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Conventional wisdom tells us that weight gain follows when we consume more calories than the ones we metabolize. A gram of fat contains about twice as many calories as either carbohydrate or protein. So, logic dictates that the more fat we eat, the more likely we are to consume calories that are surplus to our requirements, which then end up being stored as fat in our bodies. Another thing that adds to dietary fat’s fattening reputation is its name (it is called fat, after all).

These facts do, on the face of it, seem to incriminate dietary fat as something inherently fattening, and appear to justify a somewhat joyless life replete with low-fat foods and an absence of butter and bacon.

However, the propensity for fat to be stored in the body is not purely determined by the balance of calories going into and out of the body, but also by the impact foods have on hormones that regulate weight. A key player here, as we learned earlier, is insulin. Insulin is secreted readily in response to glucose (from sugar and starch in the diet), but dietary fat has minimal, if any, insulin-inducing effects. In theory at least, this means that fat has limited fattening potential.

The supposedly fattening effects of fat were comprehensively assessed in an extensive review of the evidence conducted by researchers from the Harvard School of Public Health in the US.21 After evaluating a mass of epidemiological studies, the authors concluded that: ‘diets rich in fat do not appear to be an important cause of body fatness.’ This epidemiological evidence fails to support the idea that fat is fattening, but the acid test here comes from intervention studies in which low-fat diets are tested for their effectiveness for weight loss.

The most comprehensive review on the subject found that low-fat diets led to small weight losses over time, and after eighteen months individuals were, on average, the same weight as when they started out.22 This review was subsequently withdrawn some years later, but only because there were no plans to update it. Since its publication, no other good new evidence supporting the effectiveness of low-fat diets for weight loss has surfaced. The review cited earlier 21 also assessed the evidence from intervention studies, where again low-fat diets were found to be ineffective, leading the authors to conclude that: ‘lowering fat in the diet will not be a solution for overweight and obesity.’

There’s perhaps something counterintuitive about the fact that eating fat is not fattening, and that eating less fat does not lead to us shedding bodily fat of our own. However, all becomes much clearer when we consider that fat has little or no effect on the secretion of the chief fat storage hormone: insulin. For a complete discussion of the science of weight control and how to achieve success here without semi-starvation or unsustainable amounts of exercise, see my book Escape the Diet Trap.

A key point I make in this book is that for any dietary approach to be successful, it needs to be sustainable. This essentially means people need to enjoy eating it and not have to endure undue hunger. Most individuals will simply not tolerate being hungry for extended periods of time (hunger also makes life harder in myriad other ways that are explored in Chapter 3).

So, what should we eat to keep the appetite nicely in check and make healthy eating easy?