Dietary impact on bone health

Good nutrition is essential in building and maintaining strong bones. Osteoporosis, rickets and osteomalacia are diseases of the bone that have a major impact on health and quality of life across the lifespan in both developing and industrialised countries.  Nutritional factors play important roles in determining the prevalence of these diseases.   

Bone is a living tissue that is continuously undergoing a cycle of resorption and formation. The process of bone formation requires an adequate and constant supply of nutrients. Inadequate intake of nutrients important to bone increases the risk of bone loss and can have a major impact on the quality of new bone being formed.

Calcium and vitamin D play important roles in improving bone mineral density (BMD) and reducing the risk of fracture. Other nutrients  that appear to also play a role in bone health include: protein, phosphorus, magnesium, zinc, vitamin C, vitamin K and fluoride. 

Calcium

Calcium is perhaps the nutrient most commonly associated with bone health. It is estimated that 99% of calcium in the body is contained in the skeleton and teeth. Calcium’s main function is to provide rigidity and structure.¹ Research highlights a beneficial relationship between calcium intake and bone health at various stages throughout the life cycle. ^2-4

The new Irish food pyramid published only last month recommends three servings of dairy products a day for the majority of the population, which will provide approximately 800mg of calcium per day. Irish children and teenagers aged 9-18 years need five servings of dairy products a day (1,200mg).⁵ The previous recommendation for women in the second half of pregnancy or those lactating to have five portions of dairy products a day has now been reduced to in line with the general adult population of three servings of dairy products a day.  Similarly the Institute of Medicine (IOM) recommends a calcium intake of 700-1,300mg/d for various life stage groups.

Milk and milk products are important sources of dietary calcium. They contribute 38% of the calcium intake in the diet of Irish adults aged 18-64 years.⁶ While the recent National Adult Survey reports that 16% of women aged 18-64 years had calcium intakes below recomendations,⁶ results from the Slán 2007 report highlight that some 61% of the Irish population eats less than the recommended minimum of three portions of milk/milk products a day.⁸

Milk and milk products contain calcium in large amounts and in a highly absorbable form. However, their fat content is often a cause for concern, particularly as cardiovascular disease and obesity are growing issues in the Irish population. Reassuringly, milk and milk products are available with varying fat content and low fat versions of dairy products have similar calcium, protein and B-vitamin levels to full fat versions.⁹ Although calcium is also available in a wide variety of non-dairy foods, the amount of calcium that can be absorbed from these foods, the amount of calcium per portion and the frequency with which they are consumed often makes them unreliable sources to optimise bone health. 

Certain components of plant foods can affect calcium absorption. These include phytates and oxalates, which are found in foods such as nuts, grains, spinach, rhubarb and beans. Phytates and oxalates cause insoluble complexes with calcium, reducing absorbability. To highlight how this impacts on the calcium content of our diet we can compare a serving of milk to a serving of spinach: to obtain the same amount of calcium as from a glass of milk, you would need to eat 16 portions of spinach.¹⁰ Approximately 30% of calcium from milk is absorbed, compared to only 5% in spinach.^11,12 Soy is a notable exception – despite the presence of oxalates and phytates, the calcium in soy is highly absorbable and comparable to milk. However, the calcium content of soy milk is only one sixth the calcium content of cow’s milk.¹⁰ In practical terms this would suggest that people avoiding dairy products should choose a calcium fortified soy milk as an alternative to cow’s milk.  

There are many plant foods with a low oxalate content that provide calcium in an highly absorbable form. These include vegetables such as broccoli, bok choy and kale.^10,13 However, the overall calcium content of these foods is low, making them an unreliable source of calcium in the daily diet. Other sources of calcium in the diet include oily fish with soft edible bones such as sardines and anchovies, but again, the frequency with which foods are eaten do not make them a reliable source of calcium.¹⁴

Ultimately, milk and milk products are the most reliable and absorbable form of calcium in the diet. Other foods high in calcium should be considered as a way to boost calcium intake in a dairy rich diet. Where milk and milk products are not eaten with sufficient frequency, calcium supplements and calcium fortified foods provide a way to enhance the calcium content of the diet.  

Vitamin D

Vitamin D is a fat-soluble vitamin that is found in certain foods. It is also produced on our skin when it is exposed to sunlight. Vitamin D is required for calcium absorption and plays a key role in calcium homeostasis – mainly by regulating calcium absorption and excretion. It also has a direct effect on bone, stimulating bone formation and resorption. Severe vitamin D deficiency causes poor quality bone to be formed leading to osteomalacia in adults and rickets in children. Low vitamin D levels are also associated with lower bone mineral density.¹⁵ Vitamin D production is affected by season, latitude, duration of exposure to sunlight, sunscreen use, skin pigmentation and the ability of the skin to form and process vitamin D. Vitamin D inadequacy is widespread worldwide. Studies in Ireland have revealed that low vitamin D status and vitamin D deficiency are widespread in the Irish population.¹⁶

Dietary sources of vitamin D include oily fish, fish oils, eggs and foods fortified with vitamin D. The National Adult Nutrition Survey in 2011 revealed that the main sources of vitamin D in the Irish diet were meat, fish and spreads.⁶ Irish people are often reliant on dietary intake of vitamin D to satisfy their requirements due to our northerly latitude. However, foods naturally rich in vitamin D are few in number and in many cases not widely consumed. Table   provides examples of dietary sources of vitamin D.

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In Ireland the recommended daily allowance (RDA) for vitamin D is 0-10µg/day depending on how much sunlight you are exposed to. For those aged over 51 years the RDA is 10µg/day as the skin’s ability to produce vitamin D from sunlight reduces with age. In 2011 the Institute of Medicine (IOM)¹⁷ derived RDAs for vitamin D based on conditions of minimal sun exposure. The IOM recommends 400IU (15µg) per day for children and adults, 400IU (10µg) for infants and 600IU (20µg) for older adults (71 years and older).  

Currently, infants are the only group in Ireland in which routine supplementation is recommended. In 2011 the Food Safety Authority of Ireland advised that all Irish infants from 0-12 months should be supplemented with 200IU (5µg) of vitamin D3 a day.  The IOM currently makes no recommendation regarding vitamin D supplementation as data suggests that 97.5% of the population in North America can maintain normal vitamin D levels (50nmol/L) with their current dietary intake.¹⁷ The UK and Canada, countries at similar latitude to Ireland, recommend routine supplementation of a number of at-risk groups, including the elderly and pregnant and breastfeeding women. It is expected that the FSAI will look at the exact vitamin D needs of different population groups before giving advice on supplementation to the wider population and that the first groups to be looked at would be ‘at-risk’ groups, such as pregnant women and children aged 12 months to four years.

Vitamin D enriched foods can play a vital role in ‘topping up’ our levels. Common foods supplemented with vitamin D include milk and breakfast cereals. Low dose supplementation should be considered in individuals who do not have regular sun exposure. 

Vitamin D supplements are now widely available in a variety of different doses and preparations. In the absence of national guidelines on vitamin D supplementation, caution should be exercised in recommending blanket vitamin D supplementation. Vitamin D levels can be assessed by a simple blood test which can indicate if supplementation is warranted. A serum level of 50nmol/L should be considered adequate for optimal bone health, levels >75nmol/L have not been consistently associated with increased benefit and risks have been identified for some outcomes at levels above 125nmol/L.¹⁷

Protein

Dietary protein is required for the remodelling process. Studies have shown that protein can increase urinary calcium excretion when calcium intakes are low. When intakes of both these nutrients are adequate, protein appears to interact positively on bone.¹ Additionally, protein may be important in maintaining or reducing bone loss in the elderly.  

Vitamin K

Vitamin K plays a major role in the formation of proteins within the bone matrix. Data suggests that low vitamin K intakes are associated with higher rates of fractures in the elderly – however as yet there is insufficient data to draw any firm conclusions or make any dietary recommendations.

Magnesium

Magnesium is involved in bone and mineral homeostasis. In a limited number of studies magnesium intake has been positively associated with BMD and short-term increases in BMD have been observed with magnesium supplementation.¹

Phosphorus

An adequate supply of phosphorus to bone is essential throughout life. Like calcium, phosphorus is essential for the mineralisation of the skeleton. Depletion of serum phosphate leads to impaired bone mineralisation. Some 85% of phosphorus in the body is found in bone.¹⁸ Despite the important role phosphorus plays in skeletal health there is no correlation between phosphorus intake and BMD.¹⁹

Other nutrients

Many other nutrients and dietary factors may be important for long-term bone health. Based on biochemical and metabolic evidence, there may be a role for zinc, copper, manganese, boron, vitamins A and B group, potassium and sodium, but currently there is no clinical or physiological studies to support theoretical hypotheses. The evidence is not strong enough to influence dietary guidelines but the accumulating picture suggests that current healthy eating advice will most likely benefit bone health.

Alcohol

Alcoholism is associated with a reduced BMD.¹ Ethanol reduces nutrient absorption, including calcium and vitamin D, and has a direct inhibition on bone formation. The impact of alcohol intake within recommended guidelines on bone is unclear and inconsistent, but recently updated guidelines recommend that adult males should consume a maximum of 17 units of alcohol a week and women a maximum of 11 units a week (one unit equals one measure of spirits, one small glass of wine or a half pint of beer). 

Low body weight

Small skeletal size is a risk factor for osteoporosis and reduced BMD. Meanwhile, obesity is traditionally viewed to be beneficial to bone health because of the well-established positive effect of weight bearing on bone formation. However, recent data from epidemiological and animal studies strongly suggests that fat accumulation is detrimental to bone mass.¹⁹ For overall health, an ideal body weight (BMI of 20-25kg/m2) should be maintained.  

Nutrients and bone health

Bone is a complex living organism that is constantly undergoing a process of resorption and formation – an adequate supply of nutrients throughout life is essential to optimise the quality of new bone formation. While this article discusses numerous nutrients such as protein, vitamin K, magnesium and phosphorus that play a role in bone health, the dietary requirements for these nutrients can be easily met by ensuring that the basic principles of the food pyramid are followed. Particular attention should be paid to ensuring that adequate amounts of dairy products are consumed and where dairy is avoided that calcium requirements are met with appropriate supplementation. Vitamin D-enriched products should be chosen where possible to overcome the lack of natural vitamin D production at our northerly latitude.

References

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2. Hucharek M , Muscat J, Kupelnick B, McCarron DA.  Impact of dairy products and dietary calcium on bone mineral content in children: results of a meta analysis.  Bone. 2009;43:312-321.
3. Heany RP , Dawson-Hughes B, Oparil S, Berga SL, Stern JS et al.  Dietary changes favourably affect bone remodelling in older adults.  J Am Diet Assoc.  1999;99:1228-1233.
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5. Food Safety Authority of Ireland.  Recommended dietary allowances for Ireland. 1999.
6. Irish University Nutrition Alliance.  National Adult Nutrition Survey.  Summary Report.  2011   
7. Department of Health and Children.  The Food Pyramid,  2005
8. Herrington et al.  Slan 2007: Survey of lifestyle, attitudes and nutrition in Ireland.  Dietary habits of the Irish Population.  Department of Health and Children.  Dublin:Stationary Office. 2008
9. The Food Standards Agency, McCance and Widdowson.  The compositon of food.  6^th edition.  Cambridge: The Royal Society of Chemistry.  2002.
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12. Nickel KP, Martin BR, Smith DL, Smith JB, Miller GD, Weaver CM.  Calcium bioavaiability from bovine milk and dairy products in premenopausal women using intrinsic and extrinsic labelling techniques.  J Nutr. 1996;126;1406-1411.
13. Heaney RP & Weaver CM.  Calcium absorption from Kale. Am J Clin Nutr.  1990;51:656-657.
14. Hansen M, Thilsted SD, Sandstrom B, Korgsbak K, Larsen T, Jensen M et al.  Calcium absorption from small boned fish. J Trace Elem Med Biol.  1998;12:148-154.
15. Cashman KP, Hill TL, Cotter AA, Boreham CA, Dubitzky W, Murray L et al.  Low Vitamin D status adversely affects bone health parameters in adolescents.  Am J Clin Nutr. 2008;87:1059-1044.
16. O’Sullivan M et al.  High incidence of vitamin D insufficiency in Irish Adults.  Ir J Med Sci. 2008;177:131-134.
17. Minerals and Trace Elements. In:  Thomas B &Bishop J editors.  Oxford: Blackwell Publishing. 2007.
18. Cao JJ.  Effects of obesity on bone metabolism.  J Orthop Surg Res.  2011;15(6)30.