Dr Jason Fung is a Canadian nephrologist. Seeing many diabetic patients with kidney complications, he has taken special interest in obesity and diabetes. He has pioneered the use of therapeutic fasting regimens for weight loss and type 2 diabetes control, with great success, at his Intensive Dietary Management clinic in Toronto. This talk was given at the Public Health Collaboration conference 2017 in Manchester.

The current international consensus is that a caloric deficit (weight gain = calories in – calories out) is required to achieve weight loss, and that this is achieved by eating less and exercising more. But when looking at the scientific literature, it is clear that this approach is not very well evidenced at all. For example, the TODAY Study found that over 60 months, metformin plus lifestyle changes (in the form of the eat less, exercise more approach) was no more effective for weight loss than metformin alone, with neither group losing any weight. In fact, according to the UK General Practice Database 2004-2014, the probability of an obese person achieving a normal weight by following ‘eat less, exercise more’ advice is 0.6%. In addition, 80% of people managing a 10% reduction in weight will regain this weight within a year. This effectively means that the vast majority of calorie restriction or ‘eat less, exercise more’ strategies are doomed to fail.

So why does the current model not work? The main reason for the failure of calorie restriction is that the body, as a survival mechanism, lowers its basal metabolic rate (BMR) when calorie intake is reduced. Therefore, when the body is taking in less calories, it’s also burning less too. This leads to reduced energy levels, increased hunger (which involves changes to satiety hormones) and trouble staying warm. The worst part of this is, when the BMR falls enough to match caloric intake, weight loss stalls or even reverses. We have actually known this for around a century, with the findings being reiterated fairly often since. In addition, exercise has also proven an ineffective tool in preventing and reversing weight gain in the long term. The potential reasons for this are increased hunger following exercise and compensation for increased exercise with longer periods of rest. Thus, neither manipulation of calorie intake, nor the amount of exercise, is managing to achieve weight loss.

The key to losing weight is breaking into the body’s fat stores for energy, and the main hormone that controls this is insulin. Eating food, particularly carbohydrates, throughout the day maintains raised levels of insulin, even if the overall calorie intake is reduced. This instructs the body to switch off lipolysis (fat burning) whilst activating lipogenesis (fat creation), resulting in fat gain. On the other hand, abstaining from caloric intake altogether, for an extended period of time, lowers insulin levels and allows lipolysis to be switched back o, thus, the body is permitted to break into its fat stores for energy. This practice is called intermittent fasting and has physiological effects distinct from those of calorie restriction. During periods of fasting, not only is BMR maintained via increased noradrenaline levels, but there is also no net increase in hunger, measured via ghrelin levels. Furthermore, intermittent fasting also preserves lean body mass, and so is said to be muscle-sparing.

People cannot just make a decision to burn more fat calories, or to feel less hungry. These are physiological issues and must be addressed via physiology. It is time to stop scolding people for lack of willpower, and to start showing them how to lose weight in a healthy fashio, without torturing themselves.

Get our free newsletters

Stay up to date with the latest news, research and breakthroughs.