What should we eat?

Answers to this simple question often appeal to science. Nutrition science, we are told, can tell us what we should or shouldn’t eat if we want to be healthy, fit and prevent disease. But are these appeals to nutrition science legitimate? We think in many cases the answer is “no”.

Some of the most egregious offenders misusing nutritional science are listicles – “10 Unbelievable Diet Rules Backed By Science,” the “14 Things You Should Never Eat,” or “10 Foods Science Says Are Healthier For Your Hair.” But even more legitimate sources of advice can be prone to misuse science and are guilty of what we label nutritional scientism.

In a recent article in the Journal of Bioethical Inquiry we critiqued three types of nutritional scientism: (1) the oversimplification of complex science (including suggesting causation from probabilistic conclusions from observational studies) to increase the persuasiveness of dietary guidance, (2) superficial and honorific references to science in order to justify cultural or ideological views about food and health, and (3) the presumption that nutritional value is the primary value of food. We discuss the first and third of these here.


Widespread concern over diet-related chronic diseases partly explains the appeal of nutritional scientism. We want to give and receive clear dietary guidance. We want to know how diets affect our health and use that knowledge to control and prevent disease. However, the relationship among food, health and chronic disease is complex.

Diet and Causality

While scientific knowledge about the relationship between diet and chronic disease is unclear, scientific knowledge of nutrition has led to improved health dramatically with respect to nutritional deficiency diseases. For example, the relationship between dietary vitamin C and scurvy is causatively simple.  Vitamin C is an essential nutrient for all humans. If someone eats a diet that does not contain sufficient amount of Vitamin C for long enough, they will contract the deficiency disease. For nutrient deficiency, cause has a direct, non-probabilistic meaning, in the sense that if I hit my hand with a hammer, I will cause it to hurt every time.

The success of nutritional science in addressing problems of nutrient-deficiency disease might be so impressive that the aura of scientific success is extended to nutrition and chronic diseases, which are far more complex phenomena.

Chronic diseases are considered to be multifactorial.  Unlike nutrient-deficiency diseases, the meaning of causation for chronic disease is harder to understand, and thus establishment of causation is a far trickier problem.  Factors are understood in the mathematical sense, not as causes but as elements in a mathematical model.  Not everyone for whom factors are in place will get the disease, and not everyone for whom none of the factors apply can be assured of not getting the disease.  The evidence is fundamentally probabilistic, concerned with differential risk (population-level incidence) associated with the factors in the model.

Oversimplified dietary guidance may suggest that the relationship between the food and health status is more simply deterministic, an oversimplification of a highly complex situation.  Black and white dietary advice, for example to “never eat sugar” or “always avoid a particular food,” is a gross oversimplification. Invoking the science to justify oversimplified recommendations is our first form of scientism.  There are times when oversimplified recommendations are couched as a moral responsibility of the individual.

Food and diet, more than nutritional value

A second problem with nutritional scientism is that it reduces the value of food to the way it contributes to a biomedical model of health. This excludes of other values, such as pleasure, identity or culture. If we think of health more broadly, as the World Health Organization does, nutritional scientism is exposed as having a limited  understanding of the relationship between food and health.  In contrast to a focus on the biomedical model, the WHO defines health as a ‘state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity’. If health is broadly considered as social wellbeing, rather than narrowly defined as biomedical, it can be argued that an inordinate reduction of food to nutritional value is likely to be undesirable as it leads to the exclusion of values that are significant for identity, belonging, and wellbeing.

Food is central to bodily survival and nutritional health, but it is also essential for human flourishing through communal relations, religious rites, personal milestones, cultural festivals and a wide variety of everyday interactions that contribute to human well-being. When scientific understandings of food, diet, and cuisine become the dominant or exclusive mode of understanding of food and diet, the label of nutritional scientism is again warranted.

Nutrition science properly contributes to answering the question—“What should we eat?”—but its contribution should be placed in the larger context of a plurality of values that include culture, ethics, pleasure, and well-being.

By Christopher Mayes and Donald Thompson

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