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How Fat Works

December 8, 2006

Obesity is caused by eating too much and moving too little. On this most people would agree, but there is a surprisingly large amount that is still unknown - even to obesity scientists - about how this happens, and why excess weight has such a profound impact on health.

In How Fat Works , Philip A. Wood draws on his extensive knowledge of fatty acid metabolism to give an admirable insight into how genes, diet and exercise impact on fatty acid oxidation and synthesis, and how these processes affect us. Wood has conducted most of his own research using gene knockout models in mice; he begins with a concise summary of what we have learnt about fat metabolism by using this method and by studying genetic disorders of obesity. A helpful comparison between the weight of the leptin deficient mouse and its equivalent body weight in humans - 500lb (35st 10lb for UK readers) - helps to illustrate the severity of the condition and shows that this kind of metabolic deficiency is very unlikely to account for "common" overweight and obesity in humans.

In a later section, Wood describes the various monogenic disorders of lipid metabolism in humans, illustrating how impairments at various points along the metabolic pathway can disrupt functioning, and how deficiencies have highlighted the role that gut hormones and peptides (such as neuropeptide Y, ghrelin and CCK) play in appetite. He also attempts to explain current thinking about polygenic disorders, but this necessarily method-heavy chapter risks tripping over itself trying to cover so many basic principles in such a small amount of space.

Elsewhere, Wood introduces the various constituents of fat, including "good" and "bad" fatty acids, and discusses how they are ingested or manufactured by the body. Diagrams and tables showing the chemical structure of all the common dietary fatty acids will be useful for nutrition and medical students. Meanwhile, the "ambitious general reader", whom Wood also hopes to capture, will find it interesting to learn exactly why we should care about the type of carbon bond found in different fatty acids - it is because this quality determines their properties and effects in the human body.

Even when describing difficult biological concepts, Wood retains a lightness of touch and uses creative analogies. Fatty acids are passengers in a train carriage (very low-density lipoprotein), sitting three to a seat (triglyceride). Meanwhile, visceral fat is like a current account, "easy in, easy out"; whereas subcutaneous fat is a retirement account: "You may be socking it away, but it is very difficult to get back out."

Other expertly written chapters deal with disorders involving an excess of cholesterol and triglycerides; insulin resistance, metabolic syndrome and diabetes; lipid-lowering drugs; and nutrient labelling. There is also more detail on the mechanisms of making, storing and burning fat, with an emphasis on the crucial role of glucose. The chapter on exercise left me convinced more than ever of its benefits, although Wood, like many other experts, believes that exercise alone is a highly inefficient way to lose weight.

Perhaps the most interesting and surprising argument in the book is encapsulated by what Woods describes as his "heretical" statement - that "eating a diet relatively high in unsaturated fatty acids but low in total calories may actually help lower blood levels of fatty acids". In fact, this sounds a whole lot less controversial now than it would have done at the peak of the "low fat" revolution several decades ago - most dieticians now espouse the benefits of a Mediterranean-style diet high in nuts, seeds, fish and olive oil.

But the mechanisms by which "good fats" have their benefits are still being hotly debated. Wood argues that the intake of long-chain polyunsaturated fatty acids can suppress the body's synthesis of fatty acids and can even promote fat-burning (for example by activating transcription factors called PPARs, which turn on genes involved in oxidation). He is keen to emphasise, however, that the advantages are obtained only when energy intake and expenditure are balanced - the worst diet of all is the common-or-garden high-fat, high-carbohydrate, high-calorie diet, where everything will be converted to fat.

This is a fascinating and ambitious book, well divided into manageable chunks. It deals with an exciting and dynamic area, and it includes much cutting-edge research. Perhaps for this reason, it shuns a regimented, textbook approach, which makes it quite difficult to pinpoint specific sections; this is more a book to dip into. Its ambitious coverage may also explain why Wood deprives us of a grand summary at the end. However, as a non-expert in the area, I was rather looking forward to one and I felt slightly wrong-footed by the sudden ending, after an admittedly interesting aside on media coverage of obesity research. I would have liked a few colour plates, too, to illustrate the clinical disorders discussed and show what fat cells really look like.

But the outstanding strength of this book is undoubtedly Wood's ability to describe complex metabolic processes in readable English. It will therefore be enjoyed by students, health professionals, scientists and lay readers - all of whom may now think just a little harder before choosing what, and how much, to eat for lunch.

Susan Carnell is a research fellow specialising in childhood obesity, University College London.

How Fat Works

Author - Philip A. Wood
Publisher - Harvard University Press
Pages - 249
Price - £21.95
ISBN - 0 674 01947 4

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