Nutrition, bone health and the milk issue
Bone is a living, dynamic, metabolically active tissue. It is also a specialized connective tissue, composed of a collagen (protein) framework permeated with mineral salts, composed of mostly calcium and phosphate, together with trace amounts of other minerals and ions.
A healthy, balanced diet containing both macronutrients (proteins, fats and carbohydrates) and micronutrients (vitamins and minerals), plus sufficient calories, is vital for both the normal development and maintenance of the skeleton.
The importance of calcium and Vitamin D in bone health
Calcium is a major structural component of bone tissue. It is deposited in bone as hydroxy-appatite which confers strength to the skeleton. Approximately 99% of calcium is stored in the bones and teeth. Vitamin D is also essential for the development and maintenance of bone, both for its role in assisting calcium absorption from food in the intestine, and for ensuring optimal formation and mineralization of bone tissue.
A number of research studies have demonstrated the importance of nutrition in bone health. Intervention trials carried out over three years in children and adolescents have shown that supplementation with calcium, dairy calcium-enriched foods, liquid milk, or calcium-enriched milk powder enhances the rate of bone mineral acquisition, compared with un-supplemented (placebo) control groups (1-4).
In studies among adults, one three-year intervention study in healthy young women aged 30-42 years showed that supplementing the usual diet with dairy foods prevented bone loss in the spine, compared with control subjects who did not increase their dairy intake (5). In post-menopausal women and the elderly, several intervention studies have shown that calcium or milk supplementation slows the rate of bone loss (6-15).
In a study carried out in healthy, elderly women living in nursing homes, calcium (1200mg/day) and vitamin D (800IU/day) supplementation over 18months reduced the risk of hip fractures and other non-vertebral fractures (7). A similar intervention over three years of calcium (500mg/day) and vitamin D (700IU/day) was shown to reduce bone loss and the incidence of non-vertebral fractures in elderly men and women living at home and not in institutions (6). In comparative intervention studies, dairy food supplements and calcium supplements were equally effective in preserving hip-bone mass in postmenopausal women (13,15). These studies were however not designed to evaluate reductions in fracture rates.
Vitamin D is made in the skin when it is exposed to sunlight (ultraviolet B rays) and can also be obtained from food and dietary supplements. The sun provides a source of vitamin D in varying amounts for different individuals, dietary recommendations for vitamin D are therefore approximate. Emerging evidence and expert opinion suggests that the minimum blood level of 25-hydroxyvitamin D optimal for fracture prevention is 70-80 nmol/l (16). To achieve this, the average older person would need a vitamin D intake of at least 800-1000 IU/day.
Dietary or supplemental vitamin D increases in importance during the winter months (especially in the Cape Province and those areas that experience winter rainfall) and for elderly, institutionalised people in whom the capacity for skin synthesis of vitamin D is reduced. Block-out sunscreens and darker skins also cause less absorption of Vitamin D. Mild degrees of vitamin D deficiency are very common, and in the long term can predispose one to osteoporosis. Without a sufficient supply of vitamin D from the skin or diet, the metabolism of calcium is disturbed. A low vitamin D level causes inefficient absorption of calcium from the intestine. This causes an increase in PTH levels, leading to over-extraction of calcium from the skeleton, bone loss and ultimately osteoporosis. Poor vitamin D status in the elderly also increases the likelihood of falling (and therefore fractures), because vitamin D is also important for the correct functioning of the muscles and nervous system (17).
Milk: good or bad for bones?
Milk makes a very valuable contribution to a healthy diet as it contains all the macro-nutrients (protein, carbohydrates and fat). Milk protein also contains the essential amino acids needed by the body. The carbohydrate, lactose, supplies energy and aids the absorption of calcium. Milk is considered globally to be the best source of calcium because of its high calcium content, the absence of factors that may influence the absorption thereof (such as phytates and oxalates in green, leafy vegetables) and the presence of lactose, which aids in the absorption of calcium.
Milk also contains significant amounts of minerals and trace-elements like calcium, phosphate, magnesium and selenium, and vitamins A, D and B (riboflavin and niacin), needed for bone health as well as general health.
Does the protein in milk cause calcium loss?
In the Framingham study, elderly men and women with low protein intakes had greater rates of hip and spine bone loss than those consuming higher amounts of protein (18). There is also evidence that increasing protein intake has a favourable effect on bone mineral density in elderly men and women receiving calcium and vitamin D supplements, suggesting a synergistic effect of these nutrients in improving skeletal health (19).
Despite this evidence, there has been speculation that a higher dietary intake of protein could have negative effects on calcium metabolism and could possibly induce bone-loss. This relates to the hypothesis that the ‘acid-base balance’ of the diet is a potential risk factor for osteoporosis and if a diet contains predominantly acidic foods (which include key protein sources) and does not contain sufficient alkali-rich basic foods (fruits and vegetables), the alkaline salts of the skeleton may be drawn on to buffer this effect and in the long term lead to bone loss (20). Although there is some evidence from observational studies that a more alkaline diet is beneficial to bone health in pre- and post-menopausal women (21), the theory has not been proven in more definitive clinical trials.
Vitamin K is needed for the production and functioning of osteocalcin. Osteocalcin is the second most abundant protein in bone after collagen, and is required for bone mineralization. Some evidence suggests that low dietary intake of vitamin K or low vitamin K status could contribute to low BMD and increased risk of fragility fractures in the elderly because of the impaired functionality of osteocalcin (22,23). Good food sources of vitamin K include leafy green vegetables such as lettuce, spinach, cabbage and kale, liver, and some fermented foods such as fermented cheeses and natto (fermented soybeans).
The mineral magnesium is important for calcium homeostasis and in the formation of hydroxyapatite (bone mineral). Severe experimental magnesium deficiency results in
impaired bone structure and function (24), but this level of depletion is rarely observed in generally well-nourished human populations. Magnesium is fairly widespread in the food chain; particularly good sources include green vegetables, legumes, nuts, seeds, unrefined grains, and fish. The elderly could potentially be at risk of mild magnesium deficiency, as magnesium absorption decreases and renal excretion of magnesium increases with age. Older people are also more likely to be taking certain medications that can increase magnesium loss in the urine, such as loop and thiazide diuretics, cancer medications or antibiotics. Alcoholics and diabetics are also more prone to magnesium deficiency. However, there are no studies to date which demonstrate that magnesium supplementation is useful either in preventing bone loss or reducing fracture risk.
The mineral zinc is a constituent of the hydroxyapatite mineral crystals of bone, and plays a role in the regulation of bone turnover. Zinc is also needed for the correct functioning of alkaline phosphatase, which is required for bone mineralization. Mild degrees of zinc deficiency have been reported in the elderly and could potentially contribute to poor bone status.
An observational study in middle-aged and elderly men showed that men with the lowest blood zinc levels had lower BMD than those with the highest blood zinc levels, and that dietary zinc intake and blood levels were lower in men with osteoporosis than in those with a normal BMD (25). Another study in postmenopausal women indicated that the bone-trophic effects of zinc could be mediated, at least in part, via a decrease in the blood levels of insulin-like growth factor-I (IGF-I) (26). Lean red meat and meat products, poultry, whole grain cereals, pulses and legumes are good sources of zinc.
Good nutrition alone will neither prevent nor cure osteoporosis, but in the context of a bone friendly lifestyle (exercise, stop smoking, limit alcohol intake) it is probably the more pleasurable and easiest task on the list!
Teréza Hough CEO, National Osteoporosis Foundation South Africa (NOFSA), 2010
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