Athletes often undertake periods of weight loss in an attempt to reduce fat mass (FM) while retaining fat free mass (FFM). Although absolute body weight loss may be the primary outcome of concern for individuals with overweight or obesity, it is important that weight loss strategies adopted by athletes minimise losses of FFM, so as not to compromise training and performance [
1]. Altering body composition in such a manner may be advantageous to the athlete for various biomechanical, aesthetic, and locomotive reasons, thereby increasing the likelihood of competitive success in a target weight-class (e.g., combat sports, weight lifting), weight-sensitive sports (e.g., endurance events, ski jumping), or aesthetically judged sports (e.g., gymnastics and bodybuilding) [
2]. Previous literature indicates that athletes commonly reduce their absolute body weight by 5–10% over a number of months prior to competition [
3,
4], although more dramatic weight losses of ≥7% of body weight within 24 h have also been observed [
5,
6]. To achieve the desired weight loss (and to maintain sports performance), typically combinations of nutritional and exercise interventions are recommended for athletes [
1]. Within this population, the most common nutritional weight loss strategy implemented is continuous energy restriction (CER), for the duration of the weight loss phase [
7,
8]. Specifically, CER involves reducing energy intake every day relative to weight maintenance energy requirements [
9]. Despite being currently accepted as an evidence-based dietary intervention for weight loss, CER is accompanied by a number of behavioral, metabolic, and endocrine responses that collectively threaten dietary adherence, oppose the continuation of fat loss, and predispose the individual to weight and fat regain upon completion of the period of energy restriction (ER) [
10,
11,
12,
13,
14,
15]. Furthermore, it is well documented in athletes that ER, in conjunction with high training loads, can lead to loss of FFM and decreased performance via reductions in muscle strength, reflexes, and glycogen stores and increased irritability [
2,
16,
17,
18]. Elite athletes competing at a reduced body mass in weight-class and aesthetic sports also experience increased risk of injuries and chronic fatigue, and impaired immune function, which can lead to more frequent episodes of illness [
19]. In these sports, failure to reach target body weight or body composition in the days or weeks prior to contest through the use of ineffective or suboptimal dietary strategies may cause athletes to depend on more rapid weight loss techniques that could jeopardise performance and possibly be dangerous. Such techniques include acute “water weight” loss, a practice that often involves severe dehydration via restriction of fluid intake, and actively pursuing sweating through exercise (often in combination with “sweat suits”) or the use of saunas and hot baths [
20]. Dehydration is known to adversely affect athletic performance by reducing body water, electrolytes, and glycogen, which alter a number of physiological processes including metabolism, the regulation of body temperature, and cardiovascular function [
7,
20]. An additional challenge that athletes face when embarking on weight loss is the multitude of diet subtypes, the propagation of unfounded fad diets by the media, as well as conflicting nutritional research, all of which contribute to confusion regarding optimal manipulation of dietary variables for athlete weight reduction [
21]. As such, there is an inherent need to review current and novel dietary strategies as a means of providing athletes with sound, evidence-based guidelines that facilitate the realisation of their body composition goals, without jeopardising health or performance.
Intermittent energy restriction (IER) is one nutritional strategy that has gained recent research attention [
22,
23,
24,
25,
26,
27] and which could potentially be of relevance to athletes wishing to reduce weight. IER involves alternating periods of ER with periods of greater energy intake (often referred to as ‘refeed’ periods or ‘diet breaks’), within the weight loss phase. Of note, these ER and refeed periods have also been referred to as ‘fast’ and ‘feed’ phases in some previous publications [
9]. The proposed goal of implementing refeeds during periods of ER is to briefly stimulate the release of some regulatory hormones that play a positive role on fat loss and satiety and increase metabolic rate [
2]. While a conceivable metabolic and hormonal model to attenuate ER-induced adaptations through the use of an IER regime exists, recent literature has not been definitive. The concept of integrating periods of greater energy intake within a weight loss phase received research attention following work by Wing and Jeffery [
28]. Investigators examined the effects of disturbing the momentum of weight loss in an attempt to induce dietary relapses during a 14-week weight loss program. Surprisingly, prescribed diet breaks did not lead to a backsliding of progress, with participants who adopted either a six-week diet break at Week 7 of the program or two-week diet breaks after Weeks 3, 6, and 9 of the program not demonstrating any less weight losses (at 0–5 months or 0–11 months) when compared to the control group who dieted continuously for the 14-week program. These findings caused some researchers to speculate that diet breaks or refeeds could encourage greater adherence to longer-term diets in individuals needing to lose significant amounts of body fat.
Previous research in the realm of IER has focused on populations with overweight or obesity [
29]. However, the metabolic and training status of athletes is considerably different from that of people who are overweight or who are sedentary. Athletes are typically of a healthy body composition and undertake high levels of physical activity and consequently have high energy expenditures, as well as a low probability of experiencing metabolic diseases or preliminary states of diseases, which are often observed in populations that are overweight and inactive [
30]. Therefore, it is possible that these factors could influence the response to IER. Additionally, it is conceivable that IER strategies could be more likely to benefit athletes (or lean people in general) as evidenced by leaner individuals demonstrating two- to three-fold greater protein losses [
31], greater reductions in testosterone levels [
32], and a higher proportion of weight loss from FFM during ER when compared to individuals with a BMI in the overweight or obese range [
33]. To our knowledge, no published research is currently available on IER in athletes. Thus, the purpose of this review is to discuss the existing body of literature on IER, outline its potential as an alternative weight loss strategy for athletes, and set a platform for future investigation in athletes. This review will also utilise the available evidence to develop theoretical recommendations for athletes considering IER.