**1. Introduction**

The last decade has been characterized by the search for alternative dietary ways to achieve optimal body composition while maintaining or improving physical fitness and sports performance to promote healthy lifestyle and prevent chronic diseases [1,2]. Current trends in sports nutrition are increasingly reaching for the minimization of the carbohydrate component with ketogenic diet becoming a very popular approach, in particular in endurance athletes [3,4].

According to current definitions, carbohydrate intake within the range of 50–150 g per day can be described as non-ketogenic low-carbohydrate regimens [5]. Ketogenic diet is most commonly defined by a daily carbohydrate intake below 50 g per day or energy provision from carbohydrates for up to 10% of total energy intake [6]. Out of the frequently used approaches, targeted ketogenic diet allows carbohydrates to be consumed immediately around exercise to sustain performance without affecting ketosis [7]. The cyclical ketogenic diet (CKD) alternates periods of ketogenic dieting with periods of high-carbohydrate consumption [8]. The period of high-carbohydrate eating is supposed to refill muscle glycogen to sustain exercise performance [9].

The influence of ketogenic diets on sports performance is still the topic of an ongoing debate [10,11] with often conflicting results [12]. The overreaching mainstream nutrition philosophy for endurance athletes emphasizes a carbohydrate-dominant, low fat paradigm. Under these dietary conditions, carbohydrates are utilized as predominant fuel source to cover high volumes of aerobic exercise [13]. The appeal of low carbohydrate high fat diet for endurance athletes is likely due to the shift in fuel utilization, from a carbohydrate-centric model with limited glycogen sources to predominant fat utilization with much bigger and longer-lasting fat stores [14]. This metabolic shift, seen after a period of dietary alteration, is often referred to as being "fat-adapted", which has been well-documented in studies since the 1980s [15]. Substantial reduction in carbohydrate intake promotes utilization of ketones and, according to some studies, it may enhance physical performance due to minimizing the reliance of body metabolism on carbohydrates [16,17] and reduce lactate deposition leading to enhanced recovery [18]. Importantly, ketogenic diets are, in particular in the short-term run, a very efficacious way to reduce body weight not only in physically active subjects but also in patients with obesity, type 2 diabetes and other chronic lifestyle diseases [19–21]. Nevertheless, it has to be noted that long-term compliance and efficacy of ketogenic diet is not optimal and most of the studies had rather limited duration [19,22].

Here we performed a randomized controlled trial to compare the effect of the cyclical ketogenic reduction diet (CKD) vs. nutritionally balanced reduction diet (RD) on body composition, muscle strength, and endurance performance in healthy young males undergoing regular resistance training three times/week combined with aerobic training three times/week. We hypothesized that CKD will be more efficacious in inducing fat loss as compared to RD while maintaining aerobic performance. To this end, we explored the effect of eight weeks of CKD vs. RD combined with regular exercise on body composition, and measures of strength and aerobic performance.
