Accurate, consistent dietary advice seems increasingly hard to find. For instance, a widely reported study recently claimed that people don’t need to reduce their consumption of red and processed meat for health reasons. The report sharply divided scientific opinion, with some experts praising it as a “rigorous” assessment, others questioning it.
Nutrition sceptics could cite many similar examples of conflicting opinions, such as the safety or dangers of saturated fat or of nutrition supplements. Such contradictions only further deepen public mistrust in nutrition research.
But reliable advice matters, especially for the increasing numbers of people who respond differently from the general population. Dietary advice usually draws on scientific research that has pooled results from studies on large populations, but this can mask huge variations in risk between individuals within those populations.
One size does not fit all
A study that doesn’t differentiate high-risk individuals in a low-risk general population can produce a skewed overall risk estimate. It also mistakenly reassures high-risk individuals that their risk is the same as everyone else’s. But a one-size-fits-all policy in nutrition makes no more sense than calculating the average shoe size in a population and recommending that everyone wear that size. Even statisticians agree that “the mean is an abstraction. Reality is variation.”
A simple public health campaign is warranted if the strength of the association between cause and effect is high for the whole population, as it is with smoking and lung cancer. But most individual nutrients and foods have only weak associations with risk when evaluated across a whole population. It is individuals in high-risk subgroups who need to be most concerned.
For instance, people who are overweight or obese tend to respond differently to others. A high-carbohydrate diet increases their risk of coronary heart disease, yet is of far less concern for those who are lean.
Overweight and obese people also often have higher blood insulin levels. This is a sign of being metabolically unhealthy, increasing the risk of many chronic diseases.
Obesity is often considered a risk factor for colon cancer. Yet it is not obesity itself that increases a person’s risk of getting colon cancer, it is high insulin levels. There is no increased risk of colon cancer for obese people who retain normal insulin levels. So studies that only measure associations between obesity and colon cancer – without taking into account the degrees of metabolic health in these obese people – may produce lower estimates of risk for everybody, inadvertently downplaying the real dangers for some of the group studied.
Unfortunately, few people know their insulin status, though it is at least as important for disease risk as body mass index (BMI).
Another group whose risk from diet can be dramatically higher than average are people with pre-cancers (for example, abnormal cells that are associated with an increased risk of developing into cancer). Folic acid, for instance, may help protect against colon cancer in people free of the disease, but existing precancerous growths in the colon are more likely to become full-blown cancer with a high intake of folic acid. Yet few people know if they have a pre-cancer and so assume their risk is the same as everyone else’s.
Most public health campaigns fail to take these differences between individuals into account, but you can take them into account – at least to some extent. Personalised nutrition programmes that include genetic screening are now available, though only a few genes are sufficiently understood to be of any real value. It may be more useful to have a basic health risk assessment that measures biomarkers for disease risk, such as cholesterol, blood sugar, insulin and inflammation.
A more general strategy – but one supported by a huge amount of evidence – is to hedge your bets with a proven healthy dietary pattern, such as the Mediterranean diet. Many nutrients act together in this diet and the outcome is less likely to be influenced by variations between individuals. Instead of joining the nutrition sceptics pouring scorn on contradictory advice, it is better to protect yourself by adopting a proven healthy eating pattern that has stood the test of time.
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