Reducing energy intake while maintaining nutrition, so-called caloric restriction (CR), is one of the most robust interventions for increasing lifespan in a variety of species of insects and rodents, as well as in Rhesus monkeys and for inhibiting and delaying the onset of most age-related diseases [1]. The beneficial effects of CR on the cardiovascular and cerebrovascular systems; on insulin sensitivity; on resistance to various types of stress including heat, oxidative, and metabolic stresses; on enhanced immune function; and on the regulation of body weight has been evidenced to contribute to increasing health span [2]. Energy restriction is accompanied by changes in circulating hormones, mitochondrial efficiency, and energy expenditure that serve to minimize the energy deficit, attenuate weight loss, and promote weight regain [3]. CR induces mitochondrial biogenesis and bioenergetic efficiency [4], reduces mitochondrial oxygen consumption and membrane potential, and generates less reactive oxygen species, while mitochondria are still able to maintain their critical ATP production [4]. CR is associated with improving physical fitness in rodents with respect to ad libitum-fed mice [5].

Optimal body composition provides a competitive advantage in a variety of sports. Athletes aiming to improve their strength-to-mass ratio and locomotor efficiency commonly have to reduce their weight [3]. The inclusion of an exercise component in weight loss programs for overweight and obese subjects is now standard [6]. A CR intervention could be useful for athletes looking to control their body weight and also to enhance their physical performance. It has been pointed out that in addition to weight reduction, CR can also improve a cyclist’s power-to-weight ratio without compromising endurance cycling performance [7]. Evidence of the ability of CR to enhance physical performance in athletes is scarce [7], although there are many studies on the use of CR diets for weight reduction in obese men and women [8].

CR in humans has been carried out in a variety of ways. It has been done through a reduced consumption ration and a nutrition plan adjusted to balanced-diet guidelines. Intermittent fasting (IF), that is, diets with reduced meal frequencies such as every-other-day fasting or every-other-day feeding, can have similar effects on life span and health as they provide a reduction in energy intake while maintaining nutrition [2]; it could be easier for athletes who train daily to adhere to this kind of diet rather than CR diets based solely on low daily caloric intakes. Restricting dietary energy intake could influence the total intake of essential nutrients such as vitamins, minerals, and essential amino acids, but principally essential fatty acids, rather than negatively influencing the physical performance and health of athletes.

The aim of this study was to evaluate the effects of every-other-day feeding CR interventions on the body composition and physical performance parameters (during maximal exercise tests) of well-trained athletes. We also estimate the possible impact of this CR intervention on the dietary deficiencies of micronutrients and essential fatty acids. We have previously provided evidence that a well-balanced diet supplemented with almond and olive oil-based docosahexaenoic acid and vitamin E-enriched beverages for one month does not alter athletes’ performance parameters during maximal exercise tests [9].

The main feature of the present study is that after six weeks of participating in a CR program based on intermittent partial fasting, athletes’ physical performance was enhanced. Previous results obtained in a similar study provide evidence of the null effects of one month of training on performance parameters determined during a maximal exercise test on athletes that consumed well-balanced diets or even supplemented their diets with a functional beverage [9]. In addition, the CR via every-other-day fasting allowed for body weight to be controlled, as mainly body fat was lost, although some lean mass loss was also observed. The CR decreased lipid, carbohydrate, and protein intake by 35%, 33%, and 25%, respectively. The six-week CR resulted in slight lean body mass loss, similar to what was found in other CR trials with a 40% calorie reduction and protein intake around 1 g/kg of body weight for two weeks [24]. The loss of lean body mass during weight reduction is considered a negative effect that could compromise performance [25]. Lean body mass loss during the CR via intermittent partial fasting could probably be avoided by increasing protein intake to around 2.3 g/kg of body weight [24] or by supplementing the diet with branched-chain amino acids that maintain lean mass while promoting the loss of fat mass [26]. Regardless of how they do it, athletes must aim to preserve lean body mass during weight reduction [24, 25]. To increase dietary protein intake while in a negative energy balance would come at the “expense” of another macronutrient, such as fat.

The CR led to a micronutrient and vitamin intake below RDAs for athletes, which could have compromised their exercise performance. In this sense, CR intervention programs might consider supplementing diets with vitamins and micronutrients, such as iron, magnesium, potassium, zinc, folate, riboflavin, pyridoxine, and vitamins A and C. However, blood markers of nutritional status in athletes, such as those related to iron metabolism, calcium and vitamin D, glucose, markers of nitrogen handling, or those related to tissue damage, maintained the same levels before and after the intervention. This result has been observed in other trials with hypocaloric diets [24].

The low fat intake associated with the CR could alter the availability of omega-3 and omega-6 polyunsaturated fatty acids and lead to not meeting daily requirements. The fatty acid composition of erythrocyte is a good marker for assessing the efficacy of nutritional intervention trials in incorporating dietary fatty acids [21]. The presence of different fatty acids in the diet and lifestyle factors, such as exercise and obesity, influence the incorporation of the acids into different tissues and erythrocyte membranes [21]; the erythrocyte content of the omega-3 and omega-6 essential fatty acids were maintained or even increased after the six weeks of CR. The possible lack of fatty acid availability during six-week CR did not affect fatty acid content in erythrocyte membranes. The 35% reduction in lipid intake seen under the CR program caused lower triglyceride and cholesterol plasma levels than were observed with the unrestricted diet, but erythrocyte maintained its omega-3 and omega-6 polyunsaturated fatty acids content. Additionally, the CR lessened oxidative damage in plasma lipids. It has been pointed out that a CR decreases mitochondrial electron flow as well as proton leaks in mammalian cells, and attenuates muscle damage caused by intracellular reactive oxygen species [4, 27]. We actually provide evidence that a CR reduces oxidative damage in circulating lipids and blood vessels along with reducing circulating triglyceride and cholesterol levels.

The CR intervention significantly reduced body, trunk, arm, and leg weights; it mainly reduced body fat mass, but a small yet significant reduction in lean body mass was also observed. Reducing body weight is a goal for many athletes [25]. Either rapid or gradual body weight reduction techniques have been used to control body weight in athletes with varied results on physical performance. Aerobic endurance capacity decreases after rapid body weight reduction. A severe CR of 67% to 90% of energy demands and dehydration during 48 h reduces exercise capacity in the heat compared to an adequate energy control trial [28]. It also causes detrimental effects in a 30-min treadmill time-trial session in temperate conditions [29]; affects health, muscle performance, and energy; and alters perceived exertion and dynamic postural control [30]. Severe CR also results in a large reduction in body mass that appears to be mostly explained by a rapid reduction in body water stores [31]. Gradual body weight reduction via a smaller CR induces more consistent body weight losses over a CR period of more than one week. In our study, a CR of 33% via intermittent partial fasting over six weeks led to about 0.9 kg/week of weight loss. This finding is in line with others who have observed a body weight loss of 1 kg/week with an energy intake reduction of about 25 Kcal/kg/day [25], and the losses are likely reflected in a loss of body fat and muscle [31]. We also observed a decrease in TBW content, stemming mainly from the intracellular water compartment, which represented 69% of total body weight loss. The CR also altered the water distribution between the body’s intracellular and extracellular compartments: the intercellular compartments lost water while extracellular water content increased after the intervention.

Sedentary, overweight men and women, at 55 years of age, with confirmed metabolic syndrome saw body weight losses after a 12-week 30% CR. A co-intervention with moderately intense aerobic exercise enhanced the weight losses induced by the 30% CR; the exercise led to a greater reduction in central adiposity and trunk fat mass, and it improved maximal oxygen consumption [32]. Short-term hypoenergetic weight loss programs could maintain lean body mass in young healthy athletes who have a protein intake around 2.3 g/day/kg of body weight [24].

It has been demonstrated that VO2max is increased after a body weight reduction [25]; in this sense, we have detected an improvement in physical performance markers, such as heart rate, lactate levels, fatigue perception (Borg index), and the energy expenditure required to run a meter, that are not dependent exclusively on changes in total body mass [33]. Previous studies with well-balanced diets or even after one month of consuming functional beverage supplements provided no evidence suggesting that these nutritional habits have an effect on physical performance parameters such as lactate levels [9]. Additionally, we observed a decrease in lean body mass as a consequence of the CR; however, this decrease did not involve any impairment of physical performance parameters. This fact could indicate that athletes should have an ideal lean body mass in order to produce maximal physical performance. To sum up, higher muscle mass does not necessarily indicate better physical performance. Furthermore, we detected several changes in energy expenditure and in energy efficiency as a consequence of the CR; this fact could be explained by considering that CR might cause increased mitochondria efficiency [4].

This study was limited by the reduced sample size of the population studied and the participants’ characteristics as Olympic athletes, as well as the duration of the study (weeks).

So, we observed that a 33% CR via every-other-day fasting significantly reduced daily micronutrient intake to 90% of RDA values; it also reduced body weight by 4.4%, fat body mass content by 15.1%, and lean body mass by 2.91%. Moreover, the CR reduced plasma triglycerides by 14.1% and cholesterol by 4.3% with respect to control values, and it also reduced MDA plasma levels by 15.7%. Physical performance parameters, such as heart rate, lactate levels, fatigue perception (Borg index), were significantly improved as a consequence of the CR, which ameliorated the onset of the anaerobic phase of exercise. Moreover, the CR decreased the energy expenditure required to run one meter and improved energy efficiency. However, when implementing a CR, a micronutrient supplement should also be considered.