Title: Peak Bone Mass.
Key words: osteoclasts, osteoblasts, osteocytes, mineralisation, calcium, Vitamin D, calciferol, menopause, bone loss, fractures, Vitamin K, caffeine, hypercalciuria, bone mineral density, osteoporosis, oestrogen deficiency,
Date: Sept 2000
Category: The Body
Author: Dr Van Rhijn
Peak Bone Mass.
Adult bone health is governed by maximum attainment of peak bone mass (PBM), and bone loss rate with ageing. Skeleton is dynamic, constantly resorbed (osteoclasts) and replaced (osteoblasts), coupled by locally acting communicating cells (osteocytes), under careful control by genetic, hormonal and mechanical factors. Bone ossification (osteoblasts) and mineralisation (Ca) is an extremely complex process where PBM, usually attained during the first 25-30 years, entirely replaced every 7 years, starts to decline between 3-5 years after menopause. This paper reviews the role of nutrition and physical activity in this process of constant remodelling1, required to maintain the integrity and maximum mechanical skeleton strength (PBM).
Role of Nutrition
Skeleton calcium (Ca=1Kg)2, is in equilibrium with plasma Ca (2.25-2.6 mmol/L), and gut absorption (active & passive), enhanced by vitamin D metabolite, calciferol (1.25(OH)2D3), in balance with faecal and urine losses3. Numerous factors (intake, absorption, hormones and physiological states) finely regulate Ca plasma pool making it difficult to verify each individual contribution. Various Ca supplementation trials, show positive effects on bone mass4,5,6,7,8 and skeletal growth9,10, with a threshold evident above which additional Ca has no further bone gain effects11, although effects may be temporary12. Postmenopausal Ca supplementation has little effect13,14, or some effect15,16 in reducing bone loss, but prevents hip fractures in presence of vitamin D17,18,19,20,21. Vitamin K may have a positive effect on bone development and maintenance through its role in promoting carboxylations of the matrix protein, osteocalcin22,23.
Caffeine24 and dairy protein (acid producing) has negative effects on bone mineral density by increasing calcium excretion, resulting in more hip fractures25,26, but does not ally to dairy products27,28, as phosphorus negates protein induced hypercalciuria29. Potassium bicarbonate benefits skeletal metabolism30,31 by improving acid-base homeostasis and explaining Ca mobilisation and loss in the elderly. Bone contains magnesium (65% of total body amount)32 and phosphorus (80% of total body amount)33 which are closely associated with calcium absorption, metabolism and bone density. Other elements such as boron, fluoride, manganese, copper, lithium, molybdenum, silicon and vanadium are concentrated in bone, and although their role is as yet unknown deficiency contributes to osteoporosis. Increased sodium intake may contribute to increased Ca excretion34,45 and promote bone demineralisation36.
Role of Physical activity
Bone mass regulation (mechanostat model)37 responds to regular weight bearing exercise38,39,40,41, (increased PBM) and skeletal bone loss occurs in its absence42. This positive relationship exists in all ages43, though youth44,45,46 onset, high impact exercise is more important than Ca intake47,48 and debatably49 provides protection against risk of osteoporosis in later life - as is the case in sports women who have increased bone density50.
However, women involved in high intensity physical training risk low bone mineral density. Factors contributing to the failure achieving maximum PBM51,52 are low body weight (stress / negative energy balance) related amenorrhoea and subsequent hypothalamic ovarian suppression. This oestrogen deficiency suppresses the osteoblasts53, as opposed to osteoclast over activity seen in menopausal women. There is a correlation between energy deficit and reduced bone formation serum markers54, making these athletes prone to premature bone loss and stress fractures55. The American College of Sports Medicine encourages recognition, prevention and management of this female athletic triad of under-nutrition, amenorrhoea and osteoporosis56. In contrast, age matched, low body weight gymnasts have a significantly higher bone mass57 and further studies are underway to elucidate this observation58.
Nutrient intake role on bone health remains undelineated59 due to difficulty isolating individual nutrient contributions amongst multi-factorial control on PBM, and requires more research60. The COMA Subgroup on Nutrition and Bone Health recommends a healthy lifestyle, with varied, adequate diet and regular weight-bearing activity to achieve PBM and maintain bone health.
Prevention of osteoporosis should begin during pre-pubertal years61and continue throughout life, but caution against strenuous physical activity in under-nourished sports women, as this could be detrimental to their bone health.
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