The Healthy Diet U-Turn

To me, the most interesting bit of news in the last couple of years was the sea-change in attitude among nutritional scientists from an anti-fat, pro-carbohydrate set of dietary recommendations to the promotion of a lower-carbohydrate, selectively pro-fat dietary regime. The issue is important because human health and, indeed, human lives are at stake.

For years, Americans had been told by the experts to avoid fats at all costs and at every opportunity, as if these foodstuffs were the antichrists of nutrition. A diet low in fats and rich in carbohydrates, supposedly, was the way to go in order to achieve a sleek and gazelle-like body and physiological enlightenment. In consequence, no-fat or low-fat foods became all the rage, and for a long time the only kind of yogurt you could find on grocery shelves was the jelly-like zero-fat variety, and the only available canned tuna was packed not in olive oil but in water, as if the poor creature was still swimming.

Unappetizing as much of it was, many Americans duly followed this stringent set of low-fat, high-carbo dietary dos and don’ts. But we did not thereby become a nation of fit, trim, and healthy physical specimens. Far from it. Instead, we became a nation that suffered an obesity epidemic across all age groups, a tidal wave of heart disease, and highly increased rates of Type 2 diabetes. The reason for this was that once they were digested, all those carbohydrate-rich foods got converted into glucose, which raised insulin levels and, in turn, caused storage of excess bodily fat.

Nutritional scientists thus learned the dual lesson that a diet high in carbohydrates can in fact be quite hazardous to your health, and that their alarmist fat phobia was in fact unjustified by the evidence. In reality there are good fats (such as olive oil) and bad fats, healthy carbs and unhealthy carbs (such as refined sugars). As a result, many nutritionists now favor a diametrically opposite approach, allowing certain fats as wholesome and healthy, while calling for a reduction in carbohydrates, especially refined sugars and starches.

A corollary of this about-face in dietary wisdom was the realization that much of so-called nutritional “science” was actually bad science to begin with. Many of the canonical studies of diet and nutrition were flawed by selective use of evidence, unrepresentative sampling, absence of adequate controls, and shifting clinical trial populations. Furthermore, some of the principal investigators were prone to selection bias, and were loath to confront their preconceived viewpoints with contrary evidence. (These and other failings of the discipline are exhaustively documented in journalist Nina Teicholz’s book The Big Fat Surprise [2014].)  

Unfortunately, nutritional science still remains something of a backwater. NASA’s Curiosity rover explores the plains, craters, and sand dunes of Mars, and the New Horizons spacecraft takes exquisite pictures of the former planet Pluto. Molecular biologists wield superb gene-editing tools, and are in the process of resurrecting extinct species. Nevertheless, when it comes to the relatively prosaic task of telling us what foods to put in our mouths to achieve good health and to avoid heart disease, obesity, and other ailments, dietary science still has a long way to go.