A Fish Aficionado
I am going to make it through this year if it kills me
Weight loss diet studies: we need help not hype
Over the past several decades, dozens of randomised controlled trials have compared various diets for the treatment of obesity. Ideally, such studies should have provided strong evidence for clear clinical recommendations and also put a stop to societys endless parade of fad diets. Unfortunately, the evidence base remains contested and the diet wars continue unabated.
One insight that can be gleaned from the existing weight loss literature is that even the most divergent of diets seem capable of affecting a degree of short-term success, with some diets perhaps leading to marginally greater losses than others over periods of several months.1 But since obesity is a chronic condition, it is the long term that matters. An effective diet for clinical weight management needs to be established over time scales of years to decades. Studies that have lasted 1 year or more typically do not show significant differences between prescribed diets, much less any clinically meaningful differences in maintenance of lost weight.1, 2 One example is in the Dietary Intervention Randomized Controlled Trial (DIRECT), which has been hailed as proof of the superiority of low-carbohydrate diets over low-fat diets.3,4 The DIRECT investigators used a 2-year workplace intervention and found that a low-carbohydrate diet prescription led to a significant 1·8 kg greater mean bodyweight loss than the prescription of a low-fat diet.3,4 These bodyweight differences between the diets are among the largest differences that have been observed over a 2-year period. But from the clinical perspective, such small bodyweight differences do not instil confidence for prescribing one diet over another to a patient with obesity.
What is especially striking is the similarity of the long-term pattern of mean bodyweight change, irrespective of diet prescription.5 For example, figure 1A shows data from the DIRECT study in which both the low-fat and low-carbohydrate diets resulted in rapid early weight loss that plateaued after about 6 months at a likely disappointing level6 and was then followed by slow bodyweight regain. What can we learn from the physiology underlying such a bodyweight trajectory?
Complex physiological feedback mechanisms regulate bodyweight and resist weight loss. Slowing of metabolism can be substantial and persistent7 and plays a part in halting weight loss and putting subsequent weight regain into motion. However, the typical bodyweight trajectory is primarily driven by patients experiencing an exponential decay of diet adherence due to an increase in appetite in proportion to the loss of bodyweight,8 along with difficulties in sustaining changes to dietary choices and behaviours that affect patients ability to enjoy, celebrate, and socialise with food.
Fewer resources should be invested in studying whether or not a low-carbohydrate diet is marginally better than a low-fat diet, or whether intermittent fasting provides marginally better short-term outcomes than a so-called Paleo diet. Crowning a diet king because it delivers a clinically meaningless difference in bodyweight fuels diet hype, not diet help. Its high time we started helping.
In The Lancet, Kevin Hall and I Call Out Weight Loss Studies