Diet and bone health

In the sixth article in our series on nutrition, Sarah Schenker explains the importance of calcium and vitamin D

Diet is an important factor in forming healthy bones. The mineral calcium is obtained from the diet and deposited in bones and teeth. Vitamin D is required for this process.

A healthy diet providing adequate calcium at all stages of life, coupled with an active lifestyle, will help to ensure strong bones. This is particularly important during childhood, adolescence, and early adulthood when bones are developing. Peak bone mass is reached at the age of about 30 to 35. It is the stage at which the skeleton is strongest. After this age bone mass decreases. Optimising peak bone mass at skeletal maturity provides important protection against osteoporosis in later life.

Around 90-95% of peak bone mass is attained by the end of the second decade. Adolescence is a particularly critical period, with approximately 40% of peak bone mass in girls being laid down during this time. This process is under strong genetic control but other determinants include physical activity, especially weight bearing exercise, such as brisk walking, running, and climbing stairs, and nutritional factors, such as dietary calcium and blood levels of vitamin D.

Osteoporosis

Osteoporosis is an increasing problem in the United Kingdom. It causes considerable pain and disability and costs the NHS in excess of £940m to treat each year.¹ It is a disease characterised by loss of bone mass and a deterioration in structural strength, in which the bones become fragile and susceptible to fracture, particularly at the hip, wrist, and spine. Osteoporosis most frequently affects older women who have gone through the menopause but it can affect men and younger women. One in three women and one in 12 men suffer from osteoporosis in the United Kingdom. Providing the hormones--in the form of hormone replacement therapy (HRT) that the body lacks, as a result of the menopause, can help prevent bone loss in women.

Jason and the Argonaunts discover that calcium and
vitiamin D help to grow strong skeletons

Recommended intake of calcium

There is currently no international consensus as to exact recommendations for calcium intake. This is partly because some focus on meeting requirements, others for optimising bone density. Recommended intakes for young adults vary. Calcium requirements within the UK were reappraised in 1998 and the recommended intake of 700 mg of calcium a day for adults aged 19 to 50 was reaffirmed.¹ In contrast a United States report recommended 1000 mg a day.²

The National Diet and Nutrition Survey (NDNS) of people aged 4 to 18 showed that about 10% of older boys (11 to 18) and 20% of older girls had calcium intakes below the lower reference nutrient intake--that is, intakes that are likely to be inadequate.³ In the NDNS survey of British adults aged 16 to 64 the average intake of those aged 16 to 24 was significantly lower than those aged 35 to 64 years.⁴

Research findings

Intervention studies indicate that bone mineral density can be increased in the short term with increased calcium intake. In elderly people fracture rate is lower in those with a higher calcium and vitamin D intake.

Several randomised controlled trials have now investigated the link between calcium intake and bone mineral density. Bonjour and colleagues found a greater increase in bone mineral density among 8 year old girls taking an extra 850 mg of calcium compared with those on their normal diet.⁵ This study also showed that the response to calcium supplementation depended on habitual calcium intake. Girls who had previously had the lowest calcium intake (<880 mg a day) benefited most from supplementation.

Research suggests that encouraging an increase in dairy food consumption could produce significant gains in bone density in children and adolescents. Cadogan and colleagues supplemented 80, 12 year old girls with an average of 330 ml of milk a day for 18 months.⁶ Bone mineral density increased to a greater extent in the supplemented group compared with those who had no extra milk. Some of the benefit may be due to other nutrients besides calcium as milk contains several other nutrients which may be essential for bone growth, including protein, phosphorus, magnesium, zinc, and B vitamins.

A recent review concluded that an increase in calcium intake during growth increases bone mineral by approximately 1-5%, depending on the skeletal site measured.⁷

Despite good evidence for a short term benefit of calcium supplementation, it is not clear if this persists once the supplement is withdrawn. Short term increases in calcium or dairy food intake in children or adolescents may not be sufficient to sustain an increase in bone mass over several decades. Previous studies have found differences in bone mineral density to disappear 18 months to two years after the withdrawal of the calcium supplements.⁸⁹

The ongoing Cambridge Bone Study is seeking to determine whether advising young people aged 16 to 18 to increase calcium intake (to 1000 mg a day), as well as increasing exercise levels, provides an effective means of optimising bone mineralisation. This study will also measure the effects 12 to 18 months after the end of the intervention.

Dietary sources of calcium

As well as milk and dairy products, such as yoghurt and cheese, which are the major calcium providers in the diet, calcium is also obtained from bread (a statutory requirement exists in the United Kingdom that white flour should be fortified with calcium, iron, vitamin B1, and vitamin B2), pulses, green vegetables, dried fruits, such as apricots, nuts, and seeds, and the soft bones found in canned fish.

Vitamin D

Vitamin D is important for the absorption of calcium. Bone loss may be significantly reduced in postmenopausal women whose diets are supplemented with 700 IU vitamin D daily. Other nutrients may also be important, such as magnesium, potassium, fibre, vitamin C, and zinc to be associated with a significantly higher lumbar spine bone mineral density.¹⁰

Vitamin D plays a vital role in calcium homeostasis and bone metabolism. Vitamin D insufficiency causes a change in serum free calcium which in turn stimulates parathyroid hormone secretion and mobilises calcium from bone. The clinical deficiency diseases rickets, which affects infants and children when bones are growing, and osteomalacia, which affects adults whose bone growth is completed, are rare in the United Kingdom, although cases are still sporadically reported.

But poor vitamin D status has substantial public health implications since it may be an adverse factor in developing osteoporosis. An adequate vitamin D status throughout childhood is likely to influence achieving peak bone mass.

Older people

Supplementation with calcium and vitamin D in older men and women reduces bone loss at several sites and decreases rates of non-vertebral fractures.¹¹ Ensuring an optimal intake of both calcium and vitamin D is, therefore, an important strategy to maintain existing bone mass and reduce fracture in older people.

The NDNS of people aged 65 years and over found that approximately 98% had vitamin D intakes below the level recommended (the recommended nutrient intake for this age group is 10 µg a day).¹² A large proportion also had low vitamin D status (low plasma levels), particularly those living in institutions as they had little exposure to sunlight.

The NDNS of people aged 4 to18 years found a low vitamin D state in a significant proportion of those surveyed.⁴ In both boys and girls, this problem increased with age. This may be linked with a reduction in the amount of time spent playing outside (thus exposure to sunlight).

Sources of vitamin D

Diet: The best dietary sources of vitamin D are oily fish--for example, herring, mackerel, salmon, trout--fortified margarines and spreads, meat and meat products, and eggs. The vitamin can also be synthesised through the action of sunlight on the skin, and for most people this provides the major source. Vitamin D levels fall in winter as skin synthesis declines.

Sunlight: Older people are vulnerable to vitamin D insufficiency because the skin becomes less efficient at synthesising vitamin D with age as the epidermis thins. The amount of pigmentation in the skin also influences its capacity to synthesise vitamin D and those with darker skin require longer exposure to ultraviolet light. Certain ethnic groups in the United Kingdom are, therefore, vulnerable to vitamin D deficiency and advised to take a supplement. There are also several cultural characteristics among these groups that adversely affects vitamin D status, including wearing concealing clothes and excluding meat and fish from the diet.

Encouraging children to be more physically active should ensure sufficient sunshine exposure and achievement of optimal peak bone mass.

Increasing vitamin D intake from the diet or through supplementation is likely to be of benefit for vulnerable groups who rely on dietary sources as a means of achieving an adequate state, particularly during the winter months. In the United Kingdom these vulnerable groups include children from Asian communities, whose skin pigmentation reduces absorption of sunlight, those who wear clothes which fully conceal them, older people who are housebound or seldom go out, and those living in institutions.

The British Food Foundation is an independent charity, which raises funds from the food industry, the government, the EU Commission, and other sources.

Sarah Schenker, British Nutrition Foundation, London
Email: s.schenker@nutrition.org.uk


studentBMJ 2002;10:45-88 March ISSN 0966-6494

1. Department of Health. Nutrition and bone health. London: DoH, 1998.
2. National Institutes of Health Consensus Development Panel. Optimal calcium intake. JAMA 1994;272:1924-8.
3. Gregory J, Lowe C, Bates C, et al. National diet and nutrition survey: young people aged 4 to 18 years. London: HMSO, 2000.
4. Gregory J, Foster K, Tyler H, Wiseman M. The dietary and nutritional survey of British adults. London: HMSO, 1990.
5. Bonjour J-P, Carrie A-L, Ferrari S, Clavien H, Slosman D, Thientz G, et al. Calcium-enriched foods and bone mass growth in prepubertal girls: a randomized, double-blind, placebo-controlled trial. J Clin Invest 1997;99:1287-94.
6. Cadogan J, Eastell R, Jones N, Barker ME. Milk intake and bone mineral acquisition in adolescent girls: randomised, controlled intervention trial. BMJ 1997;315,1255-60.
7. Cashman KD, Flynn A. Optimal nutrition: calcium, magnesium and phosphorus. Proc Nutr Soc 1999;58:477-87.
8. Slemanda CW, Reister TK, Peacock M, Johnston CC. Bone growth in children following the cessation of calcium supplementation. J Bone Miner Res 1993;8:S154.
9. Lee WTK, Leung SSF, Leung DMY, Cheng JCY. A follow-up study on the effects of calcium-supplement withdrawal and puberty on bone acquisition of children. Am J Clin Nutr 1996;64:71-7.
10. ;New S. Fruit and vegetable consumption and skeletal health: is there a positive link? Nutr Bull 2001; 26:121-6.
11. Dawson-Hughes B, Harris S, Krall E, Dallal G. Effects of calcium supplementation on bone density in men and women 65 years of age or older. N Engl J Med 1997;337:670-6.
12. Finch S, Doyle W, Lowe C, Bates CJ, Prentice A, Smithers G, et al. National diet and nutrition survey: people aged 65 years and over. London: HMSO, 1998.