Gabel K. Is missing a meal ideal? Intermittent fasting. Presented at: ObesityWeek Interactive; Nov. 2, 2020 (virtual meeting).

Disclosures: Gabel reports no relevant financial disclosures.
November 06, 2020
4 min read

Intermittent fasting offers viable options for obesity treatment


Gabel K. Is missing a meal ideal? Intermittent fasting. Presented at: ObesityWeek Interactive; Nov. 2, 2020 (virtual meeting).

Disclosures: Gabel reports no relevant financial disclosures.
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Intermittent fasting, specifically time-restricted eating, may be a safe and viable weight loss tool for adults with obesity and lead to naturally decreased caloric intake, according to a speaker at ObesityWeek Interactive.

Adherence to intermittent fasting eating plans — which can include time-restricted eating and alternate-day fasting — seems to be high, and time-restricted eating plans in particular have the added benefit of no calorie counting required, Kelsey Gabel, PhD, RD, clinical assistant professor of nutrition and a postdoctoral researcher at the University of Illinois in Chicago, said during a presentation. However, questions remain about the best eating window for time-restricted eating, and whether fasting or daily calorie restriction is better for weight loss.

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“We need longer trials examining the effect of intermittent fasting, including time-restricted eating, alternate-day fasting and the 5:2 diet,” Gabel told Healio. “Future trials should also compare weight loss and reduction in disease risk, as well as long-term adherence to daily energy restriction. Further, for time-restricted eating specifically, more studies should examine the length and timing of the window.”

Defining intermittent fasting

Intermittent fasting has become a “giant umbrella term” encompassing several different types of eating plans, Gabel said. Alternate-day fasting is defined as eating between zero and 25% of energy needs on a “fast” day, and eating ad libitum on a “feast day.” A “5:2” diet is an eating plan with 1 to 2 typically nonconsecutive fasting days and 5 to 6 days of ad libitum eating within a week. Time-restricted eating is defined as eating all meals within a short window each day, either 8, 6, or even 4 hours.

The two most popular versions of time-restricted eating involve an 8-hour window and a 6-hour window of eating, Gabel said. The plans have no calorie restriction guidelines.

Kelsey Gabel

“An example of an 8-hour window would be to start eating at 10 a.m., all calorie intake is before 6 p.m., and then 6 p.m. starts a water fast until 10 a.m. the next day,” Gabel said. “The 6-hour eating window is also very popular. Someone would eat between noon and 6, and then start the water fast until noon the next day.”

Time-restricted eating benefits

There remain very few human trials on time-restricted eating; most books on the topic are based on mouse models, Gabel said.

In a 12-week study with 23 participants assigned to an 8-hour time-restricted eating plan, Gabel and colleagues found that participants assigned to the eating plan lost significantly more body weight, 2.6%, compared with controls.

“This doesn’t seem to be as fast as alternate-day fasting, but we need to compare those two to see what differences might lie there,” Gabel said.

Self-reported adherence to the diet was also high at 80%, Gabel said. Additionally, the researchers noticed a surprising benefit of unintentional calorie restriction — about 350 calories per day — among participants assigned to the time-restricted eating plan. Findings were similar among participants assigned to a 6-hour time-restricted eating plan, she said.

“Just by shortening the energy-intake window, our participants naturally decreased daily caloric intake by 20% to 30% at 8 or 6 hours, respectively, even though there were no restrictions on quality or quantity of food,” Gabel told Healio.

Alternate-day fasting

In a yearlong trial, Gabel and colleagues analyzed data from participants assigned to an alternate-day fasting plan, a calorie-restriction eating plan, or a control protocol for 6 months each, followed by a 6-month weight maintenance phase. For the first 6 months, participants in the alternate-day fasting group consumed 500 calories on fast days, increasing intake to 1,000 calories on “fast” days during weight maintenance. Those assigned to calorie restriction reduced calories by 25% during the weight loss phase, followed by calorie restriction during weight maintenance. Controls maintained a usual diet.

At month 6, alternate-day fasting and daily calorie restriction both decreased body weight by 6%, with most of the weight loss occurring in the first 3 months of the respective diets, Gabel said. At month 12, there was weight rebound, but participants in both diet groups lost a mean 5% body weight at the end of the study.

Gabel noted that not everyone lost weight; approximately 25% of participants assigned to alternate-day fasting gained weight; 38% lost between 1% to 5% of body weight, and 42% lost between 5% and 15% body weight. Among participants in the calorie restriction group, the results were similar, she said.

Additionally, “on feast day, people are only eating about 110% of their actual needs, so there seems to be an inability to overeat on the feast days, and that is what seems to lead to this weight loss,” Gabel said. “It is pure energy in, energy out calculations.”

Practical considerations

Gabel cautioned that intermittent fasting is not for everyone. Pregnant or lactating women, people with disordered eating, shift workers and frequent snackers are not candidates for an eating plan that involves fasting, she said.

It is also important to warn patients starting an intermittent fasting plan that it is challenging in the beginning, she said.

“The first 10 days seem to be the most difficult for our participants, and that is true of any diet,” Gabel said. “The most common complaint is headaches, especially during the first 10 days. Eventually, we’ve been told by our participants anecdotally that you do feel a boost of energy on your fast days.”

Gabel said that eating 50 g of protein on fast days can help keep hunger low and participants satiated.

“So which diet to choose?” Gabel said. “Alternate-day fasting tends to result in faster weight loss, anywhere from 10 to 15 lbs in 3 months; however, it is hard to follow and you do have to count calories every other day. Time-restricted eating seems to produce slower weight loss, about 5 to 10 lbs 3 months, but it is easier to follow. You don’t have to count calories. You just have to watch the clock.”