Intermittent fasting (IF) is the diet choice that we love to hate. Honestly, as someone who has done it, I can see why. Refraining from eating for long periods of time on certain days of the week, or going without entirely for days at a time, is uncomfortable. It makes you grumpy, aloof, and easily annoyed. Food is the best, and eating one of the main pleasures in life. IF is also the subject of a great deal of online debate, with proponents arguing that it can cure or substantially treat half of all human disease while those who dislike the practice argue that it’s dangerous.
And now, evidence reportedly shows that IF is indeed killing people. According to a slew of media stories, a new scientific study has shown that IF is linked to a 91% increased risk of dying from heart disease. It seems like researchers have recently shown that people who use IF in their lives drastically increase their risk of dying of a heart attack.
Except, that’s really not what the research showed at all. This is very preliminary reporting on a study that doesn’t technically look at IF. There may be risks associated with fasting in your life, but this particular study tells us little about those potential problems.
The Study
The research that everyone is reporting on is, in fact, not a full scientific paper. Every year, the American Heart Association has an annual meeting where doctors, researchers, and others meet up to discuss everything related to hearts. It’s a huge event, with more than 15,000 attendees each year coming to discuss science and evidence around cardiology.
The new study that’s hit the headlines is a poster presentation. This means that the authors submitted a 250-word abstract to the conference, which got accepted, and were then allowed to present an A2ish-sized poster which they could then stand next to and talk about for anyone interested in the research.
This is, in a word, inadequate. You can’t reasonably understand and analyze scientific research based on a poster presentation. It’s a useful way to see what everyone else is working on, and it gives students a great way to get their research out there, but posters simply do not give us much data on what the scientists actually did in their research and the potential pitfalls in their methodology.
Based on the limited information that the poster provides, the authors took the United States NHANES dataset, which is a regular national study looking at diet and nutrition in the US, and looked at people’s reported dietary patterns. They divided individuals into five groups - people who ate for <8h a day, 8-10h, 10-12h, 12-16, and 16+. They then compared these groups on their risk of dying from any cause, from heart disease, and from cancer.
Now, it’s important to note that the NHANES dataset doesn’t specifically ask “do you do intermittent fasting?”. NHANES is a massive survey with hundreds of items, some of which are related to whether people restrict their eating to specific times. The poster doesn’t actually say which fields the authors used to derive their groups of eating times, which is why it’s dicey to make arguments from this sort of research, but we can guess. All this means that this study is very simply not about IF. The authors are relying on a question such as “How long has it been since you ate any food?” or “How much time did you spend eating/drinking yesterday?”. These are related to IF, to some extent, but are obviously not quite what people mean when they discuss the diet.
All that aside, the main finding was that there was no increase in all-cause or cancer mortality associated with different reported eating patterns in the author’s models which were corrected for age, sex, and a range of other factors. There was a statistically significant increase in the <8h group in cardiovascular mortality, but no increases in the 8-10h, 10-12h, or 16+ when compared with the 12-16 (which was what the authors used as their baseline). This equated to a 91% increase in risk for people who reported eating only within <8 hours a day when compared to people who ate for 12-16 hours.
And thus, the headlines.
Regular readers of my articles will know that this is quite clearly not a particularly solid finding. There are usually issues in this sort of observational research that make it hard to draw firm conclusions about cause and effect - the issue with the question type that I discussed above is one such example. A big problem for this particular study is reverse causality. Do people die more often because they eat in time-restricted windows, or do people who are already dying tend to eat in smaller time periods each day? The methodology used by the authors cannot really discern a difference between these two possibilities, meaning that their result could just show that people who have severe heart disease find it difficult to eat regular meals throughout the day.
This sort of study is fundamentally not rigorous enough to determine whether IF causes health issues. It’s not even looking at fasting specifically! What we really need are interventional trials, where we compare two groups of randomly-allocated people on either IF or another similar diet to see whether IF might have any serious health risks.
Interventional Research
Fortunately for all of us, there is quite a lot of interventional research looking at IF and similar diets. I’ve even written about some of it before. A 2021 systematic review from the gold standard Cochrane collaboration found that the clinical trials were generally not very good, but seemed to indicate that IF was about as useful as other types of diet, and probably helps people lose weight and improve other markers of health compared to no diet at all. Other reviews, from less well-respected groups than Cochrane, have come to similar conclusions.
The challenge with these interventional trials is that they are quite small, not particularly robust, and don’t look at hard endpoints such as death from heart disease. It’s possible that IF causes heart disease through some undiscovered mechanism, even though it seems about as good at helping you to lose weight and improve your blood pressure as any other diet.
That being said, the interventional research generally seems to indicate that it’s unlikely that the results from the recent poster presentation are causal. It’s hard to imagine how fasting could improve cholesterol and lower blood pressure when compared to no diet in interventional trials, but cause a massive spike in heart disease risk as well.
Ultimately, IF seems about as good or bad as most other diets. The main way that it works is by making you eat less calories, and as a mechanism for slightly lowering your food intake it’s pretty good. I’ve done it before, and while you do get really hungry, having set periods where you just don’t eat is a convenient way to manage caloric restriction.
different. But if it works for you, there’s no reason to stop doing it. The headlines might seem scary, but the evidence doesn’t seem to show any big risks from using fasting as part of your diet.