We know quite a lot about food science, but the big, easy discoveries, about vitamins and basic nutrition and suchlike, have already been made and what remains is simply difficult. Human metabolism is complex, there is significant variation in individuals, tracking diets is hard, there are vested interests on all sides, and most of the findings are equivocal and uncertain, or of small effects. The science involved here is not Levinovitz's chief interest, but he necessarily has to spend a lot of time on it, if only to convince readers that the mass of diet claims floating around are not based on evidence.
Successive chapters of The Gluten Lie look at gluten, fat, sugar and salt. Levinovitz is not saying that these are not responsible for real problems, but that these problems have been blown up out of all exaggeration, extended to whole populations when they affect only a tiny minority, applied without concern for dosage or quantity, and so forth.
Celiac disease and related conditions are real, but affect less than 1% of the population. The fad for gluten-free diets as a cure-all has no medical basis, but is a kind of mass sociogenic illness, kept alive by media alarms, marketing, and nocebo effects, where the expectation of symptoms can actually cause them. An earlier example of this was the alarm in the 1970s over MSG (mono-sodium glutamate).
"Mass sociogenic illness is real. Anxiety and negative expectations make people sick. These have profound implications for how we should view exaggerated accounts of the dangers posted by gluten and other foods. ...
None of this denies the existence of celiac disease or the possibility of non-celiac gluten sensitivity. Hyperbolic fearmongering, however, makes it difficult to distinguish those who suffer physiological reactions from those whose symptoms are caused by expectations of ill health, reinforced by outrage at Big Food and Big Pharma and dread of modernity."
The demonisation of fats mostly rests on magical thinking, on the idea that you are what you eat. It is also tied up with myths of virtuous vegetarianism and stereotyping of being overweight as a personal failing.
"The lesson to be learned from all this is not that eating saturated fat doesn't make you fat, or that willpower has no relationship to weight loss. Rather, it's that determining and properly weighting the effects of diet on health is tremendously difficult. Too often, "established facts" like the dangers of saturated fats are not facts at all, but intuitively plausible conjectures that attract disproportionate amounts of attention and research. And their plausibility, in large part, comes from an unacknowledged appeal to myths and magical thinking."
Too much sugar in a diet is certainly bad, but the jump to sugar being toxic rests on "monotonic" thinking, on the idea that the effects of large and small doses must be of the same kind. It can also be traced to religious taboos, to the idea that "if you eat something with a morally bad origin there will be physically bad consequences": in the early modern period, sugar was linked to slavery and supposed to have a tendency to provoke lust, among other dangers. (More generally Levinovitz explores "white-hat bias", or the distortion of information in the service of righteous ends.)
Turning to salt, Levinovitz looks at the cult-like Ricers, devoted to salvation through food, at the appeal to "paleo" arguments and links to the myth of the noble savage, and at how health agencies and organisations such as the American Heart Association maintain less extreme but still practically impossible daily sodium targets.
"The CDC's and AHA's salt recommendations are the height of policy irresponsibility. They disguise the controversy behind their numbers and set extraordinarily high bars without considering the consequences. The result is a public frightened of what we put into our mouths, incapable of seasoning food properly, paranoid about what we are served when eating out or at a friend's home. Even if the alarmist claims about salt proved true, it would require an entirely new food culture to comply with the health commandments we've been given."
A fifth chapter considers some of the other common features of nutrition myths. "Bulletproof Coffee" has as much credibility as Daoist mysticism from ancient China and, looking at what the scholars of folk and fairy tales call "tale types", Levinovitz shows how closely current alarms about carrageenans follow those about MSG fifty years ago. His own recommendations? Eat less food and be more active, and "prepare your food with care and eat it slowly, seated at a table".
Levinovitz concludes by illustrating, with a concrete example, how a fad diet is constructed and how rhetoric and the misrepresentation of information can be used to manipulate emotional responses.
As Levinovitz stresses throughout, there are real costs to obsessions about diet: they contribute to people paying higher prices for specialised foods of no benefit, and to the spread of eating disorders, stress, and a broader distrust of medicine and science apparent in such things as opposition to vaccination. The Gluten Lie is hardly going to stop this, but may free some people from obsessing about diets instead of enjoying their food.
As an attempt to situate and understand diet beliefs within a broader framework, The Gluten Lie is too tied to debates over the science. An alternative approach would have been to ignore the question of evidence and instead to trace the social, psychological, religious, media, and mythical contributions to recurring diet ideas (perhaps in the kind of way Stoczkowski treated myths of human origins in Explaining Human Origins).