There is no topic more widely discussed than what one should eat in order to stay healthy. And there are few topics where there evidence is so lacking in quality. This post isn’t about quackery, but about something much more important. it is about the real science (if it merits that description) behind dietary advice. I’m not an expert in nutrition, but I do know a bit about the nature of evidence. I’m continually astonished by the weakness of the evidence for some things that have become received truths, and nowhere is that more true than in nutrition.
The BMJ used my review of Gary Taube’s book, The Diet Delusion, to start off their new Round Table feature [full text link to BMJ].
The published version had some big cuts so I publish the original version here. Taubes was kind enough to send me a copy of the book after I’d mentioned his wonderful New York Times piece in my previous excursion into the murky world of diet and health, Diet and health. What can you believe: or does bacon kill you?
The biggest omission in the BMJ version was Taubes’ own ten point summary of his conclusions (on page 454).
"“As I emerge from this research, though, certain conclusions seem inescapable to me, based on existing knowledge
- Dietary fat, whether saturated or not, is not a cause of obesity, heart disease, or any other chronic disease of civilization
- The problem is the carbohydrates in the diet, their effect on insulin secretion, and thus the hormonal regulation of homeostasis – the entire harmonic ensemble of the human body. The more easily digestible and refined the carbohydrates, the greater the effect on our health, weight, and well-being.
- Sugars – sucrose and high-fructose corn syrup specifically – are particularly harmful, probably because the combination of fructose and glucose simultaneously elevates insulin levels while overloading the liver with carbohydrates.
- Through their direct effect on insulin and blood sugar, refined carbohydrates, starches, and sugars are the dietary cause of coronary heart disease and diabetes. They are the most likely dietary causes of cancer, Alzheimer’s disease, and the other chronic diseases of civilization.
- Obesity is a disorder of excess fat accumulation, not overeating, and not sedentary behaviour.
- Consuming excess calories does not cause us to grow fatter, any more than it causes a child to grow taller. Expending more energy than we consume does not lead to long-term weight loss; it leads to hunger.
- Fattening and obesity are caused by an imbalance – a disequilibrium – in the hormonal regulation of adipose tissue and fat metabolism. Fat synthesis and storage exceed the mobilization of fat from the adipose tissue and its subsequent oxidation. We become leaner when the hormonal regulation of the fat tissue reverses this balance.
- Insulin is the primary regulator of fat storage. When insulin levels are elevated – either chronically or after a meal – we accumulate fat in our fat tissue. When insulin levels fall, we release fat from our fat tissue and use it for fuel.
- By stimulating insulin secretion, carbohydrates make us fat and ultimately cause obesity. The fewer carbohydrates we consume, the leaner we will be.
- By driving fat accumulation, carbohydrates also increase hunger and decrease the amount of energy we expend in metabolism and physical activity.”
It is on these bases that Taubes suggests that the increase in obesity is, in part, a consequence of the recommendation of a low fat, and hence high sugar diet.
The Diet Delusion [ pp 601]
(published in the USA as Good Calories, Bad Calories)
Gary Taubes 2008
There is no topic more widely discussed than what one should eat in order to stay healthy. And there are few topics where the evidence is so lacking in quality. It is also a topic that is besieged by gurus, cranks and supplement hucksters.
You need to beware of misleading titles. Dietitians are good. Nutritionists are sometimes good. But titles like ‘nutritional therapist’ and ‘nutritional medicine’ are usually warning signs of alternative therapists and/or pill salespeople.
Gary Taubes is a journalist, but he is quite an exceptional journalist. His account of the importance of randomisation for the establishment of causality is one of the best ever and it was published not in an academic journal, but in the New York Times . His book, The Diet Delusion, is in the same mould. It is more complete and more scholarly than most professional scientists could manage. Not only does it cover the literature right back to Samuel Johnson, but it is also particularly good at unravelling what one might call the politics of science. And by politics I don’t mean the vast lobbying industry that has built up with the aim of selling you unnecessary supplements, but the politics of academia.
Obesity sounds simple. If you are fat it is because you eat too much or exercise too little, right? Well no, it’s not as simple as that. For a start, it has been shown time and time again that low fat diets, and exercise, have small and temporary effects on weight. The problem with diet and health revolves round causality. The law of conservation of energy is an inevitable truth, but says nothing about causality. It could imply that you get fat because you eat too much, or equally the causal arrow could point the other way and “we eat more, move less and have less energy to expend because we are metabolically or hormonally driven to get fat”. The assumption that positive caloric balance is the cause of weight gain has predominated since the 1970s and “this simple misconception has led to a century of misguided obesity research”.
At the heart of the problem is the paucity of randomised trials, which are the only way to establish causality. Those that there are have usually shown that diet does not matter as much as we are told. Taubes concludes
I think it can certainly be argued that the problem of causality has been greatly underestimated. We are warned constantly of the dangers of processed meat, on the basis of very unconvincing evidence .
This is one reason why we still know so little about the causes of obesity, diabetes and heart disease. For Taubes, a major villain was the US nutritionist Ancel Keys (1904 – 2004). His
It is quite possible that there was rather more to be said for the Atkins diet than was apparent at the time. The fact that Atkins was not a university scientist, that his views were extreme and that he was so obviously out to make a lot of money from it, gave him all the appearance of being yet another profiteering diet guru. He was dismissed by the medical establishment as a quack. Taubes points out that conflict of interest cuts both ways. Atkins’ sternest critics at Harvard were funded by General Foods, Coca-Cola and the sugar industry. It adds up to a sorry story of a conflict of vested interests and scientific vanity.
Taubes’ final judgement is harsh. He quotes Robert Merton’s description  of what science is, or should be.
He then comments
It took Taubes five years to write this book, and he has nothing to sell apart from his ideas. No wonder it is so much better than a scientist can produce. Such is the corruption of science by the cult of managerialism that no university would allow you to spend five years on a book . I find all ten points in his summary convincing. But his most important conclusion is that you cannot have any certainty without randomised trials.
The business of nutrition is greatly at fault for not having put more effort into organising randomised trials. Until they are done, we’ll never really know, and we shouldn’t pretend that we do.
2. Colquhoun, D. (3 May 2009) Diet and health. What can you believe: or does bacon kill you?.
3. Greenberg, S.A.. 2009 How citation distortions create unfounded authority: analysis of a citation network. BMJ ;339:b2680 [pdf file].
If length had allowed, there should certainly have been a reference here to Robert Lustig of UCSF. He is an academic nutritionist who supports the main thesis of Taubes’ book. See, for example, his 2005 review, Childhood obesity: behavioral aberration or biochemical drive? Reinterpreting the First Law of Thermodynamics [full
text link]. Lustig’s slide show, The Trouble with Fructose is available in the NIH web site.
There are a couple of other articles by Taubes that are well worth reading. The Scientist and the Stairmaster Why most of us believe that exercise makes us thinner—and why we’re wrong. Gary Taubes, in New York Magazine, and We can’t work it out, in the Guardian.
You can see Taubes in action on YouTube, for example in “on Cholesterol and Science Practices“, and “on Carbohydrates and Degenerative Diseases“. There is also a video of Taubes on medical grand rounds at Dartmouth-Hitchcock Medical Center in June 2009. You can see Robert Lustig on YouTube too: “Sugar: The Bitter Truth“.
22 December.2009, Unlike the serious questions dealt with in the Diet Delusion, this concerns merely another bit of the ubiquitous nutribollocks that crops up in the media, While writing this I was listening to the excellent early evening news programme, PM, run by Eddie Mair, when a diet-related topic came up, it was nonsense about how a cocktail made with vodka, cointreau, acai juice and pomegranate juice would not give you a hangover. I suppose it was meant as christmas fun but whenever I hear the words ‘antioxidant‘ or ‘superfood; I feel an email coming on. It seems that Eddie Mair liked the fact that the email contained the words ‘quack’ and ‘codswallop’ because the next thing I knew I was asked to give an interview on next day’s programme. The mp3 is here.