Intermittent fasting research still developing, time-restricted feeding shows potential
LAS VEGAS — Among the numerous intermittent fasting strategies being pursued for weight loss, time-restricted feeding has shown particular promise, according to a speaker at ObesityWeek.
“[Intermittent fasting regimens] are probably no better than continuous energy restriction — or, basically, conventional dieting — for weight loss, but they may improve some other aspects of health even independent of weight loss,” Courtney M. Peterson, PhD, assistant professor in the department of nutrition sciences at the University of Alabama at Birmingham, told Endocrine Today. “For time-restricted feeding, that seems to induce weight loss even when people aren’t told deliberately to lose weight. It does seem that there are weight loss independent effects of [time-restricted feeding] for improving blood sugar levels and blood pressure at least.”
Peterson noted that due to the number of different types of intermittent fasting techniques, there is “no one single quantitative definition” of the intervention, but the common thread is incorporating extended fasts of between 12 and 24 hours into the regular dietary routine. This expansive definition has allowed for a multitude of different types of intermittent fasting strategies, which Peterson divided into three groups.
The strategy that may hold the most promise is daily intermittent fasting or time restricted feeding, in which individuals eat during only certain hours each day, for example, between 10 a.m. and 6 p.m. Peterson noted that cutting calories is not necessary with this technique and that both animal and human evidence has pointed toward potential benefits for weight loss, diabetes reversal and glucose reduction, among others. The benefits appear to be more promising when incorporating early time-restricted feeding, when the window for eating is earlier in the day, compared with late time-restricted feeding, when the window falls later in the day and into the evening.
Time-restricted feeding is also an attractive approach due to its modifiable nature, easier implementation and potential circadian benefits, according to Peterson.
“The benefits are proportional to the length of the daily fasting period,” Peterson said during a presentation. “This suggests that there’s not a specific magic number that people need to hit.”
Peterson also said that time-restricted feeding provides “some social flexibility” because it still produce benefits in rodent studies even if practiced 5 days per week, but that the individual must be considered when choosing to adopt such an approach.
“A lot of patients would benefit from time-restricted feeding, but I’m not sure everyone can practically do it,” Peterson told Endocrine Today. “We don’t have a good sense of who struggles yet with time-restricted feeding. We’re still collecting that kind of data. For those individuals [who cannot do it], I might recommend more like a meal timing approach, where they eat a larger breakfast and a larger lunch and then a lighter dinner.”
Peterson also noted that there may be increased risk for low blood sugar with this approach for individuals with type 2 diabetes. “I would issue caution on that. Those individuals should work with a health care provider,” Peterson told Endocrine Today.
The second group of intermittent fasting strategies is periodic fasting, where individuals fast for at least 24 hours at a time. The most common version of periodic fasting is alternate-day fasting where an individual does not eat one day and then eats on the next day.
According to Peterson, some research on alternate day fasting has shown improvements in weight loss as well as glucose and insulin levels, but these findings have largely been “mixed” and combined with a tendency to induce more hunger, which calls into question its sustainability.
Intermittent energy restriction
The third group is what Peterson labeled intermittent energy restriction. These strategies generally involve alternating days with very low-calorie diets rather than strict fasting.
“There’s a little bit of debate about whether these interventions truly constitute intermittent fasting, but there’s an argument to be made that they do, or at least many of them do,” Peterson said during a presentation. “Because oftentimes on your modified fasting day, you’re only eating one meal or a reduced number of meals, so you are technically extending the fasting duration.”
The strategies in this group include alternate day modified fasting, the 5:2 diet, the fasting mimicking diet and weekly intermittent energy restriction.
In alternate day modified fasting, individuals eat a higher calorie diet on one day and then a very low-calorie diet on the next, with the low-calorie diet usually about 25% of normal caloric intake and the high diet at about 125% of normal caloric intake. Peterson noted that long-term evidence suggests that glucose, insulin, blood pressure, heart rate, lipids, visceral fat and weight change are unaffected when directly comparing alternate day modified fasting with continuous energy restriction, although there is some benefit shown for those with insulin resistance.
The 5:2 diet is a “lighter version” of alternate daily modified fasting and requires the consumption of a low-calorie diet only 2 days per week. Evidence from randomized controlled trials assessing this diet indicate that there are “short-term benefits for weight loss that seem to actually evaporate at the 1-year mark,” according to Peterson, who noted that these studies, much like all of those assessing intermittent fasting, may be missing important postprandial metrics.
“We may not actually be detecting differences in a lot of these intermittent fasting studies by only looking at fasting risk factors,” Peterson said during a presentation. “We really need to look at the postprandial levels as well.”
The fasting mimicking diet is another form of intermittent fasting that falls into the intermittent energy restriction camp. Peterson said that this is a “more feasible” strategy that is “gaining a lot of traction.” This diet requires the consumption of a plant-based diet of between 700 kcal and 1,100 kcal for a set period each month. According to Peterson, animal evidence has provided promising results for body weight and visceral fat reduction, and the limited data from trials in humans also provided evidence for potential weight loss and benefits in terms of BMI, body fat and blood pressure.
Last among the intermittent energy restriction diets is weekly intermittent energy restriction. In this technique, individuals typically alternated 1 to 2 weeks of eating a very low-calorie diet with eating a normal diet. Peterson said that the available evidence from studies assessing this strategy are inconclusive, although the rationale behind the strategy is appealing.
“By having these longer extended periods where participants are in energy balance, we actually may mitigate some of the metabolic adaptations that we see traditionally with energy restriction,” Peterson said during a presentation. “That’s a very elegant theory worth testing.”
Making adjustments to how these dietary strategies are assessed could go a long way in elucidating which are best to use in a real-world setting, Peterson said.
“I’m not sure we’re designing our studies optimally ... [in nearly all research trials] we literally counsel our participants to eat the same number of calories. Should we be surprised if we counsel both groups to eat the same number of calories and there’s no difference in weight loss?” Peterson said during a presentation. “I think we need to start doing intermittent fasting studies where we don’t give people particular calorie targets and see how well they do on their own.” – by Phil Neuffer
Peterson CM. Intermittent Fasting and Metabolism. Presented at: ObesityWeek 2019; Nov. 3-7, 2019; Las Vegas.
Disclosure: Peterson reports no relevant financial disclosures.