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the news is all over my social media this morning: a popular fad diet is apparently lethal, says scientific research. Specifically, one study found that calorie restriction, also known as intermittent fasting, has a 91% increased risk of death from cardiovascular disease.
Except the scientific research doesn’t say that, and not only should you not worry about this study, you also shouldn’t waste brain glucose thinking about it. Even including that 91% number, which you may remember, caused me pain, because I don’t think this result should be remembered.
The study is a type of nutritional research that is notoriously weak and is currently only available as a Press release. It is not clear from the numerous news articles about the study whether journalists actually saw the data that will be presented at an upcoming research meeting held by the American Heart Association.
So how can I, a science journalist, safely dismiss this research? It is based on observational research, and one lesson from more than 20 years of reporting on health and medicine is that one should be very skeptical of observational research, especially when it comes to nutrition.
In this case, the researchers used as a starting point a really useful research tool, the National Health and Nutrition Examination Survey (NHANES), a survey of 5,000 people a year about eating and dietary habits. The researchers linked this data to a separate database of deaths. Both the survey and the deaths database are administered by the Centers for Disease Control and Prevention.
These databases allow researchers to quickly check whether dietary choices appear to be associated with health problems. That’s great, because they can help scientists set the course for more rigorous research that could take years. But the answers you get by doing so are not necessarily reliable.
Part of the problem, the easy part to understand, is that the people who answer surveys are not always completely honest. More than that, especially with food, we often misremember what we have eaten and how much. For example, we might think we followed our diet and completely forget when we made a mistake.
But the biggest problem is that people who decide to go on a diet, or those who follow it, may be fundamentally different from those who don’t in ways we can’t measure. Perhaps people follow time-limited diets because they are concerned about their health. Perhaps people who follow these types of diets have bodies that function differently than those who cannot fast for as long. Perhaps, for some reason, people who followed the diet were different from those who didn’t simply by chance.
Researchers try to counteract these possibilities by “controlling” for known risk factors, such as body weight and biological sex, gender, or age. But the problem is that researchers can only control the factors they can identify.
Let’s look at an example where these phenomena were at play: the decades-long story of whether red wine prevents heart attacks. Originally, researchers posed a “French paradox”: that red wine allowed Parisians to eat croissants, foie gras, boeuf bourguignon, raclette, and mussels and fries without the heart attacks that researchers expected because at the time they thought any diet high in fat increased the risk of heart disease. This eventually morphed into the idea that drinking very moderately (no more than one glass of wine a day) had a beneficial effect on heart disease.
Except recently, some researchers have argued that this apparent benefit does not exist; It just seemed that way because moderate drinkers were healthier than others in ways that researchers had difficulty measuring.
The only way to get close to knowing this for sure is to take a large group of people and randomly assign them to, say, drink a glass of red wine a day or be teetotal. Then you’ll know that the two groups of people are probably the same, and if they follow your instructions, you’ll be able to see how red wine makes a difference. Ideally, you would give them fake wine (a placebo) or real wine so that not even the participants know what they are getting.
This is called a blind randomized controlled trial, and it often causes the “fair” stories that scientists tell themselves to evaporate. For example, there was a surprising story that the Inuit did not get heart disease from high-fat diets because they ate a lot of fish. This led to many studies, including randomized trials, that seemed to show that taking fish oil supplements would reduce heart disease. But higher quality randomized studies did not show this effect, until a recipe with highly purified fish oil was successful. However, some researchers also doubted that study, because the placebo the scientists used could have caused heart attacks. Yes, this is confusing, and that’s the point: with nutrition, we have to be very careful about everything we don’t know.
According to a summary of the new study provided to me by the American Heart Association, which is hosting the meeting where the results are presented, it appears that the researchers did not ask people if they were on time-restricted diets. What they did was look for people who only ate for a short period of time during the day based on two survey reports about what they ate.
“While informative, this study should be considered exploratory,” said Harlan Krumholz, a leading expert on the science of health policy improvement at Yale. “We’re still learning how people can optimize their diets, and this study is more of a call for more research than something that should scare people who find restricted eating a useful strategy.”
My conclusion is that the study means that daily calorie restriction should be studied more, but we knew that. I don’t think it tells us anything more about these diets; it just illustrates how much we don’t know about biology. Some articles posit that perhaps dieting this way leads to greater loss of muscle mass. Sure, maybe.
But my other concern is that studies like this, and the press coverage of them, could make people more skeptical about the things we do know in medicine. People tend to think of science as a process in which scientists conduct studies and discover the truth. But it’s more accurate to say that each study helps make us a little less wrong and a little more certain about what the truth might be. We live in a vast realm of darkness in which we have found scattered gems of truth.
This was an interesting finding that should prompt people working in nutrition to look further into this topic. For everyone else, it doesn’t really say anything at all.
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