IN a country with abundant sunshine and plenty of milk, inadequate vitamin D status and low intakes of calcium should be rare.
However, while precise prevalence estimates are lacking, a significant number of us are expected to manifest the consequences of deficiencies of either or both nutrients. Bone health, in particular osteoporosis, is the main natural consequence of this nutritional and lifestyle choice.
Sixty per cent of our adult population is overweight or obese, and the prevalence of type 2 diabetes (T2DM) has doubled in the past 20 years. There is an emerging body of observational research that links poor calcium intakes, inadequate vitamin D and high parathyroid hormone (PTH) concentrations with increasing prevalence of many chronic diseases including obesity, cardiovascular disease, diabetes and even some cancers.
Such associations between diet and disease offer a potential public health strategy, if causality reaches an acceptable level of probability. At present, this is where the issue lies.
Calcium and vitamin D have many biological effects, and determining the adequacy of each nutrient alone or in combination would depend on which end point is of greatest concern.
In part, the evidence for such effects also depends on emphasis paid to the various types of study designs that inform evidence-based nutrition.
An influential report by the US Institute of Medicine of the National Academies has, however, reignited debate by advocating calcium and vitamin D only for bone health.
This decision has resulted in an outcry from many prominent researchers in the field, who believe that there is sufficient evidence in support of the extraskeletal benefits of these nutrients for human health.
A role for calcium in the regulation of body weight has gained much interest since the observation in 2000 that increasing the intake of calcium (as yoghurt) increased the loss of body fat particularly from the abdominal region. Based primarily on studies with the agouti mouse model, the researchers proposed that intracellular calcium (iCa2+) held the key to fat deposition and obesity.
According to this early scheme, increases in dietary calcium would, via PTH, chronically lower iCa2+ in the adipocyte. This would then act to reciprocally reduce lipid deposition while stimulating adipose tissue breakdown.
A review of the evidence that I have been involved with has updated this model to include several other potential pathways that could influence both sides of the energy balance equation in humans. A simple analysis of a collation of currently available randomised controlled trials has led us to conclude that ingested calcium and vitamin D has two effects:
- An increase in whole body oxidation of fat. Unlike carbohydrates and proteins, absorbed fat has only two routes of disposal — oxidation or storage. It follows that increased fat oxidation would result in a less positive fat balance, other things being equal. A very recent meta-analysis by an independent group confirms that increasing calcium by 800 mg/d would favour an 11% increase in fat oxidation.
- An increased faecal fat excretion. It is now clear that even unabsorbed dietary calcium is not without benefit. In fact, within the gastrointestinal tract unabsorbed calcium links with dietary fat to form insoluble calcium-fatty acid soaps, which are then excreted. Available data predicts that for every 1200 mg/d of calcium, one can expect an excretion of 5 g/d of fat.
What is not yet clear is whether there is a plateau response to these two actions of calcium.
Greater fat oxidation and increased faecal fat excretion will not result in an instant shedding of all excess body fat. So from a clinical viewpoint, calcium plus vitamin D are not the magic combination that most overweight people long for.
Instead, from the population perspective, the projection is of a small but significant drop in body mass index. Most Australians derive their calcium from dairy, and its consumption provides several micronutrients and bioactive components.
A recent paper from Australia suggests that there could be considerable health care costs associated with the under-consumption of dairy. Interestingly, the authors opine that significant savings could be made through the maintenance of a healthy body weight, and dairy consumption may be one way.
Concerned agencies need to support an urgent reconsideration of the role of calcium and vitamin D in chronic disease. In the interim, a renewed drive to encourage the population to at least meet current dietary guidelines for these nutrients is a way forward.
Associate Professor Mario Soares is director of research training at the School of Public Health, Curtin University, Perth.
Potential conflict of interest: Some studies in which Associate Professor Soares has been involved have been funded by Dairy Australia and the Diabetes Australia Research Trust.
Posted 24 Spetember 2012