Learning analyzed data on the eating habits of 20,000 adults in the United States who were tracked from 2003 to 2018. They found that people who adhered to the eight-hour meal plan had a 91 percent higher risk of death from heart disease than people who followed a more traditional dietary pattern of eating 12 to 16 hours each day.
The scientists found that this increased risk also applies to people who are already living with a chronic disease or cancer. People with pre-existing cardiovascular disease who followed a time-restricted eating pattern had a 66% higher risk of dying from heart disease or stroke. Those who had cancer in the meantime were more likely to die from the disease if they followed a time-restricted diet compared to people with cancer who followed a diet of at least 16 hours a day.
The study results suggest that people who practice intermittent fasting for long periods of time, especially those with existing heart disease or cancer, should be “extremely cautious,” said Victor Wence Zhong, lead author and chair of the Department of Epidemiology and Biostatistics. at Shanghai Jiao Tong University of Medicine in China.
“Based on the evidence at the moment, focusing on What people eat seems more important than focusing on when they eat,” he added.
Zhong said he and his colleagues conducted the new study because they wanted to see how eating in a narrow window each day would affect “hard endpoints” such as heart disease and mortality. He said they were surprised by their findings.
“We expected that long-term adoption of an eight-hour restricted meal would be associated with a lower risk of cardiovascular death and even death from all causes,” he said.
The data do not explain why time-restricted eating increases human health risks. But researchers have found that people who follow a 16:8 time-restricted eating pattern, where they eat in an eight-hour window and fast for 16, have less lean muscle mass than people who eat during longer periods of the day . This is consistent with a previous clinical trial published in JAMA Internal Medicine, which found that people assigned to follow a time-restricted diet for three months lost more muscle than a control group that did not prescribed intermittent fasting.
Retaining muscle as you age is important. It protects you from falls and injury and can improve your metabolic health. Studies have found that low muscle mass is associated with higher mortality rates, including a higher risk of death from heart disease, Zhong said.
He emphasized that the findings are not final. The study found a link between time-restricted eating and increased mortality, but could not show cause and effect. It’s possible, for example, that people who restricted their food intake to an eight-hour daily window had other habits or risk factors that could explain their increased likelihood of dying from heart disease. The scientists also noted that the study relied on self-reported dietary information. It is possible that participants did not always accurately report the duration of their meal.
A modern form of dieting and weight control
Intermittent fasting is widely touted by celebrities and health experts who say it leads to weight loss and a number of health benefits. Another form of intermittent fasting involves alternating fasting days with normal eating days. Some people follow the 5:2 diet, where they eat normally 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 fasting all day. It also usually does not involve excessive food restriction. Adherents often eat or drink whatever they want during the eight-hour meal period—the only rule is not to eat at other times of the day.
Some of the earliest studies on time-restricted feeding found that it helped prevent mice from developing obesity and metabolic syndrome. These were followed by mostly small human clinical trials, some of which showed that time-restricted eating helped people lose weight and improve their blood pressure, blood sugar and cholesterol levels. These studies were largely short-term, usually lasting one to three months, and in some cases showed no benefit.
One of the most rigorous studies of time-restricted eating was published in the New England Journal of Medicine in 2022. It found that obese people assigned to follow a low-calorie diet and instructed to eat only between 8 a.m. and 4:00 p.m. each day did not lose more weight than people who ate the same number of calories throughout the day without restrictions on when they could eat. The two diets had similar effects on blood pressure, blood sugar, cholesterol and other metabolic markers.
The findings suggest that any benefits of time-restricted eating are likely a result of consuming fewer calories.
More questions about intermittent fasting
Christopher Gardner, director of nutrition research at the Stanford Prevention Research Center, said he encourages people to approach the new study with “healthy skepticism.” He said that while the findings are interesting, he wants to see all the data, including potential demographic differences in the study subjects.
“Did everyone have the same level of disposable income and the same level of stress,” he said. “Or the people who ate less than eight hours a day, worked three jobs, had a lot of stress and didn’t have time to eat?”
Gardner said studying intermittent fasting can be challenging because there are so many variations of it, and determining its effect on longevity requires carefully following people over long periods of time.
But he said so far the evidence supporting intermittent fasting for weight loss and other outcomes is mixed at best, with some studies showing short-term benefits and others showing no benefit at all. “I don’t think the data is very strong on intermittent fasting,” he added. “One of the challenges in nutrition is that just because something works really well for a few people doesn’t mean it’s going to work for everyone.”
He said his biggest complaint about intermittent fasting is that it doesn’t affect the quality of the diet. “It says nothing about poor food choices,” he said. “What if I have an eight-hour window to eat, but I eat Pop Tarts and Cheetos and drink Coke in that window? I am not a fan of this long term plan. I think this is potentially problematic.
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