Just because excess weight is associated with illnesses, that doesn’t necessarily mean it causes them

In fact, there did appear to be evidence that certain genes associated with obesity could also increase the risk of developing diabetes — and recent research that took into account actual BMIs then showed that this connection only develops in people who are overweight. Carriers of the gene who were within the normal weight range were found to have no higher risk of diabetes than non-carriers (Sandholt et al., 2012). As already explained in the chapter on genetics, genes just increase or decrease the probability that we will create certain conditions through our behaviour, but that behaviour is ultimately under our own control. We are not controlled by our genes like a marionette on the strings of a puppetmaster.

There are now a wide range of studies available that show that people suffering from conditions associated with obesity can achieve great improvement, or even complete recovery, by losing weight. Dhabuwala et al. (2000) found that severely obese patients who slimmed down to a BMI slightly above normal saw large improvements in their hypertension within two years. Of the 42 patients receiving drugs to treat high blood pressure, 18 were able to come off their medication completely. And of the 34 asthmatics, half were able to stop their medication. Eleven patients suffering from sleep apnoea found themselves completely cured of the condition. I will cite more studies showing improvements in various illnesses from weight loss in the section where I cover individual medical issues.

Overall, the research in this field is clear. The fact that conditions associated with obesity can be improved or cured by losing weight shows that excess weight is indeed the decisive cause of the condition, rather than bad luck or genetic predisposition.

There is also evidence to disprove the argument that losing weight might not be the reason for the improvements reported, and that it might rather be the fact that the patient is eating more healthily or doing more exercise. Wood et al. (1988) studied a group of men for more than a year as they lost weight using different methods. Some used caloric reduction, while changing the nature of their diets as little as possible. Another group were asked to continue eating as much they previously had, but to begin exercising. The two groups lost similar amounts of weight and saw similar improvements in their blood-test results. The men who lost weight by exercising did not see any better results than those men who had reduced their food intake, but, apart from the loss of weight, whose lifestyle had not changed.