During the teenage years, the sex hormones affect both bone size and bone strength. By 17, approximately 90% of the adult bone mass has been established. By 21, calcium is no longer added to the bones and by 30 loss of calcium from healthy bones may begin.
Girls as young as nine have been found to deliberately restrict essential foods for weight loss. For females, eating enough calcium is essential during childhood, adolescence, and young adulthood, when bones achieve their optimum density. Young persons involved in competitive sports and dancing are particularly prone to unhealthy dieting. National nutrition surveys have shown that many women and young girls consume less than half the recommended amount of calcium needed for growing and maintaining healthy bones. Studies have also shown that some women smoke to control their appetite.
Calcium contents of a selection of easily accessible foods
Calcium content mg/serving
- Skimmed milk: One glass (300g) 360
- Cheddar cheese: 50g 360
- Semi-skimmed milk: One glass(300g)354
- Whole milk: One glass (300g) 345
- Plain yoghurt: One pot (150g) 285
- Tinned sardines: 50g 275
- Dried figs: Four (80g) 200
- Vanilla ice cream: Two scoops 144
- Plain fromage frais: 100g 89
- Baked Beans: 135g 72
- Sesame seeds:10g 67
- Almonds: 25g 60
- Tinned salmon: 50g 47
- Watercress: 20g 34
- White bread: One large slice 33
- Wholemeal bread: One large slice 19
- Broccoli: 45g 18
- Peanuts: 25g 15
(Source: DoH 1998)
Recommended daily intakes of calcium
Recommended intake mg/day
- Birth to six months 210
- Six months to one year 270
- One to three years 500
- Four to eight years 800
- Nine to 13 years 1,300
- 14-18 years 1,300
- 19-30 years 1,000
- 31-50 years 1,000
- 51-70 years 1,200
- 71 and over 1,200
One survey in the USA found that these recommendations were achieved by only 35.1% of males age 12-19, 14.4% of females age 12-19, 45% of males 20-29 and 17.8% of females age 20-29.
As well as the dietary sources listed above, drinking water in hard water areas has been found to be rich in calcium. It has been one public health consideration that addition of calcium to drinking water may have beneficial effects on bone.
When diet is low in calcium, supplementary calcium is recommended. There have been two interesting recent findings about calcium supplements. Firstly, when calcium enriched mineral water is taken in small amounts throughout the day calcium is efficiently absorbed. Secondly, twice daily smaller half doses of calcium supplements are more effective in maintaining calcium levels than taking one large dose once daily.
Although not yet licensed in the UK as a supplement for osteoporosis prevention, the addition of fluoride salts to drinking water has been shown in some international studies to have a positive effect on spinal and femoral neck bone mass.
Childhood and adolescence are the most important times to ensure adequate calcium intake. There is evidence that young girls and teenagers are the most deficient in calcium and vitamin D. Efforts to promote heart health have emphasised the positive aspects of skimmed milk, but have failed to highlight the need for fat-soluble vitamins. Low-fat milk may have higher levels of calcium, but lacks fat-soluble vitamins. Dieting teenagers often cut out dairy products because they consider them too fatty. Milk is a good source of calcium and teenagers often fail to make up the amount lost through other food. Weight loss has been shown to be associated with increased bone resorption, as necessary calcium is taken from the bones in the absence of sufficient in the diet.
Eating disorders often start in adolescence, and more than 90% of anorexia and bulimia cases occur among females. Anorexia nervosa and bulimia nervosa affect as many as 3% of adolescent and young adult females, and the incidence appears to be increasing.
Eating disorders and overexercise syndrome may result in suppression of oestrogen levels and amenorrhoea if weight loss results in a body mass index (BMI) of less than 19. This in turn may impair the achievement of peak bone mass and cause early bone loss. In one Danish study, subjects with anorexia and bulimia were found to be two to three times more likely to suffer a fracture than subjects without an eating disorder. The consequent increased fracture risks persisted for up to 10 years after diagnosis and treatment.
Vitamin D is a fat-soluble vitamin that enables calcium to transfer from the intestine to the bloodstream and prevents excretion of calcium in the urine. Without enough vitamin D, the body begins to draw calcium from bones.
Food sources of vitamin D include dairy products, egg yolks, saltwater fish and liver. The recommended daily intake is 400-800 international units. Vitamin D is also derived from exposure of the skin to solar ultraviolet B radiation. It is manufactured in the skin following direct exposure to sunlight. Studies recommend that 10-15 minutes exposure of hands, arms and face two to three times a week is normally enough, but the efficiency of this mechanism is also dependent on the time of day, season, latitude, skin pigmentation, skin sensitivity, pollution levels and sunscreens.
There is evidence of significant problems in population groups who restrict their skin exposure for cultural and religious reasons. Dress covering the whole body has adverse effects on vitamin D status and the potential for causing secondary hyperparathyroidism in the long term.
Dark winter months in northern latitudes also increase the risk of vitamin D deficiency and consequent bone loss. One small German study of a group of 10 men and 20 women found that low-dose supplementation with calcium 500 mg per day and vitamin D 500 IU per day during the winter months effectively prevented bone loss.
Sunscreen markedly diminishes the manufacture of vitamin D in the skin, as do window glass, clothing and air pollution. Skin colour also affects vitamin D production: the fairer the skin, the more vitamin D is manufactured.
References available on request.