The primary purpose of this controlled before-and-after comparison study was to evaluate the effects of a proven non-energy-restricted KD on physical performance, body composition, and blood parameters. Our diet intervention enjoyed very good compliance and was fully ketogenic, as verified by positive testing of urinary ketosis, dietary food records, and reduced RER at resting indicating increased lipid metabolism, respectively. Furthermore, our cohort was heterogeneous regarding age, gender, BMI, physical activity; moreover, their physical activity levels remained unchanged during the study period.

Weight and body composition

Consistent with non-energy-restricted KD studies [12–14, 26], we observed mild weight loss over the entire 6-week KD period although mean energy intake did not change. Nevertheless, we cannot rule out the possibility that the 7-day food records of the last KD week were not representative for the whole KD period, and as most of the subjects reported feeling less hungry, a negative energy balance could have predominated during the KD’s first weeks. A negative energy balance could also be associated with the elevated excretion of energy-rich ketones via urine and breath [27]. Nordmann et al. showed that low-carbohydrate, non-energy-restricted diets are at least as effective as low-fat, energy-restricted diets in inducing weight loss for up to 1 year [28].

The results of both body composition assessments (ADP and BIA) were inconsistent regarding changes in FFM. However ADP, which is based on the same principles as the gold standard method of hydrostatic weighing [29], revealed that weight loss consisted in equal parts of reductions in FM and FFM. The unexpected decrease in REE, which contradicts the results of Alessandro et al. [30], could be partly explained by the slight decrease in FFM, the main determinant of REE [31]. We noted a significant positive correlation between FFM and REE (r = 0.749; P <0.001), but no relation between changes from PRE to POST in both parameters. This could indicate that the FFM loss did not comprise the metabolically active tissue compartment/muscles. This assumption is strengthened by the unaffected body cell mass, which represents the protein-rich and metabolically-active compartment [32], combined with an increased phase angle. The phase angle increase was due to a decrease in resistance R (reflecting fluid losses) and increase in reactance Xc (reflecting better cell membrane function). There is evidence that larger values are related to better outcomes in various diseases [33, 34]. Fluid loss could be related to the increased excretion of ketones and water in the urine during the state of ketosis [27]. The muscle-sparing effect during a metabolic state in which fatty acids are predominantly used as the energy source and the importance of sufficient protein intake during a KD are discussed in greater detail in a recent review by Paoli et al. [35].

Together with our result having documented a rise in hand grip strength as a surrogate marker of total muscle mass and function, we conclude that our intervention affected neither muscle mass nor muscle function negatively. The body composition changes may be regarded as positive.

Physical performance

A study in 9 elite artistic gymnasts found no influence of a 4-week KD on explosive strength performance [12]. However, their study’s main limitation is that the authors defined their diet as being ketogenic despite a mean protein energy content of 41% and without having measured ketone bodies in blood or urine. As a high protein intake diminishes ketone production by favouring gluconeogenesis from abundant amino acids [36], it appears highly questionable that their diet was ketogenic. The few remaining studies that have investigated the impact of a non-energy-restricted KD on physical performance included a total of just 25 subjects with KD periods lasting 28 to 38 days [11, 13, 14].

Our primary outcome measure was VO 2 peak as measured during graded exercise to exhaustion, which reflects an individual’s aerobic physical fitness and is a key determinant of endurance capacity [37]. We found a mild but significant decrease in absolute VO 2 peak by 2.4% but the relative VO 2 peak (normalized to body weight) remained unchanged as the KD caused a decrease in body weight mainly based on FM and fluid loss. Phinney et al. [11] and Klement et al. [14] found that aerobic capacity was not compromised by a KD, while Zajac et al. [13] found a significant relative VO 2 peak improvement. This relative improvement should be interpreted with caution since the subjects’ body weight dropped significantly. Our subjects’ maximum work load (peak power) was comprised and decreased significantly by 4.1%, consistent with results others have reported [13, 14]. Zajac et al. [13] noted a significant (−3.3%; P = 0.037) and Klement et al. [14] a trend (−1.5%; P = 0.08) toward decreased peak power, respectively. This issue is further supported by the increased ratings of perceived exertion of our subjects after cycling test at POST, as such ratings correlate well with endurance performance [38]. In addition, our cohort’s subjectively-rated physical sensations that physical activities were more exhausting during the KD what was confirmed by the loss in peak power at POST. A major explanation therefore could be lower muscle glycogen stores combined with lower glycolytic-enzyme activity, which is compensated by enhanced capacity for fat oxidation and muscle glycogen sparing [39–41]. Investigations were therefore carried out to implement the KD’s beneficial metabolic adaptations to enhance fat oxidation in endurance sports by solving the muscle glycogen issue via short periods of carbohydrate intake, described in detail in a review [42].

In summary, our results reveal a slightly negative impact of a 6-week KD on physical performance. In the light of the importance of regular physical exercise during and after anti-cancer therapy [7], any diet that may compromise an individual’s capacity to adhere to an exercise regime would be of great concern, and raise the question of our findings’ clinical relevance. The KD impaired predominantly the endurance capacity and but not the performance in the submaximum area, as VT1 remained unchanged. In addition, activities of daily living and training in the aerobic zone would not be impaired. Further study is warranted to demonstrate the impact on endurance capacity of a longer KD period and after transition to normal diet after a KD.

Blood parameters

All measured overnight fasting blood parameters at PRE and POST were within the reference ranges, but unexpectedly, approximately 40% of all parameters changed significantly. Our data and similar studies involving a non-energy-restricted KD in normal-weight, normolipidemic healthy adults confirm consistently significant increases in both total and LDL-C levels [13, 14, 43, 44] except for one study in 12 men reported decreased TG levels combined with unchanged TC, LDL-C and HDL-C levels after a 6-week KD [45]: they detected unchanged levels of HDL-C and TG and were in excellent agreement with an earlier study by Phinney et al. [44], but contrary to two other comparable studies reporting increased HDL-C and higher or lower TG levels [13, 14]. However, it is worth noting that all available comparable studies recruited small samples (8 to 12). A higher TG/HDL-C ratio has been identified as an index of the incidence and extent of cardiovascular disease and the incidence of type 2 diabetes mellitus [46] and this index decreased significantly during our study. A meta-analysis comparing the effects of non-energy-restricted low-carbohydrate (<60 g carbohydrates) vs. low-fat diets included five randomized controlled trials with a total of 447 obese subjects [28]. The authors concluded that long-term low-carbohydrate diets are associated with unfavourable changes in total and LDL-C levels, but favourable changes in TG and probably HDL-C levels. However, a 6-month non-energy-restricted KD intervention in 141 children with intractable seizures significantly increased TC, LDL-C, TG and atherogenic apoB-containing lipoproteins combined with a decrease in HDL-C, which corresponds to a potentially atherogenic blood lipid pattern [47].

There is evidence of strong associations between carbohydrate-restricted and fat-enriched diets and a significant increase in large LDL particles combined with a decrease in the number of small LDL particles [48, 49] and it has been hypothesised that large LDL particles have lower atherogenic potential [50]. Our subjects’ KD was rich in saturated fat (28% of energy) and the changes from PRE to POST of saturated fat intake and LDL-C correlated weakly (r = 0.294; P = 0.059) (data not shown). There is also evidence that a rise in saturated fat intake can elevate LDL-C levels [49]. Furthermore, Volek et al. showed that an energy-restricted KD rich in monounsaturated fat and supplemented with (n-3) fatty acids increased HDL-C and lowered TG levels [51]. As we did not assess lipoprotein subclasses, the atherogenic risk of our KD remains unclear and requires further investigation combined with the effect of a KD low in saturated but rich in monounsaturated fats.

Our subjects’ thyroid hormones changed significantly, but remained within the reference ranges. The impact of a KD on these hormones has only been marginally investigated. The sharp decline in the fT3 level and unchanged pituitary TSH in our subjects concur with those reports [14, 44, 52]. As Klement et al. [14] hypothesised, the KD-triggered increase in LDL-C is partly related to the significant drop in fT3 levels, as fT3 stimulates the expression of the LDL receptors on hepatic and peripheral cells, which are mandatory for LDL particle clearance [53]. The increase in fT4 combined with a decline in fT3 observed in our study is endorsed by a recent study in overweight men after a 4-week non-energy-restricted inpatient KD [54]. Probably due to the metabolic shift towards reliance on fatty acids for fuel at rest, and the less need for glucose uptake, we observed a significant decrease in fasting insulin, an observation consistently reported in the literature, namely large falls in the range of 34–48% during KDs [13, 45, 52, 55].

Complaints and study limitation

Consistent with other studies [15, 56], our subjects complained about headache, gastrointestinal symptoms, and general weakness mainly during the 1-week metabolic adaptation phase to a KD. However, as the present study had no control group (a limitation), it remains unclear if the reported complaints were side effects directly related to the KD. Results from a non-controlled study should be interpreted with caution. Because of the high prevalence of thyroid dysfunction in our region 5 participants took thyroxin medication. As they had been taking this medication for over at least 6 months, we did not exclude these patients. Likewise, one participant taking methylphenidate and another inhaling daily glucocorticoids were not excluded due to stable medication for months. As we included healthy individuals, there was a broad range of leisure time activity. As we were not focusing on athletes, this diversity was intentional.