The science of nutrition has been with us for a long time. So why are there so many apparent uncertainties over dietary advice today?
The nutrients in our food may have inherent effects on metabolism, but proving so requires tightly controlled experiments in lab-like conditions that are difficult to conduct in groups of healthy people. Evaluating how diet influences diseases or defining the optimal diet for good health, where effects are small and long-term, can only occasionally be done under such conditions.
Instead, most dietary studies involve large-scale observational research to identify associations 鈥� whether causal or not 鈥� between health and what we eat and drink, and research on the impact of dietary advice in people going about their daily lives. The problem is that adherence with dietary prescriptions is rarely perfect and many other factors can interfere to produce indecisive or conflicting results. Dietary guidelines can be issued only if the totality of evidence, including that from animal studies in which full control is possible, is sufficiently consistent.
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So how to provide more of the stronger, lab-like evidence? Some US researchers have come up with one possible way to resolve any on blood pressure, heart disease and strokes. , whose diets and activity could be tightly controlled for long periods.
Using prisoners in medical research is highly contentious, not just because of the appalling experiments on Nazi concentration camp inmates and Japan鈥檚 Chinese POWs during the second world war, but also because of inherent ethical and scientific problems. These include selection bias, uncertainties about the validity of consent in such a vulnerable group, the risks of harm from other inmates resenting their perceived preferential treatment, and the prevalence of mental health problems, concealed substance abuse and volatile behaviour among prison populations. From these perspectives, prisoners are not ideal research participants.
Paying back society
On the other hand, given full consultation with advocacy groups and scrupulous attention to how consent is obtained and safety ensured, the proposed study could provide valuable new information. It also offers a means for inmates to help society while they are detained.
The key question then becomes whether this study is really needed, given the current weight of evidence on salt. Although some journalists and self-publicists pushing diet books invoke doubt about the links between dietary salt and ill health, detailed review of all the evidence by independent medical scientists is rarely seriously wrong. Little doubt remains that a high salt intake over long periods from weaning onwards causes higher blood pressure, a known risk for various illnesses.
But there is an increasingly vocal sector questioning all sorts of official diet and health guidance. Which is why this week the BMJ is publishing a series of articles on food and health. It is also bringing the authors of these papers to Zurich, Switzerland, for a conference starting today to try and resolve some very public disputes in the media, notably over the widely championed 鈥渓ow-carb鈥� diet craze.
On that question, meta-analyses and painstaking controlled randomised trials have established that the biological effects of low-carb/high-fat or high-fat/low-carb diets on obesity are at best tiny. But studies in people going about their lives can have different outcomes because the persuasiveness of advice and the ease of identifying and changing specific foods varies from setting to setting.
For other chronic diseases associated with diet, some confusion has arisen through attempts to simplify messages. For example, advice to avoid the ill effects of longer-chain saturated fatty acids was simplified to cut all saturated fats, although shorter dairy fats are not hazardous.
Absolute proof is always welcome. But do we really want to go to the perilous extreme of inmate studies to quell the clouding of official advice, often linked to the promotion of diet books or the politics of food? Probably not 鈥� and even if we did, the doubters would still be doubting.
