Nutritional science is too complex to furnish easy answers about what to eat. Illustration by Ben Wiseman

In the early nineteen-sixties, when cholesterol was declared an enemy of health, my parents quickly enlisted in the war on fat. Onion rolls slathered with butter, herring in thick cream sauce, brisket of beef with a side of stuffed derma, and other staples of our family cuisine disappeared from our table. Margarine dethroned butter, vinegar replaced cream sauce, poached fish substituted for brisket. I recall experiencing something like withdrawal, daydreaming about past feasts as my stomach grumbled. My father’s blood-cholesterol level—not to mention that of his siblings and friends—became a regular topic of conversation at the dinner table. Yet, despite the restrictive diet, his number scarcely budged, and a few years later, in his mid-fifties, he had a heart attack and died.

The dangers of fat haunted me after his death. When, in my forties, my cholesterol level rose to 242—200 is considered the upper limit of what’s healthy—I embarked on a regimen that restricted fatty foods (and also cut down on carbohydrates). Six months later, having shed ten pounds, I rechecked my level. It was unchanged; genes have a way of signalling their power. But as soon as my doctor put me on just a tiny dose of a statin medication my cholesterol plummeted more than eighty points.

In recent decades, fat has been making a comeback. Researchers have questioned whether dietary fat is necessarily dangerous, and have shown that not all fats are created equal. People now look for ways of boosting the “good cholesterol” in their blood and extol the benefits of Mediterranean diets, with their emphasis on olive oil and fatty nuts. In some quarters, blame for obesity and heart disease has shifted from fat to carbohydrates. The Atkins diet and, more recently, the paleo diet have popularized the idea that you can get slim eating high-protein, high-cholesterol foods.

Still, I remained wary of the delicacies of my childhood. Surely it was wiser simply to avoid fats altogether? I wavered, though, in 2013, when The New England Journal of Medicine published an article endorsing the salubrious effects of Mediterranean eating habits. The article detailed the results of a study, the most rigorously scientific one yet conducted on the issue, which showed that following a Mediterranean diet rich in either olive oil or nuts could reduce the risk of heart attack, stroke, or death from cardiovascular causes by thirty per cent. I was elated until my wife, an endocrinologist who is an expert on metabolism, pointed out that the headline number of thirty per cent emerged from the complex statistical way that the study’s results were projected over time. If you looked at what happened to the people in the study, the picture was less encouraging: 3.8 per cent of the people consuming olive oil and 3.4 per cent of the people eating nuts suffered cardiovascular misfortune, compared with 4.4 per cent of the group on a regular diet. The true difference in outcome between the two diets was, at best, one per cent.

It’s one of many cautionary tales about assessing dietary data. Everyone wants to be healthy, and most of us like eating, so we’re easily swayed by any new finding, no matter how dubious. Publishers know this all too well and continually ply us with diet and health books of varying degrees of respectability and uplift. The most prominent on the current menu are Sylvia Tara’s “The Secret Life of Fat” (Norton) and “The Case Against Sugar,” by Gary Taubes (Knopf). Both present a range of cutting-edge dietary research, both say that fat is unfairly maligned, and both inadvertently end up revealing that the science behind their claims is complex and its findings hard to translate into usable advice.

Sylvia Tara is a freelance writer who holds a doctorate in biochemistry and an M.B.A.; she has worked at McKinsey and on the management side of various biotech companies. Drawing on insights from both science and consulting, she has produced a book that is part physiology and part marketing pitch. Tara wants us to view lipids positively. Once we stop treating fat “like a vicious enemy,” she argues, it “could become beloved once again.”

But Tara’s attitude to fat is more ambiguous than this statement suggests. She claims to be obsessed with her figure, measuring her worth by how well she fits into skinny jeans. In her telling, the spur to her investigations comes from her envy of a friend who stays svelte despite gorging on beer and burritos, drinking sugary lattes, and never exercising. Tara, who writes that she gains weight easily, is interested in the question of why some people eat like hogs and stay thin, while others expand no matter how abstemious they try to be.

The book is a useful primer on the biology of fat. Fat comes in different forms, categorized by color. White fat, the type that we seek to lose when overweight, stores energy. Brown fat, normally found in the neck, back, and around the heart, is filled with tiny structures called mitochondria, and serves as a furnace to burn energy for body heat. A third type, beige fat, was identified some five years ago; during exercise, it receives messages from our muscles to morph into brown fat. Moreover, fat should not be characterized simply as inert blubber. It is the vehicle by which our cells receive certain essential nutrients, like Vitamins A, D, E, and K. The myelin sheaths around our nerves are eighty per cent lipids, “which means fat is actually required to think,” Tara writes. Studies by Jeffrey Friedman, at the Rockefeller University, have shown that the hormone leptin travels from fat cells to the hypothalamus, a part of the brain which is involved in regulating appetite. “Friedman’s discovery redefined fat,” Tara writes. “It was a verifiable endocrine organ with wide influence to our bodies. Through leptin, fat could talk. It could tell the brain to stop eating.”

All this will be illuminating for many readers, but Tara is a less reliable guide when she uncritically embraces various new theories about the causes and effects of obesity. She trumpets the findings of a Turkish physician, Gökhan Hotamisligil, whose work suggests that a molecule known as TNF-alpha, which has potent inflammatory properties, may be the link between obesity and Type 2 diabetes—a condition arising when the body becomes resistant to insulin, a hormone that we need in order to process sugar. (Though there’s a clear correlation between diabetes and obesity, no one has yet discovered a causal link.) Hotamisligil’s experiments showed that not only is TNF-alpha produced by fat; it also can cause resistance to insulin. “This discovery was big news,” Tara writes. However, she fails to specify that the finding was in rodents, and that subsequent studies in humans, including some by Hotamisligil, did not show the same results.

Tara also speculates that viruses may cause obesity. The research she draws on here is obscure and unconvincing. It concerns a virus called Ad-36, which infects fowl and can make chickens fat. In the studies Tara cites, more overweight people appeared to have antibodies to Ad-36—suggesting that they had been infected in the past—than slim people did. There are many reasons to be skeptical: there’s no evidence that fowl can pass Ad-36 to humans, and there are many viruses that could easily be mistaken for Ad-36.