Unlike some of the more concerning fad diet trends, growing evidence suggests that there is something to intermittent fasting. Studies on animals and humans demonstrate some effects on weight loss, and now a trial done in humans gives us a much better idea of how the diet works.
A paper published Wednesday in Obesity focused on time-restricted feeding, a form of intermittent fasting in which people consume all their meals within a specific timeframe. Normally, explanations for intermittent fasting’s success hinge on calories — either that intermittent fasters eat less of them, or maybe even burn them at a higher rate. But this paper suggests that the reason the diet works may not have anything to do with calories.
Instead, when people crammed all their eating in a six-hour period between 8 a.m. and 2 p.m., their restricted eating patterns tended to decrease appetite over time.
The difference between appetite and calorie burning may seem inconsequential, but it’s a significant change from what scientists thought was happening during intermittent fasting — at least based on some animal models, explains lead study author Courtney Peterson, Ph.D., an associate professor of Nutrition Sciences at the University of Alabama at Birmingham.
“Some of the research in rodents found that time-restricted feeding helped the rodents burn more calories,” she tells Inverse. “Unfortunately, we found that time-restricted feeding does not help people burn more calories (like it does in rodents).”
“However, we did find three beneficial things,” she continues. Specifically her team’s results point toward time-restricted feeding’s effects on hunger and fat burning.
How Time-Restricted Feeding Affects Hunger
The strongest evidence from Peterson’s trial suggests that time-restricted feeding affects how the body processes hunger. Study participants who ate all their meals within that six-hour window before 2 p.m. reported a more even-keeled sense of hunger, an increased feeling of fullness, and a decreased desire to eat.
When Peterson and her team investigated blood samples taken from their participants, they got a sense of where these feelings might come from.
During fasting conditions, they observed significant decreases in the amount of ghrelin produced by their participants. Ghrelin is the hormone that’s usually produced when the stomach is empty, and it essentially tells the body that it’s time to fill up again. That’s part of the reason it’s famously known as the “hunger hormone.”
Importantly, in this new paper, the team makes the case that it’s not just this time-restricted feeding itself responsible for these hormonal changes. They argue that the actual meal timing may be partially responsible for the appetite suppression. When people eat earlier in the day, they tend to ensure that they’re eating during the “biological day” — the portion of the 24-hour circadian cycles that govern wakefulness, calorie burning, or hormone release.
Eating meals during biological night has had documented effects on weight gain in mice and night shift workers. So the idea is that timing the feeding window to coincide with biological day may be part of the reason why time-restricted feeding is a “powerful strategy for reducing appetite and losing weight.”
How Time-Restricted Feeding Affects Fat Burning
Peterson and her team also found that eating earlier in the day changed the ways that study participants’ bodies burned fuel. Primarily, they noted that fat burning increased during the day — which makes sense, considering that people in this study stopped eating at 2 p.m.
“We think the longer daily fast gives people’s bodies more time each day to dip into their fat reserves and burn fat,” says Peterson. “The body is typically maximally efficient at burning fat when people fast for at least 12-24 hours at a time.”
The timed-restricted eating schedule also increased their subjects’ “metabolic flexibility,” says Peterson. Metabolic flexibility is how well the body switches between burning different types of fuel sources, like carbohydrates, proteins or fats. In the paper, they add that increasing flexibility may help to combat obesity or decrease the risk of type II diabetes.
But as promising as that is, Peterson cautions that we can’t link these additional metabolic changes too strongly to long-term weight loss right now. Her study was more focused on the effects on appetite and calorie burning, but this finding is something that may be addressed in more detail later.
Applying These Findings to Dieting
Peterson’s trial seems like good news for the disciples of time-restricted feeding. But she’s eager to note that the six-hour eating window used in her experiment is a very extreme version of an already difficult diet. Generally, she doesn’t recommend trying to eat all your meals in a six-hour period.
“In the real world, however, we recommend targeting an 8-10 hour eating period 5-6 days per week. Both research from mine and Dr. Krista Varady’s lab suggests that this is a more realistic target.”
Still, in moderation, there’s some evidence that what time-restricted eating lacks in calorie-burning effects it makes up for in other places.
Objective: Eating earlier in the daytime to align with circadian rhythms in metabolism enhances weight loss. However, it is unknown whether these benefits are mediated through increased energy expenditure or decreased food intake. Therefore, this study performed the first randomized trial to determine how meal timing affects 24-hour energy metabolism when food intake and meal frequency are matched.
Methods: Eleven adults with overweight practiced both early time-restricted feeding (eTRF) (eating from 8 am to 2 pm) and a control schedule (eating from 8 am to 8 pm) for 4 days each. On the fourth day, 24-hour energy expenditure and substrate oxidation were measured by whole-room indirect calorimetry, in conjunc- tion with appetite and metabolic hormones.
Results: eTRF did not affect 24-hour energy expenditure (Δ=10±16 kcal/d; P=0.55). Despite the longer daily fast (intermittent fasting), eTRF decreased mean ghrelin levels by 32 ± 10 pg/mL (P = 0.006), made hun- ger more even-keeled (P = 0.006), and tended to increase fullness (P = 0.06-0.10) and decrease the desire to eat (P=0.08). eTRF also increased metabolic flexibility (P=0.0006) and decreased the 24-hour nonprotein respiratory quotient (Δ=−0.021±0.010; P=0.05).
Conclusions: Meal-timing interventions facilitate weight loss primarily by decreasing appetite rather than by increasing energy expenditure. eTRF may also increase fat loss by increasing fat oxidation.