural frame of the skeleton. Skeletal structure (or size of the bone) grows and changes only up to the point at which a human reaches the adulthood and remains essentially same for rest the life. Genetics, gender and lifestyle play a cumulative role in the overall development and appearance of the body shape. Finally, bones not only gave us a shape but a face to look into the mirror.

Genetics, diet, environment and lifestyle influence the outcome of bone structure and its function. Deficiency or dysfunctions of any above factors lead to ‘metabolic bone disease’. Bone disorders often result in weak bones that can lead to painful and debilitating fractures. The symptoms of bone disorders manifest as skeletal deformities, in some cases can be irreversible, affecting the posture and mobility of the body. Certain affected individuals can be seriously handicapped and confined to wheel chair. Some chronic bone disorders are extremely severe and life-threatening.

Osteoporosis is a skeletal disorder characterized by compromised bone strength, and increased risk of fracture. Once a bone fractures, osteoporosis is often extremely painful and crippling. A reduction in height or a fracture to hip or wrist may be the first sign of osteoporosis. Osteoporosis may significantly affect life expectancy and quality of life. This disease manifests due to an imbalance between bone formation and bone resorption. As a result, the bone mineral density (BMD) is reduced, bone architecture is disrupted, and the quality of bone matrix is altered. Advanced aging is the common underlying cause for the onset and progression of osteoporosis in both men and women.

Osteo-Arthritis (OA) is a degenerative bone disease, caused by the breakdown and eventual loss of the cartilage in one or more joints. In severe OA, complete loss of cartilage cushion causes friction between bones, causing pain during rest or pain with limited joint mobility. OA occurs more frequently with aging. Before age 45, OA occurs more frequently in males. After age 55 years, it is more common among females.

Rheumatoid Arthritis (RA) is an inflammatory bone disease that causes pain, swelling, stiffness, and loss of joint function. RA is an autoimmune condition with several clinical features that make it unique from other types of arthritis.

Genetic abnormalities can result in weak, thin or overly dense bones. The hereditary disease, osteogenesis imperfecta is due to abnormalities in the collagen protein, which weakens the bone matrix and predisposes multiple fractures. Another congenital (hereditary) disorder, osteopetrosis, tends to make highly dense bone.

Nutritional deficiencies, particularly of vitamin-D, calcium, and phosphorus, can result in the formation of weak, poorly mineralized bone. In children, vitamin-D deficiency causes rickets, with typical weak bones, bowing of the long bones and a characteristic deformity due to overgrowth of cartilage at distal ends of the bone. In adults, vitamin-D deficiency leads to softening of the bone (a condition known as osteomalacia), with increased risk of fractures and skeletal deformities.

Hormone disorders can cause serious skeletal problems. Overactive parathyroid glands (or hyper-parathyroidism) can cause excess bone breakdown and increase the risk of fractures. Growth hormone malfunction can affect skeletal development, which may lead to short body stature. Loss of gonadal function (or hypogonadism) in children and young adults can set off severe osteoporosis due to loss of testosterone and estrogen.

BOOKS 
 
  • Abrahams P (2008) How the body works. Amber Books Ltd: London, ISBN 978-1-905704-57-6
  • Blandine C-G (2007) Anatomy of movement. Seattle: Eastland Press, ISBN 978-0-939616-57-2
  • Carter DR, Beaupre GS (2007) Skeletal Function and Form: Mechanobiology of Skeletal Development, Aging, and Regeneration. New York: Cambridge University Press, ISBN 978-0521714754
  • Currey JD (2006) Bones: Structure and Mechanics. Princeton University Press: New Jersey, ISBN 978-0691128047
  • Hall BK (2005) Bones and Cartilage: Developmental and Evolutionary Skeletal Biology. San Diego: Elsevier Academic Press, ISBN 978-0123190604
  • Naidu AS (2009) Bio-Replenishment for Bone Health. BioRep Media:California, ISBN 978-0982445105
 REVIEWS AND ARTICLES 
 
  • Deng ZL, Sharff KA, Tang N, et al (2008) Regulation of osteogenic differentiation during skeletal development. Frontiers in Bioscience 13:2001-2021.
  • Weaver CM (2007) Vitamin D, calcium homeostasis, and skeleton accretion in children. Journal of Bone and Mineral Research 22:45-49.
  • Aguila HL, Rowe DW (2005) Skeletal development, bone remodeling, and hematopoiesis. Immunological Reviews 208:7-18.
  • Hadjidakis DJ, Androulakis II (2006) Bone remodeling. Annals of New York Academy of Sciences1092:385-396.
  • Li Z, Kong K, Qi W (2006) Osteoclast and its roles in calcium metabolism and bone development and remodeling. Biochemical Biophysical Research Communications 343(2):345-350.
  • Caetano-Lopes J, Canhão H, Fonseca JE (2007) Osteoblasts and bone formation. Acta Reumatologica Portuguesa 32(2):103-110.
  • Pogoda P, Priemel M, Rueger JM, Amling M (2005) Bone remodeling: new aspects of a key process that controls skeletal maintenance and repair. Osteoporosis International16:18-24.
  • .Khanal RC, Nemere I (2008) Regulation of intestinal calcium transport. Annual Review of Nutrition 28:179-196.
  • van de Graaf SF, Bindels RJ, Hoenderop JG (2007) Physiology of epithelial Ca2+ and Mg2+ transport. Reviews in Physiology Biochemistry and Pharmacology 158:77-160.
  • Currey JD (2003) How well are bones designed to resist fracture? Journal of Bone and Mineral Research 18(4):591-598.
  • Chappard D, Baslé MF, Legrand E, Audran M (2008) Trabecular bone microarchitecture: a review. Morphologie 92(299):162-170.
  • Clarke B (2008) Normal Bone Anatomy and physiology. Clinical Journal of American Society for Nephrology 3:S131-S139.
  • Khan IM, Redman SN, Williams R, et al (2007) The development of synovial joints. Current Topics in Developmental Biology 79:1-36.

II. Skeletal System Replenishment

Bone Health is achieved during the childhood and adolescence, the skeletal forming years; and is established during young adult years of life. Management and functional up keeping of the bone health status is dependent on several factors including, nutrition, lifestyle, environment and age. Diet could affect the overall outcome of bone health. Malnutrition (low calcium, vitamin-D and trace mineral intake), oral hygiene and diabetes, can influence the bone health status. Addictive habits, such as tobacco consumption, smoking, alcohol, drug abuse, extreme dieting and certain medications could adversely affect bone health. Bones work on the ‘use it or lose it’ principle. Therefore, physical activities, especially, the weight-bearing exercises influence the strength and agility of the skeletal system. Indoor living (with little or no exposure to sunlight) and sedentary lifestyle could trigger a rapid bone loss. Aging is an ongoing biological phenomenon. Environmental factors (i.e. polluted air, water and land) and stress can take a cumulative toll on the body and cause rapid aging. Skeletal system is the primary target of an aging process. Sports, occupational and accidental injuries only worsen the aging bone. [Kiebzak 1991, US-DHHS 2000, Wright et al 2003].

Lifestyle factors strongly influence bone health. Sedentary lifestyle will result in bone thinning. Our current generation is ‘immobilized’ in front of a computer screen; a severe compromise to bone function. The human race, in the name of comfort, is gradually leaning towards a sedentary lifestyle with remote controls, mechanical devices that drive, fly and float. The Nature’s law of ‘use it or lose it’, with its enforcement (or forced adaption) could cripple the human skeletal system [Salamone et al 1999].

Physical activity: In addition to adequate calcium and vitamin-D intakes, weight bearing exercises are critical to the development, repair and maintenance of healthy bones [Specker 1996]. Higher bone mass is evident among athletes than non-athletes, and in highly active children compared to those who are sedentary. A similar trend can be observed in retired dancers and gymnasts. Reaching the peak bone mass during early years of life, is likely to offset future development of osteoporosis and bone fragility. Weight-bearing exercise strongly influences bone health [Bassey and Ramsdale 1994]. An astronaut in space can develop osteopenia within 6 weeks due to lack of weight-bearing stimulus to the bone .

Several clinical trials have shown that exercise in elderly women may prevent bone loss and increase bone mineral density (BMD), significantly [Wallace 2000, Kelley et al 2001]. Brisk walking, stepping block training, resistance and strength training provide positive benefits to the BMD of spine. In these studies, physical activity has shown a small reduction in fracture risk, inconsistent reduction in risk for falls and limited evidence in improving bone quality. However, among the elderly, physical activity could increase the risk for injury from falls as it involves skeletal muscle movement that displaces the body’s center of gravity and balance. Statistically, among the elderly, walking and climbing stairs are the two most common causes of non-fainting falls, which makes up 39% and 20% of falls, respectively. However, physical exercises done with proper training can significantly improve bone density and strength. Mechanical stresses such as enduring weight or bending are essential aspects of physical activity. These activities mobilize hormones and nutrients to the regions of the body that are undergoing stress. Studies have shown mineral accumulation in such regions of the bone, which eventually increase the bone mass. Therefore, physical activity is crucial in the prevention or even reversal of post-menopausal bone loss [Wolff et al 1999].

Weight-bearing activities such as brisk walking and resistance exercises are effective in increasing bone mass and strength. But this effect also declines with age; therefore it is advised to establish an exercise schedule while young and make it a part of the lifestyle. Weight-bearing exercises seem to enhance bone mineral accrual in children, particularly during early puberty [Bass et al 2002].

Dietary practices (e.g. weight loss diets, semi-starvation diets, crash diets) mostly among women, and also some men, attempting to be fashionably thin, can cause serious bone health problems [Wardlaw 1988]. Thin individuals have thin bones. Usually, underweight persons do not consume enough calories to maintain proper body weight; which can result in low BMD. Patterns of self-imposed under nourishment often begin early in life as adolescents become weight conscious. Body weight is the best predictors of BMD. Low body weight is associated with low peak bone mass development in the young, which eventually poses the risk of increased bone loss and fragility fractures in the adulthood. Body mass index (BMI) denotes the relationship between body weights to height and is used to classify individuals as being below or over a healthy range. The risk for hip fractures almost doubles in people with BMI of 20 kg/m2, compared to those with BMI of 25 kg/m2. Overweight adults on a calorie-restricted diet to lose weight, should take proper care to prevent bone loss. Sufficient intake of calcium and vitamin-D, and weight bearing physical activity are highly recommended to individuals on weight loss diets. It is also important to avoid ‘fad’ diets that eliminate whole foods [USDA 1992].

Alcohol could cause several detrimental effects on the bone. Alcohol inhibits liver enzymes that convert vitamin-D into active form; which can hamper calcium absorption. As a result, chronic alcoholism leads to poor mineral absorption and increased excretion of important bone-building nutrients like calcium, magnesium, vitamin-C, zinc and copper. Alcohol also blocks vitamin-B6 function. Alcohol is directly toxic to bone cells, and cause a decline in the spongy inner matrix of the trabecular bone [Schapira 1990].

Currently, there is conflicting evidence on beneficial effects of moderate alcohol consumption on the bone health. Alcohol derived from wine has favorable effects on the level of high-density lipoprotein (HDL) cholesterol and inhibition of platelet aggregation. In the elderly population, there seems to be a direct correlation between wine consumption and preservation of BMD. Red wine has high levels of polyphenols that positively influence multiple biochemical systems, such as increased HDL cholesterol, antioxidant activity, decreased platelet aggregation and endothelial adhesion, suppression of cancer cell growth, and promotion of nitric oxide production.

In contrast, higher levels of alcohol intake – more than two standard units of alcohol per day, could significantly increase the risk of hip and other osteoporotic fractures. Excess alcohol intake has direct detrimental effects on bone-forming cells and on hormones that regulate calcium metabolism [Sampson 2002]. In addition, chronic, heavy alcohol consumption is associated with reduced food intake (including low calcium, vitamin-D and protein intakes) and overall poor nutritional status, which in turn have adverse effects on skeletal health. Excess alcohol use could influence body balance and predispose the dangers of trip over, thereby increases the risk of fractures. However, available data is insufficient to indicate the precise range of alcohol consumption that would maximize bone density and minimize hip fracture risk [Felson et al 1995].

Smoking and its relation to the onset of bone disorders is complex; also various risk factors often co-exist. Body wise, smokers are thinner than non-smokers, physically less active and consume poor diet. Women smokers tend to reach early menopause than non-smokers. Among smokers, fractures take longer time to repair with several complications during the healing process. Regular tobacco use and smoking cause a significant decrease in blood total alkaline phosphatase, an indicator of bone metabolism. Although not confirmed yet, exposure to second-hand smoke during youth and early adulthood may increase the risk of acquiring low bone mass.

Smoking elevates nicotine levels in the body that cause blood vessels to constrict by approximately 25% of the normal diameter. Due to this constriction, the blood flow is reduced; consequently the supply of nutrients, minerals, and oxygen to bone tissue is diminished, which may slow down the production of bone-forming cells. In post-menopausal women, it reduces the protective effect of estrogen replacement therapy and may double the risk of rheumatoid arthritis.

Nicotine and other harmful chemicals in cigarettes affect bone health in several ways. Cigarette smoke generates huge amounts of free radicals with devastating effects on the body's natural defenses. Free radicals trigger a chain-reaction that damage tissue, organs, and hormones (e.g. estrogen) that regulate bones health. Other bone-damaging effects of smoking include elevation of the cortisol levels (hormone that regulates bone breakdown); and slowing down the calcitonin (hormone that helps to build bones). Nicotine and free radicals generated by smoking also kill the osteoblasts (bone making cells). Nicotine can also damage nerves in toes and feet, which may increase the risk of falls and fractures. [Law and Hackshaw 1997, Brot et al 1999, Krall et al 1999, Kanis et al 2005].

Caffeine is a stimulant present in a variety of drinks including tea and coffee. Although linked to a number of possible health benefits for heart and memory; caffeine is often implicated in the development of osteoporosis, due to its effect on calcium absorption. Caffeine can temporarily increase calcium excretion and may modestly decrease calcium absorption, but these effects are easily offset by increasing calcium consumption in the diet. Controlled clinical studies show that although caffeine ingestion results in a small, temporary increase in calcium excretion, it has no effect on 24-hour urinary calcium loss. One cup of regular brewed coffee causes a loss of only 2-3 mg of calcium which is easily offset by adding a tablespoon of milk. Moderate caffeine consumption, (1 cup of coffee or 2 cups of tea per day), in young women who have adequate calcium intakes would not have any negative effects on their bones. Studies that examined the effects of caffeine on rates of bone loss in post-menopausal women showed that caffeine intake had no detrimental effects, as long as calcium intake is sufficient (above 800 mg/day). However, if calcium intake is low, caffeine intake equivalent to about 3 cups of brewed coffee per day is associated with significant bone loss. A standard can of Cola drink contains 34-38 mg of caffeine. The potential risk of an acute caffeine toxicity may be greater with the consumption of “energy drinks” (stimulants and boosters) than conventional dietary sources of caffeine, like coffee and tea. Caffeine intoxication has been linked to a number of symptoms like nervousness, anxiety, restlessness, insomnia, gastrointestinal upset etc. which closely resemble symptoms of anxiety and mood disorders. [Massey 2001, Sakamoto et al 2001, Heaney 2002].

Soda drinks, high in phosphate content, are perhaps the most pervasive habit that promotes a calcium drain in the body. Phosphorus, an acid-forming mineral in the cola drinks, can interfere with calcium absorption by the bone and set off calcium loss through urinary excretion. Some studies have reported that high carbonated soft drink consumption either increased the fracture risk or decreased the bone mineral density. A recent study of soft drink consumption in adolescents suggested that teenage girls who drink lots of soda are predisposed to the risk of developing bone fractures and osteoporosis. These drinks also contain large amounts of refined sugar or equally dangerous sugar substitutes, which can trigger bone loss. During the teenage years, 40 to 60 per cent of peak bone mass is built, and therefore, it is very important to avoid or limit soda intake and change to a natural calcium-rich diet.

On the other hand, studies done with controlled calcium-metabolic methods indicated that the net effect of carbonated soft drinks, including those colas with phosphoric acid on calcium retention is low. An ‘acidic diet’ causes minerals to be drawn from the bones to neutralize the impact of the acid on blood pH. The body normally produces 50 to 100 mEq of acid a day during metabolism. The acid load imposed by a 20-ounce cola is only about 4.5 to 5.0 mEq, substantially less than the amount produced by eating a moderate protein breakfast. Phosphorus is a key constituent of bone mineral along with calcium, and there is no evidence for detrimental effects of phosphorus intake on bone health or osteoporosis risk in healthy individuals. The possible adverse effect of carbonated beverages may be due to substitution of milk in the diet by these drinks, which reduces calcium intake. Carbonation itself is also not responsible for the calcium depletion, as many commercial mineral waters are carbonated, and some are rich in calcium and other minerals. [Heaney et al 2001, Fitzpatrick and Heaney 2003].

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