Does the vitamin D status of Australians and New Zealanders need improving and how do we do it?

Vitamin D insufficiency in Australia and New Zealand was thought to be limited to certain high-risk groups such as institutionalised elderly, veiled women and their breastfed infants. The remainder of the population was assumed to receive sufficient vitamin D from sunlight exposure. Recent surveys in both countries have cast doubt on this assumption. (1-4) For example, almost 50% of New Zealand children and adults were deemed to be vitamin D-insufficient in national surveys, based on low serum 25 hydroxyvitamin D concentrations (<50 nmol/L), the best indicator of vitamin D status. (3,4) Vitamin D concentrations were much lower in the winter than summer and in Pacific People than Europeans, suggesting that skin colour and season are important determinants of vitamin D status. These findings are consistent with reports from several other countries. (5,6)

Do low circulating 25 hydroxyvitamin D concentrations pose a health risk to Australians and New Zealanders? Although reportedly on the rise the prevalence of rickets and osteomalacia is probably low outside of high-risk populations. (7) Of greater public health concern is that vitamin D insufficiency, not severe enough to cause rickets or osteomalacia, may increase the risk of osteoporotic fracture and other negative health outcomes. Vitamin D insufficiency leads to poor calcium absorption, a compensatory rise in parathyroid hormone leading to accelerated bone loss and increased risk of osteoporotic fracture. Low vitamin D status has also been associated with a number of non-skeletal outcomes such as increased falls, poor dental health and increased risk of type 1 diabetes as well as certain types of cancer. (8) Is vitamin D status causally related to these non-skeletal outcomes? Ultimate proof usually requires controlled supplementation trials. Unfortunately there have been few randomised trials for non-skeletal outcomes. In the Women's Health Initiative Trial vitamin D supplementation with or without calcium had no effect on colon cancer incidence over an average of seven years of follow up. (9) Even for osteoporotic fracture prevention, where a role for vitamin D is generally assumed, a benefit of vitamin D supplementation is not always reported. (10) To date, clinical trials have generally been three to seven years duration and carried out in high-risk populations. Maintaining optimal vitamin D status over several decades or during critical life periods such as childhood or adolescence may be the relevant exposure for disease reduction. (11) However, it is possible that the inverse association between vitamin D and risk of some cancers and diabetes may be the result of confounding. People who exercise more are likely to be outdoors, get more sunlight and synthesise more vitamin D. Thus, the lower risk of disease may not be the result of high vitamin D but the fact that people who have high vitamin D exercise more. The evidence for exercise lowering the risk of diabetes and some cancers is convincing.

What level of 25 hydroxyvitamin D is required in the population? While most experts agree that a circulating 25 hydroxyvitamin D of at least 15-25 nmol/L is needed to prevent rickets and osteomalacia, (12) there is a lack of consensus on the amount of 25 hydroxyvitamin D required for optimal health. Cut-offs for 25 hydroxyvitamin D required to minimise fracture risk are mostly based on the concentration of 25 hydroxyvitamin D above which there is no further suppression of parathyroid hormone, somewhere between 20 and 70 nmol/L. (13) A cut-off of greater than 50 nmol/L is often used to define a normal 25 hydroxyvitamin D; (13) however, cutoffs greater than 90 nmol/L have been proposed for some non-skeletal outcomes. (14,15) Most cut-offs have been derived from older European populations. Thus, caution is warranted when applying these cut-offs to other age and ethnic groups.

Even if a conservative cut-off is used, it is clear that a substantial proportion of New Zealanders and Australians might benefit from improved vitamin D status, particularly during the winter months. How do we improve the vitamin D status of the population? One approach is to recommend daily UV exposure from sunlight. However, this would be a confusing message for the public because it is at odds with 'Sun Smart' messages to reduce UV for skin cancer prevention. Further, recommendations for duration of sun exposure would need to vary by the amount of skin exposed, season, latitude and skin colour. In the winter months, when 25 hydroxyvitamin D is lowest, long periods of sun exposure would be required for adequate vitamin D synthesis. Indeed someone in Dunedin or Hobart could stand naked in the sun all day in July and not make sufficient amounts of vitamin D to meet their requirements.

In the absence of sufficient UV exposure from sunlight vitamin D must be obtained from dietary or supplementary sources. Until recently there has not been an Australian/New Zealand Nutrient Reference Value for vitamin D. (16) The current adequate intakes (AIs) of 10 [micro]g/day for those aged 50-70 years, 15 [micro]g for those aged over 70 years and 5 [micro]g for everyone else are not easily obtained from typical Australian/New Zealand diets. We are unlikely to change dietary patterns to increase vitamin D intake as good sources of natural of vitamin D such as oily fish are not frequently consumed and increased consumption of others sources such as eggs, butter and fortified margarine are not consistent with current healthy eating guidelines. Moreover, there is concern that current AIs may not be sufficient to maintain 25-hydroxyvitamin D concentrations in the healthy range. The amount of vitamin D required to achieve a certain hydroxyvitamin D concentration is not known precisely. However, intakes of vitamin D of 18-25 [micro]g/day were required to increase 25 hydroxyvitamin D to 70-100 nmol/L in studies of both younger and older adults, somewhat higher than the current AIs. (15)

Vitamin D supplementation is one option to improve vitamin D status, especially for high-risk groups. However, routine supplementation of the general population is not practical because of cost, compliance, and safety issues. Mandatory vitamin D fortification may be required as the Australian and New Zealand governments have approved AIs for vitamin D that are unobtainable from current typical diets. Mandatory fortification must not be implemented without careful consideration as to the choice of food vehicle(s), the amount of vitamin D added and plans for population monitoring. For instance, vitamin D is added to milk in North America; however, adding vitamin D to milk here will not improve the vitamin D status of Pacific People as they are typically low dairy consumers. (17) Detailed dietary modelling is required to ensure that target groups receive sufficient vitamin D while ensuring that others in the population are not exposed to excessive amounts of vitamin D. Safety is always a concern especially given that vitamin D is toxic at high doses. Fortunately there is a reasonable margin of safety between current AIs and the tolerable upper intake level of 80 [micro]g/day. (16)

Mandatory vitamin D fortification, if it occurs, is still several years off. How should dietitians advise their clients in the meantime? High-risk groups would still require supplementation even after mandatory fortification. Individuals in the general population who are concerned should consider taking a vitamin D supplement containing between 10 and 25 [micro]g/day during the winter months. Voluntary fortification of a few foods is permitted under food regulations and a number of milks are currently marketed in Australia and New Zealand as vitamin D-fortified. Regrettably under current food standards milk can only be fortified with 1 [micro]g per 200 mL. In order for an older person to meet their Al of 15 [micro]g/day they would need to consume 3 L of this milk. Immediate consideration should be given to increasing the amount of vitamin D allowed in milk and other permitted foods in the Food Standards Code.

Tim Green, PhD

Senior Lecturer, Department of Human Nutrition

and

C. Murray Skeaff, PhD

Associate Professor, Department of Human Nutrition University of Otago Dunedin, New Zealand

REFERENCES

1 Jones G, Blizzard C, Riley MD, Parameswaran V, Greenaway TM, Dwyer T. Vitamin D levels in prepubertal children in Southern Tasmania: prevalence and determinants. Eur J Clin Nutr 1999; 53: 824-9.

2 McGrath JJ, Kimlin MG, Saha S, Eyles DW, Parisi AV. Vitamin D insufficiency in south-east Queensland. Med J Aust 2001; 174: 150-51.

3 Rockell JE, Green TJ, Skeaff CM et al. Season and ethnicity are determinants of serum 25-hydroxyvitamin D concentrations in New Zealand children aged 5-14 y. J Nutr 2005; 135: 2602-8.

4 Rockell JE, Skeaff CM, Williams SM, Green TJ. Serum 25-hydroxyvitamin D concentrations of New Zealanders aged 15 years and older. Osteoporos Int 2006; 17: 1382-9.

5 Looker AC, Dawson-Hughes B, Calvo MS, Gunter EW, Sahyoun NR. Serum 25-hydroxyvitamin D status of adolescents and adults in two seasonal subpopulations from NHANES III. Bone 2002; 30: 771-7.

6 Ruston D, Hoare J, Henderson L, Bates C, Prentice A, Birch M. National Diet and Nutrition Survey: Adults Aged 19-64years. Volume 4: Nutritional Status (Anthropometry and Blood Analytes), Blood Pressure and Physical Activity. London: TSO, 2004.

7 Robinson PD, Hogler W, Craig ME et al. The reemerging burden of rickets: a decade of experience from Sydney. Arch Dis Child 2006; 91: 564-8.

8 Holick MF. Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. Am J Clin Nutr 2004; 80: 1678S-88S.

9 Wactawski-Wende J, Kotchen JM, Anderson GL et al. Calcium plus vitamin D supplementation and the risk of colorectal cancer. N Engl J Med 2006; 354: 684-96.

10 Jackson RD, LaCroix AZ, Gass M et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med 2006; 354: 669-83.

11 Heaney RP. Long-latency deficiency disease: insights from calcium and vitamin D. Am J Clin Nutr 2003; 78: 912-19.

12 Ringe JD. Vitamin D deficiency and osteopathies. Osteoporos Int 1998; 8 (Suppl 2): S35-9.

13 Lips P. Which circulating level of 25-hydroxyvitamin D is appropriate? J Steroid Biochem Mol Biol 2004; 89-90: 611-14.

14 Dawson-Hughes B, Heaney RP, Holick MF, Lips P, Meunier PJ, Vieth R. Estimates of optimal vitamin D status. Osteoporos Int 2005; 16: 713-16.

15 Bischoff-Ferrari HA, Giovannucci E, Willett WC, Dietrich T, Dawson-Hughes B. Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr 2006; 84: 18-28.

16 Commonwealth Department of Health and Ageing (Aust), Ministry of Health (NZ) and National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand Including Recommended Dietary Intakes. Canberra: NHMRC, 2006.

17 Russell D, Parnell W, Wilson N. NZ Food: NZ People. Key Results of the 1997 National Nutrition Survey. Wellington: Ministry of Health, 1999.

(See paper by Shrapnel and Truswell, pages 206-212)