Not All Intermittent Fasting Is as Healthy as You Think
Research into circadian rhythms shows that time-restricted eating is not the panacea many thought it was.
Intermittent fasting is a kind of diet that predominantly focuses on the when — instead of the what — to eat. Its most common form is time-restricted eating (TRE), which usually comprises eight hours of eating and 16 hours of fasting in each 24-hour cycle. But everyone seems to do TRE differently: some only have one meal a day, while others eat in a six- or ten-hour period. Some eat earlier, while others later in the day.
Either way, as scientific evidence accumulates, some experts now believe that abstaining from food for extended hours could be one of the healthiest lifestyle choices people can make. More and more studies show that certain forms of fasting can restore metabolic health and reduce chronic inflammation in the body (Manoogian et al., 2022)— the two most crucial factors of every chronic disease. Time-restricted eating can also lead to weight loss and potentially prevent type 2 diabetes and cardiovascular disease (Moon et al., 2020), among others:
“The combination of improved glucose regulation, improved lipids, and [lowered] blood pressure is really a powerful combination for targeting some of the most common diseases that we have,” says Emily Manoogian, a staff scientist at the Salk Institute in La Jolla, California, where she leads clinical trials on TRE.
Although TRE has shown robust benefits in animal studies over the past decade, human research is still in its infancy. Most of the studies are small, Manoogian says, and use varying protocols, which makes extrapolating the results more difficult. Nevertheless, positive findings in recent years flood the media, as athletes, celebrities, and science influencers have also taken up fasting, which makes distinguishing the facts from the hype more challenging.
Yet the science is becoming clear that not all protocols offer the same benefits. Studies that investigated late TRE — only eating in the afternoon and evening — saw reduced benefits (Xie et al., 2022a, Xie et al., 2022b). Some research also indicates that skipping breakfast might increase the risk of cardiovascular disease (Chen et al., 2020).
So is the conventional wisdom — that we ought to eat breakfast like a king, lunch like a prince, and dinner like a pauper — more than pseudoscience?
What does our internal clock tell us?
Research into circadian rhythms — our internal biological clock — indicates it is. For example, the hormone melatonin, which controls our sleep cycle and a myriad of other mechanisms in the body, builds up in the evening and is present until the following morning. Melatonin helps us fall asleep, but it also inhibits insulin secretion6, so snacking late at night or eating early in the morning impairs how the body can use glucose:
“A calorie is interpreted very differently at different times of the day. You are just not able to process food the same way.” Manoogian says.
Then, as melatonin wanes in the morning, another hormone, cortisol, wakes the body up. We are more sensitive to insulin and glucose earlier in the day, meaning if we eat breakfast, our body can use the fuel more efficiently6.
“It suggests our body was designed to be ready for fight or flight in the morning after we had rested,” says Eric Ravussin, an expert on metabolic health at Louisiana State University.
Similar morning-specific patterns guide the hormones that signal hunger and satiety and those that switch between glucose and fat as the body’s energy source (Poggogialle et al., 2018). Many of these circadian patterns are disrupted in people suffering from obesity or type 2 diabetes (Oosterman et al., 2020), and in shift workers, whose irregular sleep-wake cycle predisposes them to all sorts of chronic diseases (James et al., 2017).
And it’s not only that the body can process food differently at different times of the day, but the converse is also true: eating has “acute effects on our circadian rhythms,” Ravussin tells me. “The onset of melatonin before sleep is totally deranged by eating, and the circadian rhythm of cortisol is also disturbed.”
Observational studies examining Muslim populations fasting for Ramadan confirm this: only eating after sundown can lower their evening melatonin (Al-Rawi et al., 2020) and their next morning cortisol (Riat et al., 2021). The former leads to disrupted sleep at night and decreased alertness during the day.
The explanation is that “food is also an arousal cue” for both our brains and metabolism, Manoogian says. When we introduce energy into the body, our circadian rhythm assumes that we need to ramp up activity to use that energy. “But your body isn’t prepared to digest food when it’s planning on resting. So eating close to bedtime can compromise your sleep, and it’s also going to lead to negative metabolic effects,” she adds.
The research on TRE has also picked up on these findings (Charlot et al., 2021). A few small-scale studies suggest more health benefits to early TRE than to a later protocol (Hutchison et al., 2019, Kim and Song, 2023, Zhang et al., 2022). “The hypothesis is that, yes, in terms of metabolic health, you would benefit more from early time-restricted eating,” notes Ravussin.
These interventions have shifted the eating window completely to the early part of the day, from wake-up until 3 or 4 pm. Yet even to the most health-conscious of us, having to eat two large meals at work and finish by early afternoon seems drastic and unattainable. Especially since dinner, in most cultures, is the most important meal of the day. It’s often the one we get to spend with loved ones. It’s a ritual. And skipping it and any other dinner commitment would be detrimental to our social lives and emotional well-being.
How to go about fasting
However, we don’t need to stick to the most rigorous schedule to benefit from fasting. Recognizing these constraints, the leading circadian rhythm lab of Satchin Panda at the Salk Institute put forward three principles that they believe can help people attain most of the circadian benefits without having to adopt such a rigorous schedule (Manoogian et al., 2022):
1) Don’t eat in the first hour after waking up to allow for the night’s melatonin to clear before you stimulate insulin secretion.
2) Finish dinner at least three hours before bedtime to not disrupt the release of melatonin needed for restful sleep.
3) Aim for consistent meal times across your days, including the weekends, so your circadian rhythm can settle into a stable cycle and work more efficiently.
These principles offer plenty of room to maneuver. Ravussin suggests: “Try to progressively reduce your window of eating to eight hours and accommodate it to your lifestyle and your social life.” Avoid compulsive and neurotic meal planning. He warns: “You don’t want to become miserable from being very strict.”
