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blog post 1 19 March 2014


Myths about obesity

John Menzies, Edinburgh, 19th March 2014.

‘Drunken Silenus supported by Satyrs’ (~1620)attributed to Anthony van Dyck, The National Gallery, London


In Greek mythology, Silenus was a satyr, one of the caprine companions of Pan, and tutor to Dionysis. He is often depicted as “jovial”, perhaps a euphemism for overweight and tipsy.


There is something to be said about folk wisdom. An apple a day keeps the doctor away; comer hasta enfermar y ayunar hasta sanar; Jedes Böhnchen gibt ein Tönchen, and so on. But we ought to rely on a scientific evidence base to guide us when we decide when and how to eat. However, some misguided ideas about diet, exercise and obesity have found wide acceptance, not only with the public but also with some scientists, health professionals and policy makers.

Recently David Allison and colleagues set out to examine these preconceptions. In an article in the New England Journal of Medicine they take on several common ‘myths’ around obesity. read the full paper

First they discuss the appealing idea that large losses of body weight can be achieved with fairly small changes to diet or exercise levels. This idea is based on the concept that we need to burn, or not eat, around 7800 kcal to lose one kilogram in body weight.

This rule of thumb was derived from studies where very large reductions in daily calories intake were imposed on young male volunteers. Sadly, there is no evidence that smaller changes add up over time. The amount we eat on a given day can vary hugely - between 70% and 130% of what we eat on the ‘average’ day - and any small reductions are likely to be balanced by later increases.

But it is still unclear how we compensate for under-or overeating and balance our body weight. One study showed no compensatory increase in eating after a 36 hour fast [article], but another showed compensatory undereating after drinking something as seemingly trivial as a single milkshake. [article]  Nonetheless, it’s accepted that if we want to lose a lot of weight, big and sustained changes are needed, not small ones.

Next , they question the assumption that dieters who set modest goals will be more likely to succeed, but those who set ambitious goals will be more likely to fail. This has its basis in psychology - that attainable goals lead to better performance in tasks and encourages behaviour toward that goal. In other words, if an obese individual does not lose the desired amount of weight they may give up that goal. However, in the context of obesity there is no correlation between the ambitiousness of the weight-loss goal and failure. Indeed the reverse may be the case . [article]

Third, the authors ask if slow, steady weight loss is more successful in the long term than a more rapid weight loss. This relates to the question of attainable goals but there is no evidence that rapid versus slower weight loss produces differences in the longer term. In any case, in the long term weight loss is usually not sustained - dieters tend to regain the weight they lose. Indeed, the best predictor for weight gain is weight loss! [article] 

Physical exercise is, of course, a good thing - it improves cardiovascular fitness and mood - but does it impact on obesity? We accept that weight loss can occur not just by eating less but also by doing more. But what effect does physical exercise have, particularly in schools? A recent meta-analysis of interventions designed to increase physical activity in schools showed improvements in fitness but no effect on body weight . [article]

Breastfeeding has many benefits for babies and infants, but it is often also suggested to be advantageous for the mother. It is commonly believed that breastfeeding will protect against postpartum weight gain in the mother and obesity later in the child’s life. The latter does not seem to be the case - a very large prospective study showed no protective effect of breastfeeding on body weight . [article]

Lastly, it has been said that us humans are creatures of inherently base urges, and when we’re not thinking about food, we (perhaps men, at least) are thinking about sex. Perhaps the two can balance out in some way - the physical activity surely uses some of the calories consumed? It does, but not very many. There is, of course, a large variability in the frequency, duration and intensity of sexual behaviour between individuals, but taking the average duration of a bout of sexual activity, a disappointing six minutes [article], a rough calculation shows this will use around 20 kcal - about the same as playing a board game for 12 minutes. Perhaps more fun, but not a way to lose weight.

Schönen Dank to Gabi Wagner and Andrea Johnston.

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