The short answer to any question about – does diet X work – is always going to be, yes and no. Intermittent fasting is no different. Diets statistically work in the short term because any time you pay attention to your calorie consumption and exercise you are likely to consume fewer calories, which can result in weight loss. But diets notoriously don’t work in the long term for about 95% of people. But every new diet is presented as if it finally represents the behavioral “hack” everyone has been looking for.
Recent studies looking at intermittent fasting demonstrate the basic reality of the situation and the challenges of doing research in this area. One study recently published in the Annals of Internal Medicine compared time-restricted eating (8 hours per day without explicit calorie counting) to unrestricted eating but with calorie counting, to a control group with no intervention. They found:
Compared with the control group, the time-restricted eating group reduced on average 425 more kilocalories per day and lost 4.61 kg more weight at 12 months. Similarly, the calorie restriction group reduced an average of 405 more kilocalories per day and lost 5.42 kg more weight than the control group. There were no significant differences between the time-restricted eating and calorie restriction groups in terms of reduced calorie intake and weight loss achieved.
Of note, all participants had access to a dietician once per week. It’s hard to know what this study is actually showing us. Studies of diet and weight loss are all challenged by background effects that are likely to obscure the effect being studied. There is an effect just from paying attention to how much you are eating and exercising. There is an effect from being weighed on a regular basis, which gives you concrete feedback about your calorie balance. There is an effect from having a support structure, such as the infrastructure of any clinical trial. There is an effect from having regular diet counseling.
From this point of view, this study tells us that if you regularly meet with a dietician, weigh yourself, and pay attention to your eating (let’s call these general effects), you are likely to lose weight. It also showed that the details of your diet strategy (specific effects), in this case intermittent fasting vs calorie counting, don’t really matter.
If we look at the dieting and weight loss research in total over decades, there is a common and persistent theme. The general effects I listed above are dominant, and the specific effects of dietary details either don’t matter at all or have only a small effect. Often strategies are proclaimed to be superior based upon tiny and short term effects.
About 95% of weight loss, in terms of behavioral strategy, is about paying attention to how much you are eating, weighing yourself regularly, maintaining or increasing your physical activity, and having some psychological support or encouragement. Yet there is a vast literature focusing on the 5% – the kind of calories (macronutrients) and the timing of eating. Research consistently shows that these factors either don’t matter, or there is a small effect, and/or the effect is short lived and sometimes comes with a long-term deficit.
The research also shows that sustained weight loss is really hard. Most people lose only a small amount of weight and don’t keep it off long term. Three to six month studies are presented as “proof of concept” studies, but may be ultimately useless if the goal is long term weight loss and maintenance. Intermittent fasting is just the latest attempt to come up with the one trick that will finally work, but the research is finding essentially the same thing. Yet studies and headlines obsess over small effects from, say, the duration of fasting, or the timing of fasting. Is it better to eat in the morning or later in the day?
It seems as if we just don’t want to admit to the big picture – none of this really matters. Weight maintenance for many people is simply really hard. Distracting ourselves with these small details won’t change that.
So what will work? On an individual level, we really still don’t know, beyond the general effects I listed. The evidence also suggests that obesity is not an individual failure but a societal problem. Developed nations generally have readily available calorie-dense food in large proportions. We are simply not evolved to deal with this abundance of food engineered to maximally stimulate our senses. Brute force of willpower is simply not up to the task of living in this world without overconsuming calories (for most people most of the time).
There are medical options. Various forms of gastric bypass are effective. But surgical intervention is generally reserved for the severely obese with medical complications. There are medications available, such as Ozempic. These are primarily for diabetes, but are shifting to treatments for obesity itself. However, these medications are expensive and have to be taken indefinitely, with a risk of rebound if they are stopped. Medical interventions therefore have a role to play, but they are not for everyone.
The bottom line is that we still haven’t cracked this problem. We are nibbling around the edges, debating about the 5%, looking for the next hack. Meanwhile, we actually know what the solution is, but there doesn’t seem to be the collective will to do anything about it. The environment needs to change. The food industry needs to change. Until that happens it feels like we are rearranging deck furniture on the Titanic.