Intermittent fasting — when people only eat at certain times of day — has exploded in popularity in recent years. But now a surprising new study suggests that there might be reason to be cautious: It found that some intermittent fasters were more likely to die of heart disease. The study The scientists found that this increased risk also applied to people who were already living with a chronic disease or cancer. The study results suggest that people who practice intermittent fasting for long periods of time, particularly those with existing heart conditions or cancer, should be “extremely cautious,” said Victor Wenze Zhong, the lead author and the chair of the department of epidemiology and biostatistics at the Shanghai Jiao Tong University School of Medicine in China. “Based on the evidence as of now, focusing on what people eat appears to be more important than focusing on the time when they eat,” he added. Zhong said that he and his colleagues conducted the new study because they wanted to see how eating in a narrow window each day would impact “hard endpoints” such as heart disease and mortality. He said that they were surprised by their findings. “We had expected that long-term adoption of eight-hour time restricted eating would be associated with a lower risk of cardiovascular death and even all-cause death,” he said. Losing lean muscle mass Losing lean muscle mass The data didn’t explain why time-restricted eating increased a person’s health risks. But the researchers did find that people who followed a 16:8 time-restricted eating pattern, where they eat during an eight-hour window and fast for 16, had less lean muscle mass compared to people who ate throughout longer periods of the day. That lines up 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 was not assigned to do intermittent fasting. Holding onto muscle as you age is important. It protects you against falls and disability and can boost your metabolic health. Studies have found that having low muscle mass is linked to higher mortality rates, including a higher risk of dying from heart disease, said Zhong. He stressed that the findings were not definitive. The study uncovered a correlation between time-restricted eating and increased mortality, but it 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 might explain their increased likelihood of dying from heart disease. The scientists also noted that the study relied on self-reported dietary information. It’s possible that the participants did not always accurately report their eating durations. A trendy form of dieting and weight control A trendy form of dieting and weight control Intermittent fasting has been widely touted by celebrities and health experts who say it produces weight loss and a variety of health benefits. Another form of intermittent fasting involves alternating fasting days with days of eating normally. 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. Some of the earliest studies on time-restricted eating found that it helped prevent mice from developing obesity and metabolic syndrome. These were followed by mostly small clinical trials in humans, 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, typically 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 people with obesity who were assigned to follow a low-calorie diet and instructed to eat only between the hours of 8 a.m. and 4 p.m. daily lost no more weight than people who ate the same number of calories throughout the day with no 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 likely result from eating fewer calories. More questions about intermittent fasting More questions about intermittent fasting Christopher Gardner, the director of nutrition studies at the Stanford Prevention Research Center, said he encouraged people to approach the new study with “healthy skepticism.” He said that while the findings were interesting, he wants to see all the data, including potential demographic differences in the study subjects. “Did they all have the same level of disposable income and the same level of stress,” he said. “Or is it that the people who ate less than eight hours a day worked three jobs, had very high stress, and didn’t have time to eat?” Gardner said that studying intermittent fasting can be challenging because there are so many variations of it, and determining it’s impact on longevity requires closely following people for long periods of time. But he said that 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 are very strong for 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 that his biggest complaint with intermittent fasting is that it doesn’t address diet quality. “It doesn’t say anything about choosing poorly when you’re eating,” he said. “What if I have an eight-hour eating window but I’m eating Pop Tarts and Cheetos and drinking Coke in that window? I’m not a fan of that long term. I think that’s potentially problematic.” Do you have a question about healthy eating? Email [email protected] and we may answer your question in a future column. 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