Touted as a holy grail of weight loss and longevity, intermittent fasting is an eating strategy built on the idea that the human body shifts into a healthier metabolic state when it goes without food for 16 consecutive hours. The concept is so popular that 1 in 9 Americans tried intermittent fasting in 2024, according to the International Food Information Council.
But does popularity guarantee efficacy? Recent research suggests it does not.
A widely-reported review of the research, which analyzed 22 studies, has found that intermittent fasting was no better than a traditional low-calorie diet for weight loss. Although fasting participants did lose more weight than non-dieters, they didn’t lose more weight than people following other types of calorie-restricted eating.
These findings line up with other research that found that, while intermittent fasting does improve cardiovascular health measures, it doesn’t appear to outperform traditional dieting. The health benefits may be linked more to the amount of weight lost than to the timing of your eating.
Still, intermittent fasting may work well for some people. The answer, for now, may lie in the nuances of the research and in whether this style of eating fits your lifestyle and preferences.
Intermittent fasting works in lab mice.
Intermittent fasting captured the attention of scientists decades ago when studies showed that fasting mice lived longer. The resulting theory posits that periods without food push the body into self-preservation mode and, in doing so, improve a number of health markers.
Animal studies have consistently shown that fasting can prevent or reverse many aspects of metabolic syndrome, including abdominal fat, inflammation, high blood pressure, and insulin resistance — all risk factors for cardiovascular disease and other illnesses. Studies have also linked fasting to a reduced risk of neurological conditions like Alzheimer's disease and dementia.
But what happens in mice doesn’t necessarily translate to humans. For one, the metabolism of a mouse is much faster than that of a human, which means the effects of fasting may be more dramatic in rodents. Also, lab studies often require mice to fast for one to three days in a row, a prolonged period of food deprivation that may elicit more dramatic outcomes than the 16-hour fasting window commonly recommended for people.
Intermittent fasting may be good — but not better.
Most human studies find that intermittent fasting can lead to weight loss and improvements in metabolic health, but it doesn’t appear to be better than traditional calorie-restricted eating.
For example, a 2022 review of the research found that people following intermittent fasting lost significantly more weight than those in control groups who were not dieting. They also had decreased triglyceride and cholesterol levels. But the study couldn’t determine whether these health benefits resulted from the timing of eating or simply from consuming fewer calories overall.
Several recent studies have shed some light on this question. One 12-month study asked two groups of people to follow a low-calorie diet, but one group could only eat during an eight-hour window. The groups had similar changes in body weight and cardiometabolic risk factors, suggesting that fasting did not enhance their benefits.
Intermittent fasting works for some people.
The secret sauce of intermittent fasting may be a decreased consumption of calories overall; though there are no restrictions on what can be eaten in the non-fasting window of roughly eight hours. For some people, this approach feels simpler than a traditional diet. You focus on when you’re eating, not what you’re eating, which can reduce the stress around decision-making and calorie-counting as well as food noise.
Still, for other people, fasting for 16 hours can be difficult. One study reported that less than half of the participants would continue with intermittent fasting after the study ended. A sense of deprivation may lead to overcompensating when people finally enter their eating window.
Timing may matter.
There’s also a growing body of research on the effects of eating — and not eating — late at night. As we nod off, the body’s circadian rhythm sends signals to the digestive system and metabolism to slow down.
Some researchers believe that ending the eating window earlier in the evening, and reducing the load on a sluggish GI tract, could improve metabolic health. There’s evidence supporting this idea. Late-night eating has been linked to weight gain, insulin resistance, impaired glucose regulation, and poorer sleep.
Researchers at Northwestern University recently tested this concept by asking participants to begin fasting three hours before their bedtime. Compared with the control group that didn’t fast, the study participants had improved insulin response, heart rate, blood pressure, and cortisol levels after six weeks. Their circadian rhythm was also more aligned with the 24-hour clock.
The decision to try intermittent fasting may come down to personal preference. It’s clearly not the holy grail of weight loss or longevity, but it is effective for some people. If you struggle with counting calories, restricting specific foods, or food noise, intermittent fasting may be a good fit. If the idea of going 16 hours without food sounds miserable, then don’t despair.
