A Comprehensive Guide to the Ketogenic Diet

History of the Ketogenic Diet

The ketogenic diet was originally studied and developed in the early 1900’s for the use in children who suffered from intractable generalized seizures and continues to be the main focus of this diet. Many of these children were not responding to conventional pharmaceutical drugs, and there was evidence to show that a ketogenic diet was able to reduce frequency of seizures in these cases. 

Today, the ketogenic diet still serves as a method of treatment for children with intractable seizures, and along with it, scientific research in the area has been performed at an increasing rate. Overall, the results look promising in many cases.

If you’re reading this for fitness purposes, you may be thinking, “What does this have to do with me?”. If you’re about to change your diet for any purpose, we believe it’s always important to know the origins of the diet prior to jumping right in. This can be regarded as a safer approach, but also, it can help you decide whether or not it’s worth it.

Let’s take a look at what a ketogenic diet actually consists of, and then we’ll dive into any available evidence for/against its use in the fitness domain.

What is a Ketogenic Diet​?

Simply breaking down the word “ketogenic”, we see that it implies the generation, or creation, of ketones. So what are ketones? Ketones, often called ketone bodies in a technical sense, are produced after your body starts breaking down fat for the purpose of generating energy. Therefore, they aren’t really a substance that acts on your body, but rather, the result (a by-product) of this type of process.

​So why is this unique? More often, in healthy individuals, the body uses glucose as the fuel for energy production. This is the basis for the vast majority of diets for athletes and other fitness gurus. For example, you may have heard of athletes eating something like whole-wheat pasta the day of, or the night before, an event. Among other things, the purpose of this is to provide the body with enough fuel in the form of glucose to run off during the event.

  • Side Note: You may have heard about the Glycemic Index (GI) when it comes to meal preparation. For endurance events, athletes typically prepare by eating foods with a low GI level. This will cause a gradual rise and fall in blood glucose levels, allowing them to sustain that fuel for a longer period of time. Athletes requiring shorter bursts of energy may opt for higher GI foods, depending on what the actual event is. GI could be considered a “subsection” to a diet incorporating carbohydrates, and thus glucose.

So what happens if your body is low on glucose? Do you just completely shut down and become incapable of movement? The answer is no (at least not yet). The body has many different energy cycles it can use to create energy, and the cycles it chooses are often based in what type of activity is being performed, but just as importantly, how much fuel is available for each cycle. If your body becomes low on glucose, it can transition to breaking down fats for the production of energy, and thus, ketones are produced.

Therefore, a ketogenic diet represents the preference to use fats for the production of energy, which is why it is often considered a form of low-carb/high-fat diet. Fat is actually packed with energy (calories are a unit of energy), and thus, it has been proposed that this could theoretically be a way to lose weight or boost your fitness by unlocking a key energy source that burns off fat mass in the process. If you are considering a ketogenic diet, theory isn’t enough to go off, so let’s see what any available scientific evidence has to say about this.

The Ketogenic Diet and Fitness

The first thing we want to make clear is that research into the fitness effects of a ketogenic diet pales in comparison to that of medical purposes where it was first identified. This doesn’t necessarily mean that scientists don’t think it’s worth studying, but more likely that this is a new enough topic that there just hasn’t been enough time for lots of comprehensive long-term studies to take place. So does that mean it’s cutting edge, or bogus? Let’s find out.

** ​For the sake of practicality, as well as accounting for advancements in the research, we will only consider scientific studies from the last 15 years.

Scientific Evidence Supporting the Ketogenic Diet

Study 1: Losing Weight by Decreasing Fat Mass

A study performed by Jabekk et al. (2006) included a group of 16 women between the ages of 20-40 who were classified as being overweight, that is, with a BMI greater than 25 kg/m^2. They were assigned to one of two groups. The first was a group that participated in a 10-week resistance-training program performed twice a week (e.g. leg press, shoulder press, etc) with their normal diet, while the other group maintained a ketogenic diet.

So what was the ketogenic diet in this case? The 8 participants in the ketogenic group were encouraged to limit their intake of carbohydrates as much as possible. They were confirmed to have maintained their state of ketosis through urine samples that confirmed positive tests for urinary ketone bodies (remember, ketones are a by-product of fat-breakdown). Otherwise, there were no limits on protein, fats, or fatty acids. For example, these subjects in the ketogenic group were still allowed to eat meats, fish, cheese, margarines, butters, and oils.

The results? As one might expect through a resistance training program, the normal diet group saw in increase in lean body mass, but no decrease in fat mass. On the other hand, the ketogenic group saw a statistically significant reduction in fat mass, but no statistical change in lean mass.

The conclusion – a low-carb ketogenic diet may be beneficial in reducing fat mass in overweight women who participate in a resistance-training program. The reason we say “may” is because it’s important to acknowledge that this study wasn’t attempting to probe into the underpinning mechanisms of these differences, and there was certainly some variability between individuals. For some, the ketogenic diet had more positive affects than others. While they stuck to a general type of diet, habitual dietary intake could not be accounted for, as well as any other day-to-day or pre-existing factors that could slide under the radar and influence the results.​

Nevertheless, the statistical significance of thee results shows some promise, particularly since blood lipid profiles of the ketogenic group did not seem to be negatively influenced during the study.

Study 2: Weight Loss and Performance in Weight Category Athletes

This second study (Rhyu and Cho, 2014) was performed in South Korea on Taekwondo athletes. The participants were high school students who regularly practiced Taekwondo, and the study was performed at one of their Taekwondo summer camps. There were 20 participants in total, with 10 being randomly assigned to the ketogenic diet group, and 10 being assigned to the control group.

What is interesting about this study, and different from the first study we mentioned, is that the investigators had more control over caloric intake. Participants were instructed to record their caloric intake over a 3-day period and the average amount of calories consumed per day was recorded. Then, at the start of the study, the participants were all assigned meal plans that reduced their caloric intake to 75% of their normal intake for the purposes of weight loss.

The menu for the ketogenic diet group consisted of foods like beef, pork, fish, beans, eggs, and cheese, but excluded items like bread, rice, noodles, coffee, and tea. They were limited to 22g of carbs per day, and they stuck to a ratio of 55% lipids (fat), 40.7% protein, and 4.3% carbs. In contrast, the proportions for the control group were 30% lipids, 30% protein, and 40% carbs. This diet stage lasted for 3 weeks, and both groups participated in their usual training program, with an emphasis on strength improvement.

The results? After the 3-week protocol, the groups both lost weight, but did not differ significantly from each other in terms of weight loss, loss of fat vs. fat free mass, BMI, or sprint/maximum exertion exercise performance. Often times, there was actually a slight decrease in performance for both groups, most likely due to the relatively sudden weight loss and caloric restriction.

However, there were a couple notable differences that were statistically significant. First, the ketogenic diet group was able to finish a 2000m sprint in less time than the control group, and they experienced less aerobic fatigue as measured by the Wingate test. Here is what the authors had to say about this result:

“This result indicates that although the ketogenic diet may cause reduction of fat-free mass, when exercise is prolonged, it can be effective for improving aerobic capacity and fatigue-resistance capacity by compensating for decreased carbohydrates and causing lipids to be used as an efficient energy source.” - Rhyu and Cho, 2014

Additionally, from blood profiles, the researches also noticed an anti-inflammatory effect in the ketogenic diet group that was also noticed in previous studies as well.

“In conclusion, the ketogenic diet can be helpful for weight category athletes, such as Taekwondo athletes, by improving aerobic capacity and fatigue-resistance capacity, and also by exerting a positive effect on the inflammatory response.” - Rhyu and Cho, 2014

Study 3: Metabolism and Performance in Off-Road Cycling

This third study, conducted in 2013 by Zajac et al., set out to determine the effects of a long-term ketogenic diet on aerobic performance and exercise metabolism in off-road cyclists. The participants included 8 male cyclists with at least 5 years of sport-specific training and a VO2max of at least 55 ml/kg/min. Given the low sample size to begin with, this study employed a crossover design, meaning that participants were randomly assigned to one of two groups (ketogenic diet or non-ketogenic diet), and then they switched after 1 month.

In this case, the ketogenic diet consisted of 70% fat, 15% protein, and 15% carbohydrates. The non-ketogenic diet consisted of 30% fats, 20% protein, and 50% carbohydrates. Both diets were isocaloric, meaning calorie intake was controlled and maintained. Lots of biochemical testing on blood samples were performed to gain insight into metabolic changes and maintenance of a state of ketosis. Comprehensive fitness testing was performed prior to the diets being assigned, which provided baseline measurements of performance, and then the training programs maintained a high volume and moderate intensity.

The results? The metabolic effects of the ketogenic diet group were often pronounced, indicating that this diet could promote fat oxidation at moderate intensity and at rest. It could also slow down the rate of carbohydrate utilization and enhance endurance performance in long distance events lasting 2-5 hours. The ketogenic diet group saw statistically significant effects on body composition, as reflected in a lower body fat percentage, and there were also indications of reduced post-exercise muscle damage.

In terms of performance, the ketogenic diet group displayed statistically significantly higher VO2max and VO2 at lactate threshold than the control group. It must be noted that the authors acknowledge that many of the positive effects in performance cannot be guaranteed to be a result of purely the ketogenic diet, but perhaps other factors that go along with it, such as changes in body composition. Furthermore, they did see a decrease in high-intensity performance with the ketogenic diet, likely due to a lack of available fuel for those sorts of tasks (e.g. glucose).

In conclusion, the authors summarize all of their combined results in the following way:

“…long-term, high-fat diets may be favorable for aerobic endurance athletes, during the preparatory season, when a high volume and low to moderate intensity of training loads predominate in the training process. High volume training on a ketogenic diet increases fat metabolism during exercise, reduces body mass and fat content, and decreases post-exercise muscle damage. Low carbohydrate ketogenic diets decrease the ability to perform high-intensity work, due to decreased glycogen muscle stores and the lower activity of glycolytic enzymes, which is evidence by lower lactic acid concentration and maximal workload during the last 15 minutes of the high intensity stage of the exercise protocol.” - Zajac et al., 2013

Scientific Evidence - Mixed Reviews

Study 1: The Atkins Diet and Obesity

So it seems that a ketogenic diet can have some benefits for particular athletes, and we have also seen some effects in young people who are technically considered to be overweight. What about those who are classified as obese? Can this diet be used to promote health outcomes in obese and largely non-active individuals? One study set out to try and determine if this could be plausible.

The study we would like to highlight that shows some positive outcomes was performed by Foster et al. (2003) and was published in the New England Journal of Medicine. This journal has the highest known impact factor, essentially meaning it has to be a really well designed study that provides significantly useful information. To be clear, this study provided a focus on the Atkins Diet, which is considered a low-carb high-fat diet, but not necessarily in the true ketogenic sense, as we will describe shortly.

This study was one year long and included 63 obese men and women (BMI > 30). Potential participants were excluded if they had type 2 diabetes or were taking any medications that could interfere with the results. 33 were randomly assigned to the low-carb/high-fat group. This diet consisted of limited carb intake to 20g per day during the first two weeks, and then gradually increasing until a desired weight was achieved. The rest of their diet was in the form of protein and fat, as advised in the Atkins plan (more detailed information presented in the journal publication itself). The conventional/control diet group consumed 60% of their calories from carbs, 25% from fat, and 15% from protein.

Outcomes included body weight, blood pressure, cholesterol levels, insulin levels, and urinary ketones. It appears no exercise plan was put in place, but the subject met regularly with a registered dietician to ensure adherence to the program and avoidance of any negative health issues with relation to the study. Different measurements were performed at different frequencies throughout the study, but in general, all measurements were performed on a regular basis and rather frequently throughout the study.

So what did the results show? The low-carb/high-fat group lost significantly more weight than the conventional group at the 3- and 6-month marks of the study, but after a full year, the difference was not statistically significant. While there were no differences in blood pressure or LDL levels (“bad cholesterol”), there were continuous increases in HDL levels (good cholesterol) for the low-carb/high-fat group and decreases in triglycerides (a good result).

Now, the important thing to consider in this study, especially in relation to a ketogenic diet, is that the percentage of low-carb/high-fat participants that tested positive for urinary ketones were significantly higher only at the 3 month mark, which is most likely associated with the time point of the lowest carb intake. Furthermore, there were no statistically significant associations between weight loss and ketosis at any point during the study.

So what does this all mean? Remember, this study is taking the perspective of obese individuals who may be trying to lose weight and improve their cardiovascular health. While there some outcomes that would have positive outcomes for cardiovascular health, the investigators warned that others did not change, and that this diet could potentially discourage one from consuming healthy foods like fruits and vegetables, which could negate any positive effects.

Furthermore, the lack of subjects reaching a state of ketosis, combined with the absence of a training program, suggests that this diet, and maybe ketogenic diets as well, may or may not be overly beneficial for improving cardiovascular health in obese individuals. Overall, it’s inconclusive.

Scientific Evidence Not Supporting the Ketogenic Diet

Study 1: Strength Performance in Elite Artistic Gymnasts

This research study, conducted by Paoli et al. (2012), aimed to investigate the effects of a ketogenic diet on explosive strength performance. 8 high-level male artistic gymnasts who averaged 21 years old were recruited for the study, some of which were on the Italian national team. In this case, since there weren’t many participants, they all took part in the ketogenic diet along with a 30-day training period monitored by the investigators. Then, after a 3-month break from the diet, they were tested again with the same training program and their conventional diet.

The ketogenic diet was quite comprehensive with strict guidelines, but otherwise, the athletes could consume the foods as they wished. Athletes mainly consumed beef, veal, poultry, fish, eggs, seasoned cheese, infused tea, moka coffee, and herbal extracts. They avoided food and drinks that included alcohol, bread, pasta, rice, milk, yogurt, soluble tea, and barley coffee. They were also provided with other high quality protein supplements with virtually zero carbohydrates. Very detailed information about the herbal extracts and supplements can be found in the journal publication itself.

The results showed that the diet was successful in reducing body weight via reducing fat mass, thus decreasing the percentage of body fat, which is farily impressive considering these athletes already had a low body fat percentage to begin with. However, while the difference was not statistically significant, the athletes experienced a slight drop in lean body mass as well. There were no differences in any of the performance outcomes.

The authors of this study concluded that while a ketogenic diet is likely beneficial for reducing body fat percentage, there were no beneficial effects on explosive strength performance. Furthermore, some athletes reported not being able to complete some of the training tests on the ketogenic diet, but this only occurred during the first week when the athletes were becoming accustomed to their drastically different diet. The sample size was also quite low.

Lastly, one major thing that struck us while reviewing this study is that there did not appear to be any blood or urine measurements for detection of circulating ketone bodies, so it can’t be confirmed that these athletes were actually in a state of ketosis during the strength testing. Given the comprehensive protocol for the ketogenic diet, one could assume they were, but science leaves little room for assumptions.

Study 2: Ketogenic vs. Non-Ketogenic Diet in Sedentary Individuals

This is another well-controlled study. Published in 2006 in The American Journal of Clinical Nutrition by Johnston et al., this study focused on overweight men and women who were defined as being sedentary (non-active, between 20-60 years old). This study was similar to the Atkins study that was inconclusive. In this case, the control group was still consuming a diet low in carbs, but not to the extent of the ketogenic group. Perhaps this will help shed some light on the effect of a ketogenic diet for sedentary individuals looking to lose weight.

It’s a little confusing how this group decided to design the diets, but for most of the study, the ketogenic diet group was limited to under 5% of their diet being carbohydrates, 60% being fat (21% was saturated fat), and the remainder being protein. Gradually, their carbohydrate limit was increased over the course of the study.

Unlike the previous Atkins study, this one showed a significant increase in ketone bodies in the ketogenic diet group for a slightly longer proportion of the study. Only the very last time point (Week 6) were no statistical differences between groups detected. Lots of body composition and biochemical measurements were taken regularly throughout the study, with a focus on indicators of cardiovascular health.

So what did the results show? Well, not much to be honest. If anything, there were some adverse health effects associated with the Atkins-style ketogenic diet. There were no differences between groups for body weight or insulin resistance. Furthermore, blood LDL levels (bad cholesterol) were elevated and were calculated to be associated with higher ketone bodies, and there was also an additional inflammatory risk. In summary, the authors concluded the following:

“The ketogenic low-carbohydrate (KLC) and the non-ketogenic low-carbohydrate (NLC) diets were equally effective in reducing body weight and insulin resistance, but the KLC diet was associated with several adverse metabolic and emotional effects. The use of ketogenic diets for weight loss is not warranted.” - Johnston et al., 2006

Our Conclusion!

There are many other studies out there that investigate the ketogenic diet to some extent, but we feel this is a good and recent representation of the overall results. In fact, enough evidence has emerged to warrant the recent publication of a review study in 2015. A review study involves an expert in that area of research finding all available literature on the particular topic and addressing the state of affairs. In this case, the review written by Antonio et al. seems to line up with the evidence we presented here. This review was appropriately titled: The Ketogenic Diet and Sport – A Possibe Marriage?

First, let’s consider the overall themes of the results we summarized in this article, and then we will combine this with the perspective of Antonio et al. Generally speaking, all the studies that showed benefits of following a ketogenic diet also included exercise. Furthermore, the performance benefits associated with a ketogenic diet were most pronounced in those participating in long duration endurance events where maximum exertion (i.e. explosive strength, anaerobic power, etc) was not required.

In the case of those sports that relied heavily on maximum/explosive performance, the ketogenic diet did not show any benefits, which makes sense considering our explanation of the diet at the very beginning of this article. Reductions in muscle glycogen stores should not be advantageous for quick-burst activities, but switching over to a fat oxidation cycle during, such as those during endurance events, should theoretically hold more potential. The evidence does seem to support this.

Lastly, it seems apparent that the ketogenic diet is NOT appropriate for sedentary individuals looking to drop weight through a diet program. It does seem to help with shedding some pounds in certain cases, but the whole purpose of losing weight is it keep it off and improve your health, and a ketogenic diet does not seem to facilitate this, and sometimes, may even be detrimental.

So what does the review study have to say? They do offer a large list of practical considerations and future directions, so here are a couple that stood out to us. If you are at all interested, we highly suggest taking a look at the study for yourself to see everything they had to say, as it is very comprehensive.

  • A ketogenic diet should include less than 5% carbohydrates, and no more than 20g per day.
  • For athletes who need to maintain lean mass (i.e. muscle mass), the daily intake of protein should be at least 1.2-1.7 grams per kilogram body weight, up to a maximum of 2.5g. Otherwise, you may risk losing muscle mass, and may even perform lower on tests like the VO2max.
  • Athletes should ensure they are receiving adequate vitamin and mineral intake while on a ketogenic diet, which may be compromised if not carefully planned.
  • There is no way to confirm that you are in a state of ketosis without a urine or blood sample being performed. Blood is a more reliable indicator of ketosis than urine.
  • If you are a young athlete, be careful. You may not realize it at first, but often times other people can pressure you into taking extreme measures to improve your performance, whether it be parents, coaches, recruiters, or friends. Your health always comes first, so an extra-cautious approach should be taken for young people.​

Final Note

We really like this review article that we just summarized (perhaps over-summarized – it’s a big article). It is very recent and considers multiple perspectives, and appears to be largely unbiased. They offer some other very specific guidelines for athletes in general, but from the studies we have seen, plus this review study, we would like to offer our own three-point take home message:

  1. The ketogenic diet holds the most potential for highly competitive endurance athletes who have time to stick to the diet (you may notice drops in performance the first week while your body adjusts). If you don’t fall into this category, it’s probably not worth it.
  2. If you plan on following the ketogenic diet, we highly encourage you to consult with a registered dietician. It can be a difficult diet to adhere to, and maintaining a state of ketosis can be difficult in general, especially if you want to be consuming adequate amounts of vitamins and minerals at the same time. A registered dietician will make the process way easier for you, which can make it well worth the cost.
  3. If you’re an overweight or obese individual looking to improve your health by losing weight, be highly skeptical of the Atkins Diet. While it seems like a lot of work, you will have the best chance of success if you take a multifaceted approach and consult with a few different professionals: i) a personal trainer can help you design an effective, practical, and fun training program, ii) a registered dietician can help you find a diet that is healthy and helps you lose weight, but is still tasty and easy to manage, and iii) a doctor should be consulted from time to time to make sure you aren’t experiencing any adverse health events, and it can also be a good way to track your progress (e.g. blood pressure measurements).​

We hope you found this guide to the ketogenic diet to be useful. We wish you the best of luck with all of your health, fitness, and performance goals!