There are many potential health benefits to intermittent fasting and high-intensity interval training (HIIT), particularly for people with overweight or obesity. Now, a new study offers fresh evidence that adopting both of these approaches at once may be better for attacking risk factors like high blood sugar and excess fat around the midsection that can lead to heart attacks and strokes.
“Isolated time-restricted eating and HIIT have received increasing attention for being effective and feasible strategies for at-risk populations,” senior author Trine Moholdt, PhD, head of the exercise, cardiometabolic health, and reproduction research group at the Norwegian University of Science and Technology in Trondheim, said in a statement. “We wanted to compare the effects of the combination of time-restricted eating and HIIT with their isolated effects, and to determine whether time-restricted eating and HIIT would act synergistically in improving health in individuals with risk for cardiometabolic disease.”
In the United States alone, an estimated 47 million adults are living with a cluster of interrelated conditions known as cardiometabolic disorders that increase the risk of type 2 diabetes, heart attacks, and strokes, according to the American College of Cardiology (ACC). These conditions, which often appear in combination and are more common in individuals with overweight and obesity, include: high blood pressure, elevated blood sugar, high cholesterol, and an excess amount of fat accumulation around the midsection that’s known as abdominal obesity.
For the study, researchers randomly assigned 131 women with overweight or obesity to one of four groups: HIIT alone, time-restricted eating alone, a combination of both interventions, or a control group of no intervention at all. All of the participants had one or more cardiometabolic conditions that are risk factors for type 2 diabetes and cardiovascular disease.
In the groups that did HIIT, participants exercised at 90 percent of their maximum heart rate for 35 minutes three times a week. These workouts typically combine both aerobics and strength training and break up intervals of vigorous exercise with brief periods of lower-intensity movement.
Participants assigned to time-restricted eating, a form of intermittent fasting, consumed all of their daily calories within a 10-hour window. They were not given any restrictions on what foods to eat or how much to eat within this window.
Women stuck with their assigned eating and exercise routines for seven weeks. To assess the impact of HIIT and intermittent fasting alone or in combination, researchers measured several risk factors at the start of the study and again seven weeks later including blood pressure, body mass index (BMI), fat and cholesterol levels in the blood, and several measures of blood glucose and insulin levels.
Based on several of these outcomes, the combination of HIIT and time-restricted eating was more effective than either intervention on its own. Participants who did both HIIT and time-restricted eating had bigger improvements in blood sugar, fat mass, abdominal obesity and what’s known as cardiorespiratory fitness — the ability to use oxygen efficiently during exercise.
“This finding highlights the importance of changing both dietary and physical activity habits for individuals who wish to rapidly improve their health and lower their disease risk,” Dr. Moholdt said.
However, there were no meaningful differences between groups in appetite hormones, levels of fats in the blood, or vital signs, researchers reported in Cell Metabolism.
Beyond the short duration of the trial, another limitation of the study is that it was conducted during the COVID-19 pandemic, when stay-at-home orders may have dramatically influenced participants’ lifestyles in ways that impacted the results. In addition, 22 women dropped out of the study, including 10 due to COVID-19.
Roughly two-thirds of the women who were assigned to do HIIT or time-restricted eating alone or in combination stuck with their programs throughout the entire study. This level of adherence is unusually high for a new diet or exercise program, and suggests that the approach tested in the clinical trial might work well in real life.
“We recommend this kind of program for people who wish to have a relatively simple way of changing diet and exercise habits and improving their health,” Moholdt said. “Time-restricted eating is a less tedious and time-efficient method to lose weight compared with daily calorie counting, and HIIT is tolerable and safe for previously sedentary individuals and can be completed within 30 to 40 minutes.”
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