IMPORTANT NUTRIENTS IN A BONE HEALTH

Summary

• Bone undergoes continuous remodeling, and an adequate supply of nutrient substrate is needed for this process.
• Dietary recommendations are designed to promote bone health and strength, and to maintain normal nutritional status.
• Evidence supports the important roles adequate intakes of Calcium, Protein, Vitamin D and Vitamin play in optimal Bone Health.

Background

Bone undergoes continuous remodeling and an adequate supply of nutrients is needed for this process. In addition to their roles as substrates for bone formation, dietary calcium, phosphorus and protein also play active roles in bone metabolism. Nutrients such as vitamin D, vitamin K, magnesium and zinc are essential for the many metabolic processes involved in the bone remodeling cycle. There are also a vast array of other micronutrients that are required for optimal bone health. In 1997, the Food and Nutrition board of the National Academy of Science, USA, released the new Dietary Reference Intakes (DRI) based on the latest understanding about nutrient requirements for optimizing health. For the bone related nutrients – calcium, magnesium, phosphorus, and vitamin D, these recommendations were designed to promote bone health and strength and to maintain normal nutritional status for individuals at different stages of life. The recommendations for other nutrients important to bone health (e.g. vitamin K) were based on other parameters, such as usual intake in healthy individuals and biochemical markers of starts. These recommendations were recently revised in 2000-2002.

Nutrient That Affect Bone

Calcium

Calcium is one of the main bone-forming minerals and 99% of the body’s calcium resides in the skeleton. There is now substantial evidence that adequate dietary calcium maximizes peak bone mass early in life and prevents bone loss later. A recent review of the benefit of calcium on bone throughout the lifespan showed that most trials increasing calcium intake led to increased calcium balance and bone gain during growth, reduced bone loss in later years and reduced fracture incidence. Meta analysis of trials has shown that calcium supplementation reduces the relative risk of hip fracture by 25-70%, vertebral fractures by 23% and non vertebral fractures by 14%. Calcium requirements have been set at levels associated with a positive calcium balance; the development of maximal peak bone mass during growth; and the lessening of bone loss thereafter.

Vitamin D

Vitamin D maintains serum calcium levels by increasing the efficiency of intestinal calcium absorption. Insufficient vitamin D increases the risk of fractures, especially in the elderly. This is due to the decreased efficiency of skin synthesis and intestinal absorption, and reduced sun exposure and intake. Hypovitaminosis D is now a major world wide public health problem, even un young adults. Studies show that vitamin D and calcium supplementation decreases bone loss and fractures in postmenopausal women and the elderly. Vitamin D supplementation without calcium has been shown to protect against bone loss in onl a few studies. The AI for vitamin D will maintain adequate serum 25-hydroxyvitamin D levels for individuals who have limited sun exposure and vitamin D stores, multiplied by a safety factor to account for those unable to obtain any sunlight. This also allows for seasonal fluctiations in vitamin D status.

Vitamin K

Vitamin K is a cofactor of ã-carboxylase, an enzyme necessary for the ã-carboxylation of glutamic acid residues on special proteins, including osteocalcin (OC). OC is the principal non-collagenous protein found in bone and although its precise function are unclear, recent work suggests that OC stimulates bone mineral maturation and maybe responsible for important aspects of bone mineralisation. Vitamin K deficiency increases the levels of undercarboxylated osteocalcin (unOC), which binds poorly to the hydroxyapatite crystal lattice of the bone. Population studies have reported an association between low vitamin K intake and lower BMD or higher fracture risk. Conversely, vitamin K supplementation reduces unOC levels, urinary calcium excretion, and increases BMD. Rexommended intake levels for vitamin K were based on reported consumption levels of ‘healthy’ individuals. These levels are consistently lower compared to the required levels suggested by studies relating vitamin K and bone health.

Protein

Protein is part of the organic matrix of the bone collagen structure and is also essential to the production of hormones and growth factors that modulate bone synthesis. A high protein diet can increase urinay calcium excretion, but a high calcium intake offsets this negative effect, allowing the protein-induced increase in growth factors I (IGF-I) to domintae. Studies show that a diet high in protein and calcium induces positive effects on bone in postmenopausal women and the elderly, probably through an increase in IGF-I, and improves outcomes after hip fracture.

Phosphorus

Phosphorus is required for the appropriate mineralisation of the skeleton. Phosphorus deficiency impairs mineralisation, but diets usually provide ample phosphorus. It is important to note that high dietary phosphorus, combined with low dietary calcium, increases parathyroid hormone (PTH) levels, ehich increases bone resorption. However, this effect is negated with adequate dietary calcium. In fact, phosphorus intake was positively correlated to bone in post menopausal women and shoen to be low in those with osteoporosis. Phosphorus intake recommendations were based on maintaining phosphorus balance and normal serum phosphate lebels in adults. Intakes that maintain serum lebels above the lower end of the normal range are considered adequate to meet cellular and bone formation needs in healthy individuals.

Magnesium

Magnesium influences bone metabolism indirectly through its role in ATP metabolism and as a cofactor for over 300 enzymes; and directly by dictating hydroxyapatite crystal size. Magnesium deficiency affects calcium metabolism, bone growth and fragility. Magnesium supplementation has been shown to increase BMD in pre and post menopausal women and reduce bone turnover in men. The new intake recommendations were set to meet tissue needs and offset losses. Studies in menopausal women; elderly women; and men have shown that intakes close to the DRI are related to greater BMD, lower bone resorption and suppression of bone turn over.

Zinc

Zinc is involved in osteoblastic activity, collagen synthesis, and alkaline phosphatase activity. Low serum zinc levels and excessive zinc excretion appear to be related to osteoporosis and decreased bone strength. The elderly may be at higher risk of zinc deficiency because this group usually has low serum zinc values. In young women, zinc was related to higher BMD and in postmenopausal women, zinc and other nutrient supplementation resulted in greater gain in bone compared to calcium alone. Since a sensitive indicator of zinc nutritional status is not readily available, the recommendation for zinc is based on a number of different indicators of zinc nutritional status.

Vitamin C

Vitamin C is a cofactor for the production of lysine and proline. These amino acids are required in the cross-linking of collagen present in bone. It also stimulates alkaline phosphatase activity, a marker for osteoblast formation. Studies have reported beneficial effects of vitamin C on BMD in pre and post menopausal women consuming at least 500 mg of calcium. The DRI for vitamin C was recently increased for both men and women. It is still based primarily on the avoidance of deficiency, rather than the prevention of chronic disease and the promotion of optimum health. However, intakes higher than the DRI have been related to improved bone health.

Vitamin A

Osteoblast and osteoclasts both have nuclear receptors for retinoic acid. Therefore, vitamin A is deemed to be essential for bone remodelling. Vitamin A deficiency leads to morphological changes in bone, while excessive intakes are related to lower BMD and higher fracture risk. Both high and low intakes of vitamin A are associated with lower BMD, while vitamin A intake close to current recommendations is associated with greater BMD. The latest DRI for vitamin A is based on the amount needed to ensure adequate stores of vitamin A in the body in order to support normal reproductive an immune function, gene expression, and vision.

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