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Osteoporosis prevention: building stronger bones

Samantha Stear
PhD MSc BSc RPHNutr
Science Director
The Sugar Bureau
Duncan House
Dolphin Square
London SW1V 3PW
W:www.sugar-bureau.co.uk

Member
Scientific Advisory Committee
National Osteoporosis Society

Osteoporosis literally means "porous bones" and is characterised by severe bone loss and deterioration of the internal bone structure. This results in increased bone fragility and is associated with a high risk of fractures. More than one-third of adult women and one in 12 adult men will sustain one or more osteoporotic fractures in their lifetime.(1)
The burden of pain, disability and risk of premature death for sufferers and the financial drain on health services caused by osteoporosis is considerable. In the UK, 310,000 osteoporotic fractures, predominantly of the wrist, hip and spine, occur each year, costing the health and social security annual budgets in excess of £1.7b.(2) The consequences of these fractures can be devastating - one in five people who sustain a hip fracture die from complications, and only half of the survivors regain full mobility and are able to live independently.(3,4) In the UK, more women die from the consequences of osteoporosis than from cancers of the ovaries, uterus and cervix combined.

Prevention of osteoporosis
Fracture risk depends ultimately on the strength of the bone and the tendency to trauma, with risk varying between fracture sites. For instance, almost all wrist fractures are associated with a fall, whereas fewer than a quarter of spine fractures follow a fall. Bone mineral strength in later life is a function of the maximum amount of bone mineral attained in early adult life (peak bone mass) and of subsequent age-related bone loss. Consequently, there are two main strategies that could potentially reduce the incidence of osteoporosis: reducing bone loss and the impact of falls in the middle-aged and elderly; and increasing peak bone mass in young people before bone loss begins.
Skeletal mass increases progressively during growth, with peak bone mass being achieved in young adult life (25-35 years). In general, men have a greater bone mineral mass at skeletal maturity than women because of their larger body size. It is well accepted that bone mineral mass is largely controlled by familial and genetic factors. However, lifestyle factors such as diet and physical activity are also considered to be important influences at all ages.

Nutrition
A healthy balanced diet is necessary to supply the body with all the energy and essential nutrients it needs. The influence of nutrient intake on bone mineral status is largely undefined, although a large number of dietary components have been proposed as possible determinants of osteoporosis. The role that nutrition has to play in osteoporosis has recently been reviewed.(5,6) Much of the work examining the effect of nutrition on bone has focused on calcium and phosphorus, as these are major constituents of bone tissue. However, some trace elements, such as zinc, manganese and copper, are also necessary for the growth, development and maintenance of healthy bones. Several vitamins, including vitamins A, B(6), C, D and K, have also been linked to bone health. The majority of nutrient effects are probably due to their influence on calcium absorption and excretion and hence on calcium balance.

Calcium
Calcium is the fifth most abundant mineral in the human body. Over 99% of total body calcium is found in the bones and teeth. Absorption of dietary calcium depends on its interaction with other dietary constituents and on physiological factors such as calcium-regulating hormones and the stage of the lifespan. Calcium losses occur through urine, faeces and sweat.
The only external source of calcium for the body is the diet, which has led to assumptions that dietary calcium influences the health of the skeleton. The major food sources of calcium in the UK are dairy products. Other sources include green leafy vegetables, products of white flour (due to its fortification with calcium carbonate), tofu and the soft bones of fish. Table 1 gives examples of some calcium-rich foods.

[[NIP10_table1_57]]

Average intakes for dietary calcium have declined over time, mainly due to a decline in milk and bread consumption. The debate about whether we are eating enough calcium has been ongoing for many years, and the uncertainties that remain are reflected by the differences in the recommended daily allowances in various countries. What is clear is that calcium intake must be sufficient to meet the biological requirement if optimal bone development is to be achieved, and therefore must be sufficient to provide an adequate amount, once absorption efficiency and obligatory losses have been taken into account. The current UK recommendations are given in Table 2.

[[NIP10_table2_57]]

Data from British surveys suggest that 10-15% of adolescents, younger women and women over 75 years who are not in institutions are consuming diets inadequate in calcium. The best way of ensuring that the nutritional status for calcium is adequate for bone health is to consume a healthy, varied and balanced diet. Providing that energy needs are met and the diet incorporates a wide variety of foods, it is reasonable to assume that calcium intake will be adequate. A diet that is inadequate in energy is often also deficient in nutrients. Restrictive eating and being underweight are undesirable at any age, and osteoporosis is more likely if individuals have a low body weight.

Bioavailability
The absorption of calcium from food depends on its bioavailability, which is the accessibility of a nutrient to participate in metabolic and/or physiological processes. Therefore, one has to take into account not only the total calcium content of the meal, but also the presence of constituents that modify calcium absorption or excretion. Bioavailability of calcium from nondairy sources is poor, which has led to concern that vegetarians, especially vegans, may have low calcium intakes and compromised bone mineral status. Several studies have demonstrated that vegetarians in Caucasian populations who eat dairy products are able to meet recommended calcium intakes and do not have compromised bone mineral density. However, vegans are at an increased risk of not meeting their calcium needs, especially during the rapid growth phase in adolescence.
A further source of calcium for those individuals who avoid dairy products is calcium-rich mineral water. Calcium from this source is as well absorbed and retained as that from milk. However, very few surveys have included calcium intake from drinking water, and hence population intakes of calcium are likely to be higher than surveys suggest. It is also worth noting that high intakes of calcium, greater than 2g per day, may have adverse effects on the absorption of other nutrients eaten at the same time, such as iron, zinc and magnesium, and although calcium supplements may be useful therapeutically, they have no demonstrable role in improving bone health in the general population.

Physical activity
Physical activity is important for bone health and helps protect against the risk of fracture, by maintaining and improving bone density and by enhancing neuromuscular competency. There is evidence that a lack of physical activity plays a major role in the deterioration of bone quality that accompanies osteoporosis. The loading of bone, either from gravitational forces or from muscular tension, influences its functional strength, particularly at the loaded regions of the skeleton. Both arise through weight-bearing activities such as running, jumping and climbing stairs, whereas activities such as weight-training and swimming involve muscular tension alone.
The skeletal response to exercise is greatest at the site of maximum stress; therefore weight-bearing exercises such as jogging can increase bone density in the spine and hips, and armloading exercises such as weight training and racquet sports (stroke side only) can increase bone density in the wrist. Recently, several studies have shown that one of the most effective ways of loading the skeleton, particularly at the clinically important sites of the hip and spine, is by doing brief bouts of high-impact exercise such as jumping and jogging. However, it is important to take into account any ­previous injuries or particular contraindications, such as osteoarthritis, before advising high-impact exercises.
It has also been demonstrated that prolonged exercise is not necessary to strengthen the bones. A limited effect will be achieved through nonweight-bearing activities, such as swimming and cycling, but they still have important cardiovascular benefits. In addition, although there is unlikely to be any improvement in bone density from low-impact activities, such as walking at a normal pace, they may contribute to improved balance and coordination, which in turn could help prevent the falls that precipitate fractures.
Participation for about 30 minutes in varied physical activities with a weight-bearing component would be expected to promote stronger bones at all ages. However, the activities need to be carried out at an intensity that is appropriate to both the age and capability of the individual.
There is an urgent need to identify means of reducing the incidence of osteoporosis. It is not an inevitable part of ageing but is preventable. The Royal College of Physicians guidelines state that possible measures to reduce fracture risk in the general population include increasing the level of physical activity participation at all ages, reducing the prevalence of smoking, and eating a calcium-rich diet.(7) Preventive strategies and health promotion should focus on these key lifestyle factors.

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References

  1. Cooper C. Epidemiology of ­osteoporosis. Osteoporos Int 1999;Suppl 2:S2-8.
  2. Torgerson DJ, Iglesias CP, Reid DM. The economics of fracture prevention. In: Barlow DH, Francis RM, Miles A, editors. The effective management of osteoporosis. London: Aesculapius Medical Press; 2001. p. 111-21.
  3. Copper C, Atkinson EJ, Jacobsen SJ, Ofallon WM, LJ Melton. Population-based study of survival after ­osteoporotic fractures. Am J Epidemiol 1993;137:1001-5.
  4. Eddy DM, Johnson CC, Cummings SR, et al. Osteoporosis: review of the evidence for prevention, diagnosis, treatment and cost-effectiveness ­analysis. Osteoporos Int 1998;8 Suppl 4fully .
  5. Prentice A. Is nutrition important in osteoporosis? Proc Nutr Soc 1997;56:357-67.
  6. Department of Health. Nutrition and bone health: with particular reference to calcium and vitamin D. London: HMSO; 1998.
  7. Royal College of Physicians and Bone and Tooth Society of Great Britain. Osteoporosis: clinical guidelines for prevention and treatment. Update on pharmacological interventions and an algorithm for management. London: RCP; 2000.

Resources
British Nutrition Foundation
High Holborn House
52-54 High Holborn
London
WC1V 6RQ
E:postbox@nutrition.org.uk
W:www.nutrition.org.uk

National Osteoporosis Society
Camerton
Bath BA2 0PJ
T:01761 471771
W:www.nos.org.uk