Intermittent Fasting — What Is It and Does It Work? : ACSM's Health & Fitness Journal (2024)

WHAT IS INTERMITTENT FASTING?

Intermittent fasting is defined as eating during a certain time period, or, fasting for an entire day. For example, intermittent fasting can be defined as eating foods within an 8-hour period, but fasting for the remaining 16-hour period. This is often referred to as “time-restricted feeding” and/or “16/8 intermittent fasting,” referring to hours one fasts (16 hours) and the hours during which one consumes foods (8 hours). Alternate day fasting also is considered intermittent fasting and is defined as not consuming food for an entire day. With alternate day fasting, for example, a person may eat for 3 days, then fast for 1 day, then consume foods for another 2 days, then fast for another day. The “5:2” diet is one where an individual would eat normally for 5 days of the week, but restrict energy intake for the remaining 2 days of the week. The proposed advantage of intermittent fasting is that individuals do not have to count calories; rather, they are limited to the time in which they can consume foods.

The proposed advantage of intermittent fasting is that individuals do not have to count calories; rather, they are limited to the time in which they can consume foods.

It is recommended that individuals consume healthy food choices during their periods of food consumption. In addition, fasting may result in dehydration, and thus, it is usually recommended that individuals consume water, unsweetened coffee, and/or tea, during the fasting periods (¹).

DOES INTERMITTENT FASTING RESULT IN WEIGHT LOSS AND/OR HEALTH BENEFITS?

Although much of the research reported on intermittent fasting has been conducted in animals, more and more human studies are being conducted and published, with some positive results. Gabel et al. (²) examined time restructured feeding that did not include counting kilocalories (kcal), hypothesizing that it would result in weight loss in individuals who were obese. They asked their participants to use the 16/8 intermittent fasting pattern for the study. There were 23 individuals who participated in the intermittent fasting group and 23 individuals in the control group. Over the course of the 12-week study period, those in the 16/8 intermittent fasting group were asked to consume foods, ad libitum, between 10 a.m. and 6 p.m. They were allowed to consume water during the 16-hour fasting period.

The researchers reported a 2.6% weight loss and a reduction of about 340 kcal per day in the time-restricted fasting group compared with the control group after the 12-week intervention. These results were significant (P < 0.05). They also reported a significant reduction in systolic blood pressure in the intermittent fasting group compared with the control group (P < 0.05). The researchers did not report significant differences between groups in body composition (fat mass, lean body mass, deep [visceral] body fat), diastolic blood pressure, and insulin sensitivity markers. They also did not report significant differences between groups in high-density lipoprotein (HDL) cholesterol, low-density lipoprotein cholesterol, fasting glucose, fasting insulin, or hom*ocysteine concentrations. Gabel et al. (²) concluded that an 8-hour time-restricted fast could be helpful for weight loss and in decreasing systolic blood pressure.

Although Gabel et al. (²) reported positive weight loss results, their study was conducted over a 12-week period. Would longer term intermittent fasting be possible and result in similar findings? Sundfør et al. (³) evaluated whether intermittent fasting (using the 5:2 method) or continuous energy restriction resulted in weight loss or maintenance and improved cardiometabolic risk factors in adults with abdominal obesity and one or more of the other risk factors for metabolic syndrome. They had 112 individuals, 21 to 70 years of age, participate in their study (50% women and 50% men). Individuals were randomized to either an intermittent fasting group or a continuous energy restriction group. The intervention was 6 months in duration and included 10 nutrition counseling sessions with dietitians. This was followed by a 6-month maintenance period, which did not include any nutrition counseling sessions. Those who were in the 5:2 intermittent fasting group were asked to consume between 400 (women) to 600 (men) kcal on 2 nonconsecutive days of the week.

The researchers reported that both groups decreased kilocalorie intake by approximately 26% to 28%. There were no significant differences in weight loss between the two groups (8.0 vs. 9.0 kg in the intermittent fasting vs. continuous energy restriction group, respectively). Improvements in blood pressure, waist circumference, and triglyceride and HDL-cholesterol concentrations were reported in both groups, with no differences between groups. The regain in body weight was small in both groups, with no significant differences between groups. However, those who were in the intermittent fasting group reported being hungry more of the time than those in the continuous energy restriction group (P = 0.002). This longer term study demonstrated no differences in weight loss or other markers after 1 year, between intermittent fasting and continuous fasting. The greater feelings of hunger in the intermittent fasting group may deter individuals from participating in this type of eating over a longer period.

Although intermittent fasting results in weight loss, and some improvements in cardiometabolic health, will it still improve cardiometabolic health without weight loss? Sutton et al. (⁴) conducted a controlled feeding trial to examine if intermittent fasting would have cardiometabolic benefits without weight loss in men who had prediabetes. They used “early time-restricted feeding,” which has a 6-hour feeding period and dinner must be consumed before 3 p.m., which the researchers stated would be “in alignment with circadian rhythms in metabolism” (⁴). They compared this to a control schedule, which had a 12-hour eating period. This was a cross-over study, whereby all participants were on each eating schedule for a 5-week period. Sutton et al. (⁴) reported that the early time-restricted feeding improved insulin sensitivity, blood pressure, oxidative stress, beta cell responsiveness, and appetite. These results occurred without weight loss among the participants. Although the results were positive, this was a short-term study, and the early time-restricted feeding may prove to be difficult to maintain over time, in particular, with respect to consuming an early dinner.

DOES INTERMITTENT FASTING IMPAIR ATHLETIC PERFORMANCE?

Intermittent fasting may be something athletes choose to do, or it may be a result of a religious practice, such as Ramadan. During the month of Ramadan, Muslims fast daily from sunrise to sunset. Does the practice of fasting during Ramadan affect athletic performance? Zarrouk et al. (⁵) assessed this on muscle fatigue and neuromuscular performance in eight karate athletes, approximately 17 years of age. They evaluated the athletes 1 week before Ramadan (control session) and then again during the first and fourth weeks of Ramadan. At each session, they evaluated dietary intake, anthropometry, maximal voluntary isometric contractions, and submaximal contractions (75% of maximal isometric contractions). They also used electromyography to analyze neuromuscular changes.

Zarrouk et al. (⁵) reported that total kilocalorie intake and water intake were significantly greater during Ramadan than at baseline. Despite the increased energy intake, there were no changes in body mass during Ramadan. Neuromuscular activity also was maintained among the athletes during Ramadan. They concluded that Ramadan did not affect athletic performance in the eight karate athletes.

In a meta-analysis, Aird et al. (⁶) assessed the effects of fasting on exercise performance, specifically, the effects of fasting compared with pre-exercise feeding on aerobic and anaerobic exercise performance. A search was performed using the MEDLINE and PubMed search engines. Of the studies analyzed, the researchers reported that pre-exercise food consumption positively affected prolonged aerobic exercise (P = 0.012). However, pre-exercise feeding did not positively affect shorter duration aerobic exercise performance. They concluded that more research is needed to evaluate how pre-exercise feeding (or fasting) may influence exercise performance and physiological adaptations.2

SUMMARY

Intermittent fasting has become increasingly popular within the past 5 years. Based on the research, it seems to be beneficial, not only to weight loss, but with respect to cardiometabolic risk. Although it may lead to positive results, the effects on exercise training and performance have not been well-studied. More research with respect to the different types of fasting on exercise performance is warranted.

Intermittent fasting has become increasingly popular within the past 5 years. Based on the research, it seems to be beneficial, not only to weight loss, but with respect to cardiometabolic risk. Although it may lead to positive results, the effects on exercise training and performance have not been well-studied.

References

Recommended Resources

• Monique Tello, Intemittent fasting: surprising update. Harvard Health Publishing. https://www.health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156
• Roger Collier, Intermittent fasting: the science of going without. CMAJ. 2013;185(9):E363–4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3680567/