The popularity of intermittent fasting — when people only eat at certain times of the day — has exploded in recent years. But now a new study suggests there may be reasons to be cautious: some of the exercisers were found to be more likely to die from heart disease.
The results were presented Monday at a meeting of the American Heart Association in Chicago and focused on a popular version of intermittent fasting that involves eating all your meals in just eight hours or less — resulting in a daily fast of at least 16 hours. hours, commonly known as a “time constraint.”
The study analyzed data on the eating habits of 20,000 adults in the United States who were followed from 2003 to 2018. They found that people who adhered to the eight-hour eating plan had a 91% higher risk of dying from heart disease compared to people who followed a more traditional eating pattern of eating food for 12 to 16 hours a day.
Scientists found that this increased risk also applied to people already living with a chronic illness or cancer. People with existing cardiovascular disease who followed a time-restricted dietary pattern had a 66% increased risk of dying from heart disease or stroke. People with cancer, in turn, were more likely to die from the disease if they followed a time-restricted diet, compared to people with cancer who followed an eating duration of at least 16 hours a day.
The study results suggest that people who practice intermittent fasting for long periods of time, particularly those with heart problems or cancer, should be “extremely cautious,” warns Victor Wenze Zhong, the lead author and chairman of the department of epidemiology and biostatistics at Shanghai Jiao Tong University School of Medicine, China.
“Based on current data, it seems more important to focus on what people eat rather than when they eat it,” he adds. Zhong says he and his colleagues conducted the new study because they wanted to see how eating within a narrow window every day would impact “hard endpoints” like heart disease and mortality. And he says they were surprised by the conclusions.
“We expected that the long-term adoption of eight-hour food restriction would be associated with a lower risk of cardiovascular death and even death from all causes”, he explains.
Loss of lean muscle mass
The data does not explain why time-restricted eating increases a person’s health risks. But researchers found that people who followed a time-restricted eating pattern, in which they eat during an eight-hour window and fast for 16, had less lean muscle mass compared to people who ate during longer periods of the day. This is in line with a previous clinical trial published in JAMA Internal Medicinewhich concluded that people who followed a time-restricted diet for three months lost more muscle than a control group that wasn’t assigned to intermittent fasting.
Maintaining muscle as you age is important. It protects people against falls and disabilities and can improve metabolic health. Studies have found that reduced muscle mass is associated with higher mortality rates, including a higher risk of death from heart disease, says Zhong.
The researcher emphasizes that the results are not definitive. The study revealed a correlation between dietary restriction and increased mortality, but was unable to demonstrate cause and effect. It’s possible, for example, that people who restricted their food intake to an eight-hour daily window have other habits or risk factors that could explain their greater likelihood of dying from heart disease. The scientists also noted that the study was based on self-reported dietary information. It is possible that participants did not always accurately report the duration of their meals.
A modern way to diet and control your weight
Intermittent fasting has been widely promoted by celebrities and health experts who claim it produces weight loss and a variety of health benefits. Another form of intermittent fasting involves alternating days of fasting with days of normal eating. Some people follow the 5:2 diet, in which they eat normally for five days a week and then fast for two days.
But time-restricted eating is generally considered the easiest form of intermittent fasting for people to follow because it doesn’t require full-day fasts. It also does not normally involve excessive dietary restriction. Fans usually eat or drink whatever they want during the eight-hour period — the only rule is not to eat at other times of the day.
Some of the first studies on time-restricted feeding found that it helped prevent rats from developing obesity and metabolic syndrome. Small clinical trials in humans followed, some of which showed that dietary restriction helped people lose weight and improve blood pressure, blood sugar and cholesterol levels. These studies were mostly of short duration, typically one to three months, and in some cases revealed no benefit.
One of the most rigorous studies on time-restricted eating was published in New England Journal of Medicine in 2022. This study concluded that people with obesity who were assigned to follow a low-calorie diet and instructed to eat only between 8 am and 4 pm daily did not lose more weight than people who ate the same number of calories throughout the day without restrictions on the time they could eat. The two diets had similar effects on blood pressure, blood sugar, cholesterol and other metabolic markers.
The results suggest that any benefits from time-restricted eating likely result from eating fewer calories.
More questions about intermittent fasting
Christopher Gardner, director of nutrition studies at the Stanford Prevention Research Center, says he encouraged people to approach the new study with “healthy skepticism.” He says that although the findings were interesting, he wants to see all the data, including possible demographic differences in the study subjects.
“Did everyone have the same level of disposable income and the same level of stress?” he asks. “Or could it be that people who ate less than eight hours a day worked three jobs, had very high stress and didn’t have time to eat?”
Gardner says studying intermittent fasting can be challenging because there are many variations of it, and determining its impact on longevity requires closely following people over long periods of time.
But he adds that so far, the evidence supporting intermittent fasting for weight loss and other results is mixed at best, with some studies showing short-term benefits and others showing no benefit. “I don’t think the data is very strong for intermittent fasting,” he says. “One of the challenges of nutrition is that just because something works really well for some people, doesn’t mean it works for everyone.”
Your biggest complaint with intermittent fasting is that it doesn’t address the quality of the diet. “It doesn’t say anything about choosing poorly when you’re eating,” he points out. “What if I have an eight-hour window to eat, but I’m eating Pop Tarts and Cheetos and drinking Coca-Cola? I’m not a fan of that long term. I think that’s potentially problematic,” he concludes.
Exclusive PUBLIC/The Washington Post
Translation: Barbara Wong
