Excess body weight is recognized as a fundamental cause of numerous health issues. In addition to assessing weight, the concept of “overfat” or “normal-weight obesity” has emerged, referring to individuals with excessive body fat despite falling within the normal weight range. While there is a consensus that improving dietary habits and physical activity patterns is crucial for reducing body fat, debates continue regarding the optimal quantity and type of exercise and ideal dietary strategies. Among the various dietary methods proposed for fat reduction, the ketogenic diet (KD) has recently gained popularity.
The ketogenic diet (KD) is a dietary regime that focuses on reducing carbohydrates and replacing them with healthy fats. It has proven to improve health and has resurfaced as a trendy weight loss method. The KD, which restricts carbohydrate intake and emphasizes the consumption of fat and proteins, can potentially affect body fat, muscle mass, and exercise performance by altering the body’s energy supply and metabolic processes. While controversial, it is undeniable that the KD has the potential to affect body fat, muscle mass, and exercise performance. Although scientific evidence has not yet been fully established and academic consensus has not been reached, this review provides an overview of the research on the effects of the KD on these parameters.
Understanding the Ketogenic Diet
In a conventional diet, our bodies mainly utilize carbohydrates and fats as primary energy sources. However, when following the KD, a significant metabolic shift occurs. The body transitions from relying on carbohydrates to using fats as the primary energy source, achieved by depleting carbohydrate stores. The central nervous system, specifically the brain, typically relies on glucose as its primary energy source owing to the exclusive utilization of glucose by the blood-brain barrier. This protective barrier prevents the entry of various substances into the brain, including large fatty acids. Debates regarding the potential risks associated with the KD center on the brain. After a few days on the KD, one of the brain’s primary energy sources, glycogen, becomes depleted. This depletion can lead to stress and other issues. However, proponents of the KD argue that despite the need for an adaptation period, this dietary approach induces a state of ketosis. During ketosis, as fats are broken down, three types of ketone bodies are produced: acetoacetate, ß-Hydroxybutyrate, and acetone. These ketone bodies increase within the body, allowing the brain, heart, muscles, and other tissues to use them as an energy source.
Types of Ketogenic Diets
The KD is characterized as a high-fat, very-low-carbohydrate diet. Several variant KD that show similar efficacy to that of the original form has been developed to date, and offer flexibility to increase compliance with the regimens. There are four major types of the KD with proven efficacy: the classic long-chain triglyceride (LCT) KD, medium-chain triglyceride (MCT) KD, modified Atkins diet (MAD), and low glycemic index treatment.
- Classic LCT KD: The most traditional type, widely used in clinical settings, incorporates a 4:1 ratio of fat (in grams) to protein plus carbohydrate (in grams). Fat provides 90% calories, and its predominant source is food-derived LCT, and a 3:1 or lower ratio may be used.
- Medium-Chain Triglyceride (MCT) KD: The dietary use of MCT oil is more acceptable and is more ketogenic than LCTs. The MCT KD has better flexibility in diet ratios than the LCT KD, and the calorie intake is calculated based on the percentage of energy derived from MCT.
- Modified Atkins Diet (MAD): Based on the Atkins diet, shares similar food choices with the classic KD, but without the need for precise weighing of ingredients. The MAD does not have a strict ketogenic ratio, which typically ranges from 1:1 to 1.5:1 and, sometimes, can reach 4:1.
- Low Glycemic Index Treatment: Based on the concept that the protective effect of the KD relies on stable glucose levels, but has a liberalized regimen with low-carbohydrate composition to minimize glycemic increases (glycemic indices <50), and is an effective antiepileptic intervention in children with intractable epilepsy.
Impact on Body Fat
Several studies have demonstrated the positive effects of the KD on body composition across various age groups and clinical conditions. For instance, a study comparing the KD with a hypocaloric diet in children and adolescents with obesity found that the KD was more effective in terms of weight loss and improvement in metabolic parameters. In another study conducted by Goss et al., older adults with obesity maintained their calorie intake at normal levels over 8 weeks, and a comparison was made between the KD and a low-fat diet. The results indicated that the KD group exhibited a greater reduction in total fat mass than the low-fat diet group. Notably, the KD group experienced a threefold greater decrease in visceral adipose tissue. Hussain et al., compared the effects of a 24-week low-calorie diet and the KD in individuals with type 2 diabetes. The KD was more effective than the low-calorie diet in significantly reducing body weight, body mass index (BMI), and waist circumference (WC). Additionally, the KD group demonstrated favorable effects on blood glucose levels and alterations in hemoglobin and glycosylated hemoglobin, total cholesterol, low-density lipoprotein cholesterol (LDL), high-density lipoprotein cholesterol (HDL), and triglyceride (TG) levels.
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Available evidence suggests that the KD potentially exerts a favorable influence on body fat reduction. Nevertheless, it remains imperative to meticulously consider individual variability in the response and long-term effects on body fat. From a long-term perspective, the success of a nutritional approach is determined by the extent of weight regain.
Ketogenic Diet vs. High-Intensity Interval Training (HIIT)
Diet and exercise play vital roles in reducing overweight status. High-intensity interval training (HIIT) has gained popularity as an efficient exercise strategy for improving cardiovascular endurance, enhancing heart metabolism, and positively affecting body composition changes. Cipryan et al. investigated the independent and interactive effects of the KD and HIIT on visceral fat reduction. The KD and KD+HIIT groups showed significant decreases in visceral fat, whereas the HIIT group showed minimal effects. However, it is important to consider that this comparison focuses solely on changes in visceral fat and excludes other potential benefits of HIIT, such as cardiorespiratory fitness and cardiovascular health improvements. Nonetheless, the KD appears to be more effective in reducing visceral and overall body fat than exercise alone without dietary intervention.
Mechanisms of Action on Body Fat
Evidence regarding the effects of the KD on weight management is compelling; however, the underlying mechanisms of its action remain unclear. Ludwig et al. analyzed the impact of the KD on total energy expenditure (TEE). They investigated the differences in effects based on the duration of application. A meta-analysis of 29 studies, found that the KD initially leads to a temporary decrease in TEE; however, after approximately 2.5 weeks, a more significant increase in TEE was observed. During this adaptation period, individuals may experience symptoms of “Keto Flu,” including headaches, brain fog, fatigue, irritability, nausea, sleep disturbances, and constipation.
Impact on Muscle Mass
Another important aspect to consider is the potential effect of the KD on muscle mass. Several studies have suggested that carbohydrate restriction in the KD may contribute to reduced muscle mass. Interestingly, the combination of the KD and exercise did not prevent a reduction in muscle mass. Some studies have indicated that during the period of KD application, exercise leads to increased utilization of muscle amino acids for gluconeogenesis due to glycogen depletion in the liver and muscles. This utilization could also be attributed to sodium and water excretion and body water loss due to glycogen depletion. However, Cipryan et al. pointed out that during the initial 4 weeks of the 12-week KD experimental period, there was a noticeable decline in participants’ muscle mass, which subsequently stabilized.
Conversely, numerous studies positively evaluate the KD’s impact on muscle mass. Wilson et al. compared the KD and a conventional diet over 11 weeks and found that both groups showed similar levels of muscle hypertrophy and increased strength. One noteworthy aspect of this study was that the participants were experienced in resistance training, and the KD group showed a remarkable increase in testosterone, a key factor in muscle synthesis, compared with the conventional diet group. The KD group experienced a reduction in muscle mass during the initial 4 weeks, but it stabilized thereafter.
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Optimizing Muscle Hypertrophy on a Ketogenic Diet
The impact of the KD on muscle mass is still under discussion; however, it is believed to be influenced by key factors such as total energy and protein intake. To maximize muscle hypertrophy through the KD, daily total energy intake needs to be optimized. For maximal muscle hypertrophy, a caloric surplus is recommended, meaning that the daily total energy intake should exceed the total daily energy expenditure. Consuming 15% more calories than the total daily energy expenditure is advised. During periods of muscle hypertrophy, minimizing unnecessary fat gain by not exceeding a weekly weight gain of 0.25-0.5% of one’s body weight is essential. Furthermore, excessive protein intake can induce gluconeogenesis and disrupt the state of ketosis, so protein consumption should be optimized.
Impact on Exercise Performance
Research related to the KD has primarily been conducted in the context of weight or fat reduction. Investigations into the relationship between the KD and exercise performance are lacking. The KD may positively impact exercise performance by providing benefits related to metabolic regulation, ketone body production, mitochondrial function enhancement, inflammation reduction, and weight loss. The benefits of the KD in relation to exercise performance have been explored primarily in the context of endurance performance.
A study by Burke et al. involved elite world-class race walkers who alternated between a typical high-carbohydrate diet and the KD for 3 weeks each while undergoing intensified training. This study observed significant improvements in peak aerobic capacity and whole-body fat oxidation at various speeds and intensities when following the KD However, this study highlights that the KD impacted exercise economy during the real-life race performance of elite endurance athletes. In other words, after adopting the KD, race walkers required increased oxygen consumption to maintain the same exercise workload or speed, indicating reduced efficiency. This reduction in the exercise economy ultimately negated the benefits of intensified training.
Pathak and Baar explained the decrease in high-intensity exercise performance due to the KD. The key muscle adaptation resulting from the KD and the activation of peroxisome proliferator-activated receptor (PPAR) enhances muscle fat oxidation but can also contribute to a reduction in high-intensity exercise performance. When PPAR is activated, it promotes the expression of genes related to fat oxidation and energy production. While this adaptation can benefit endurance activities requiring sustained energy over longer durations, it is less efficient at intensities greater than 65% of VO2max. Endurance athletes on a KD ultimately require more oxygen to maintain the same exercise workload or speed as athletes on a conventional diet during high-intensity performance.
Furthermore, there have been concerns about the potential health risks associated with athletes adopting the KD. One such study by Volek et al. investigated the metabolic characteristics of 10 long-distance runners who followed a KD for over 6 months. The results indicated that runners experienced decreased body weight and body fat compared with their pre-KD state. However, the blood concentration of ketone bodies, a metabolic byproduct, significantly increased, leading to side effects such as fatigue, insomnia, and digestive issues in athletes.
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In contrast, some studies have reported improvements in high-intensity endurance performance. McSwiney et al. investigated the KD in endurance athletes over 12 weeks and conducted various performance tests, including a 100 km timed trial, a 6-second sprint, and a critical power test. The results showed that compared with athletes following a conventional diet, there was no significant difference in the 100 km timed trial performance after adapting to the KD. However, the KD-adapted athletes consistently demonstrated increased fat oxidation rates during exercise.
Anaerobic Exercise
Although there is insufficient information available on the effects of the KD on anaerobic exercise, studies have suggested that the KD may have a negative impact on anaerobic exercise. Nevertheless, a study by Paoli et al. targeted elite gymnasts and implemented the KD for 1 month without altering their training routines. The results revealed a reduction in body fat and maintenance of strength. Furthermore, Kephart et al. analyzed the impact of the KD over 3 months on exercise performance and body composition in CrossFit trainees. No significant differences in strength or muscle mass between the KD and control groups were observed. Similarly, Sawyer et al., focused on men and women who strength-trained and investigated the effects of a 7-day short-term KD on strength and power. The findings indicated that despite reductions in body weight, both sexes sustained strength and power.
Weight-Category Sports and Aesthetic Performance
Despite the ongoing debate surrounding the impact of the KD on exercise performance, it appears that the KD can be particularly beneficial for athletes involved in weight-category sports and those in whom aesthetic appearance plays a crucial role in their performance. Paoli et al. focused on natural bodybuilders over 8 weeks with equal total energy and protein intake conditions: one group followed the KD, whereas the other adhered to a conventional diet. The results indicated that the KD group significantly reduced body fat more than the conventional diet group. The conventional diet group showed a slight increase in muscle mass, whereas the KD group maintained their existing muscle mass without experiencing muscle loss. Moreover, insulin sensitivity, a critical metabolic health marker, significantly improved only in the KD group. On the other hand, inflammatory cytokines (IL-1, IL-6, TNF-a) increased in the conventional diet group but decreased in the KD group compared with baseline. Therefore, implementing the KD during a phase focused on maximizing fat reduction can serve as a strategy not only for efficient fat loss without muscle loss but also for preventing potential metabolic and immune health decline during the body fat reduction phase.
The Role of Ketone Bodies
In the liver, excessive production of acetyl coenzyme A (acetyl-CoA) and oxidation of fatty acids leads to the production of Ketone Bodies (KBs). The acetyl-CoA molecule can be utilized in the Krebs cycle or to produce acetoacetate, which is then spontaneously converted to acetone or 3-β-hydroxybutyrate by 3-β-hydroxybutyrate dehydrogenase. The KBs then enter the bloodstream and can be utilized by the brain, heart, and muscle, where they produce cellular energy in mitochondria. Higher circulating KB levels lead to ketonemia and ketonuria. The KBs constitute a more efficient energy source than glucose, metabolize faster than glucose, and can bypass the glycolytic pathway by directly entering the Krebs cycle, whereas glucose needs to undergo glycolysis. Moreover, KBs cause fatty acid-mediated activation of peroxisome proliferator-activated receptor α as well as the inhibition of glycolysis and fatty acids. Therefore, KBs reduce the production of glycolytic adenosine triphosphate (ATP) and increase mitochondrial oxidation-induced ATP generation, thereby promoting mitochondrial oxidative metabolism, with resultant beneficial downstream metabolic changes.
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