When Ramadan IF studies are excluded, there is very little research examining TRF programs. Stote et al. (50) conducted a study of TRF, which used daily 20-hour fasts in male and female participants (age: 45.0 ± 0.7; mean ± SEM). The study used a randomized cross-over design with two 8-week periods of eating either 1 meal per day or 3 meals per day. These 2 phases were separated by an 11-week washout period, and all food was provided to the subjects throughout the study. During the 1 meal per day phase, participants consumed all their calories within a 4-hour window of time in the evening. After eating 1 meal per day, as compared with 3 meals per day, lower-body weight (65.9 ± 3.2 kg versus 67.3 ± 3.2 kg) and fat mass (14.2 ± 1.0 kg versus 16.3 ± 1.0 kg) were reported. Although both treatments were designed to be isocaloric, the subjects ate ∼65 fewer calories per day during the 1 meal per day phase of the study because of “extreme fullness” and difficulty eating all the food in the allotted time window (50). It is possible that individuals would have eaten even less if they had been free to choose when to stop eating, and a lower level of energy intake could have led to even greater weight loss. The ability to adhere to this type of eating pattern is questionable, as indicated by higher ratings of hunger and desire to eat in the 1 meal per day group. The severity of these phenomena increased throughout the study, indicating that the subjects did not grow adequately accustomed to the eating pattern.
To our knowledge, only one study has examined combining an IF protocol with an exercise program (6). The study examined 4 groups: ADF, ADF plus exercise, exercise alone, and control. Twelve weeks of supervised endurance exercise on stationary bikes and elliptical machines was implemented in the 2 exercising groups. Subjects exercised 3 times per week, beginning with 25 minutes at 60% of their age-predicted maximum heart rate (HRmax) and progressing to 40 minutes at 75% HRmax over the course of the study. It was not reported whether subjects exercised on modified fasting days or on ad libitum feeding days, as well as whether subjects exercised in a fasted or fed state.
The ADF plus exercise group lost more weight and fat mass than any other group. The ADF and exercise alone groups both lost weight and fat mass but did not differ in the amount lost. There were no differences between groups for fat-free mass changes, although the ADF did exhibit a small decrease in fat-free mass. Lean mass was retained in the group that exercised and followed ADF, and the authors reported that the exercise program may have been responsible. A limitation of this study is that BIA was used to measure body composition.
Meal frequency is often a polarizing topic, and many fitness practitioners recommend a relatively high meal frequency. Although the number of studies specifically examining different IF protocols is limited, investigations of meal frequency alterations can provide some additional information about effects of decreasing meal frequency.
It should be noted that the line between decreased meal frequency and IF protocols is somewhat blurred. Intermittent fasting, by definition, is a systematic reduction in meal frequency. However, IF emphasizes extending periods of fasting or modified fasting, which is not necessarily the case when meal frequency is otherwise reduced. For example, a diet that reduces meal frequency may include meals at breakfast and dinner, which leads to a significantly shorter daytime fasting window (∼6–10 hours) than most of the IF protocols use. As discussed, this prolonged fasting window may have beneficial effects on lipolysis and lipid oxidation, which could potentially lead to improved fat loss.
The lack of research specifically examining the effects of implementing IF programs in athletes makes it difficult to provide concrete recommendations for the use of these programs in athletes. However, several points are worth considering. Intermittent fasting can be an effective means of reducing calorie intake, body weight, and body fat in nonathletes. Intermittent fasting programs can be designed to allow adequate nutrient consumption before and after physical activity (i.e., exercise does not have to be performed in a fasted state when an IF program is implemented). Some IF programs are as simple as abstaining from food after dinner and not eating again until breakfast or lunch the next day. These milder TRF programs lead to a period of fasting that is ∼12–16 hours in duration.
Most forms of IF could be modified to fit an athlete's training schedule. In ADF and WDF, the modified fasting days consisting of very low-energy intake could be used less frequently or placed on rest days or days with lighter training activities. A TRF schedule could be developed that allows the athlete to eat at the most critical times (e.g., before and after training sessions and competition). Even using a single day per week of modified fasting could help an athlete achieve a negative energy balance for the week while not disturbing the usual pattern of food intake on heavier training and competition days. Although there is scant evidence to demonstrate the ability to adhere to these types of dietary interventions long term, IF may provide an alternative strategy for athletes who are trying to lose weight or prevent weight gain.
Currently, there is a paucity of literature on the effects of IF protocols on exercise performance. Thus, it cannot be decisively concluded if these types of dietary strategies hinder or enhance exercise performance, if they affect performance at all. However, if this type of dietary strategy is used in a conservative fashion as described here (i.e., fasting one select day of the week or on nontraining days), it could theoretically play a very minor role in exercise performance because of the limited impact on most training days. However, more research and empirical data are needed to make more definitive conclusions in this area.
It should also be noted that there are data showing the importance of regularly consuming dietary protein to maintain lean muscle tissue, which is an item of concern for many athletes(31). Thus, this should be considered when using longer duration fasts. Moore et al. (40) and Areta et al. (2) have reported that ingesting 20 g of whey protein at regular intervals every ∼3 hours may be superior in regard to net protein balance and protein synthesis when compared with consuming the same total amount of protein (∼80 g) in larger, less frequent or in smaller, more frequent doses. The benefit of eating protein in this quantity and frequency may be due to the “leucine threshold” that is needed to optimize protein synthesis above baseline levels. Thus, if an athlete uses an IF protocol, he or she may choose to modify it and consume whey protein or another protein source at metered points throughout the fasting window, particularly if lean mass preservation is a major concern.
Future research specifically examining IF programs in athletes should be conducted, particularly in athletes competing in weight-restricted sports. The temporal relationship between nutrient intake and athletic activities should be considered, and any IF program implemented in athletic populations should take into consideration the specific requirements of the sport as well as individual variation and preferences.
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