Somewhat unusually in a book of this kind, it features a running commentary on the author's own health problems, including high blood pressure and over-weight. He decided that the way to solve these was to modify his diet, and he picks up on his attempts to do this from time to time in the course of the book. As in his first book, he writes very informally and colloquially, sometimes perhaps to a fault (I really wish he didn't use 'bacteria' as a singular).
As the title indicates, there is a lot of myth-busting in this book, starting with the simplistic idea, still widely advocated, that losing weight is 'simply' a matter of reducing calorie intake and increasing energy output by exercise. In practice this doesn't work, at least for any length of time, because the body isn't fooled and compensates for the change in life style by conserving energy. To explain how this happens is the point of the book, but the answer lies mostly in the microbiome.
Spector provides a detailed discussion of the roles of fats, carbohydrates and proteins in our diet. He also deals with artificial sweeteners (which are less helpful to weight loss than one might think), cocoa and caffeine, alcohol, vitamin supplements (mostly useless, including the much-touted vitamin D), and the adverse effects of antibiotics, which he advises us to avoid unless they are really needed.
The central message that comes out of the book is that nutrition is a complex subject and there is no one solution that suits everyone. People vary widely in their genetic makeup (even genetically identical twins may react differently to diets because of epigenetic differences) and their microbiomes, which can influence weight to a remarkable extent, will also be different. Unfortunately there is still much remaining to be discovered about how this works. Taking 'probiotic' supplements is unlikely to be anything like a complete answer, partly again because there is no one-size-fits-all recipe.
Faddy diets of all kinds get pretty short shrift here, including the currently fashionable "palaeo" diet. Spector points out why tthat his is bogus; we cannot know with any certainty what our remote ancestors ate, and anyway it probably varied a lot from place to place and time to time. The whole idea seems to be an example of popular pseudo-evolutionary scientism. But Spector does not entirely escape from dubious evolutionary speculation himself. In discussing possible reasons why people vary so much in their taste preferences he writes: "The considerable variations in sensitivity may perhaps have evolved so that whole tribes would not be wiped out by eating the same poisonous fruit." This depends on a group selection hypothesis, which is difficult to sustain today.
Spector's conclusion is pretty uncontroversial; he finds that the best way to try to achieve a healthy microbiome is to eat a "Mediterranean" diet, with as much variety as possible, especially in terms of vegetables, fruit, nuts and olive oil.
There is a great range of good advice in this book. I particularly liked some of what is said towards the end, where Spector tells us mostly to ignore "best before" labels on food, thus avoiding waste, and not necessarily to throw away medicines just because they have passed their expiry date; they may have lost a little of their efficacy after this but in most cases they can be taken quite safely. The book largely succeeds in its professed aim of debunking myths, although I wish Spector didn't endorse the value of taking antioxidants (see Nick Lane's explanation of why this doesn't work in Power, Sex, Suicide: Mitochondria and the Meaning of Life).