The ketogenic diet is a diet that capitalizes on a very interesting aspect of human physiology, namely ketosis.
Ketosis is a metabolic state that occurs when we consume carbohydrates in such low quantities fatty acid oxidation becomes the main source of fuel.
When fatty acid oxidation reaches a certain point and carbohydrate availability is low enough, the body begins producing ketone bodies.
These ketone bodies are then used preferentially in specific tissues, especially the brain.
The ketogenic diet allows us to survive on without food intake or on very low (down to virtually zero) carbohydrates for long periods of time.
From an evolutionary, biochemical, and physiological perspective, ketosis appears to be a survival strategy/mechanism.
For the purposes of the ketogenic diet as a fat loss tool, a focal point of the ketogenic diets is that it allows us to live for long periods of time on virtually no carbohydrate intake and theoretically lower insulin levels than normal.
As it has been hypothesized that insulin plays a role in body composition by regulating fat storage, this is one of the key points the ketogenic diet attempts to capitalize on.
History Of The Keto Diet
The history of the ketogenic diet is quite fascinating. Unlike most other diets that have arisen from health and fitness circles or popular text books, the ketogenic diet most likely arose from the medical field.
The ketogenic diet was initially used to help control seizures in children with epilepsy as brain metabolism can be regulated more “evenly” being in ketosis in some circumstances.
Over the years it has been adopted by the mainstream and utilized as a diet for fat loss.
General Overview of Components & Main Principles of the Keto Diet
Ketosis arises from a lack of dietary carbohydrates and lower intakes of dietary protein, coupled with either fasting or high dietary fat intake.
When fat metabolism reaches a level where the key substrates for the Krebs Cycle are depleted (namely oxaloacetate), the body, more specifically the liver, begins producing ketone bodies (beta-hydroxybutyrate, acetoacetate, and acetone) which then become a substantial fuel source for select tissues in the body (e.g. the heart and the brain).
Ketogenic Versus Low Carb
A low carbohydrate diet is based on the same fundamental idea of a ketogenic diet: reduce carbohydrate intake to reduce calorie intake and lower insulin levels. While they operate under the same premise, they are physiologically very different, specifically in their effect on blood ketones.
During a low carbohydrate diet ketones do not accumulate in the blood to any appreciable degree, this changes the type of substrates used in key tissue like the brain and the heart. It could probably be argued that if you go low-carb you should probably go ketogenic to help tissues adapt better; however, that remains to be debated.
What About Ketoacidosis?
The ketogenic diet differs substantially from diabetic ketoacidosis, primarily in that having low insulin (a ketogenic diet) is completely different than no insulin (diabetic keto acidosis).
In diabetic ketoacidosis, insulin is not present to regulate ketogenesis. Below are the respective levels of blood ketones for “normal” levels, nutritional ketosis, and diabetic ketoacidosis:
Normal Ketone Levels= 0.0 -0.5 mmol/L
Nutritional Ketosis Ketone Levels= 0.5 - 3.0 mol/L
Diabetic Ketoacidosis= > 5.0 mmol/L with concurrently high blood sugars and lack of insulin
How to Stay in Ketosis
In order to enter and stay in ketosis dietary carbohydrate intake needs to remain below a certain threshold. This threshold varies from person to person, but a good guide for most people is between 5-15% of daily total calorie intake should serve as a starting point.
Typically checking ketone levels can help each person find their range of intake that allows them to stay in ketosis.
Protein consumption must also stay low in order to stay in a ketogenic state. Ingesting protein above certain quantities is glucogenic, meaning it can produce glucose your body will use for fuel. This can effectively “kick” your body out of ketosis.
In practical terms, consuming protein at or above around 15% of your total daily calorie expenditure can be enough to render your ketogenic diet non-ketogenic.
On principle, the ketogenic diet does not explicitly prescribe specific timing of meals nor does it specify meal frequency. However, as it is often easier to maintain ketosis while fasting, many people often combine ketogenic diets with types of fasting, usually intermittent fasting.
That being said, if consuming a very high fat, low protein, very low carbohydrate diet, one can maintain ketosis eating quite frequently.
The ketogenic diet does not explicitly restrict any foods or food groups, but by the nature of the required macronutrient intake it often restricts large groups of food by nature.
As carbohydrate intake must be kept between 0-15% of daily energy intake, foods such as rice, potatoes, bread, pasta, fruit, and other carbohydrate rich foods become practically impossible to include in one’s diet, unless in very small quantities. Additionally, protein must also be kept fairly low, between 5-15% of daily energy intake in order to stay in ketosis.
Does it Include Phases?
The ketogenic diet does not included phases as typically prescribed and followed.
Some people will adopt a cyclic ketogenic diet in which they use a ketogenic diet and stay in ketosis for 4-5 days at a time and then have a carbohydrate refeed.
Who is it Best Suited For?
The ketogenic diet is best suited for individuals who enjoy not tracking calories, do not mind having limitations on the types of foods they eat, and who are not high-level athletes.
Individuals who have rapid fat loss goals (e.g. figure competitors prepping for a show) can have success with slight modifications to the ketogenic diet.
How Easy is it to Follow?
Asking how easy the ketogenic diet is to follow is like asking who people voted for at thanksgiving. You will get answers flung at you from both sides with fervor. Some people will say it is really difficult to follow and some will say it is the easiest diet in the world to follow.
When you really look at adherence rates to the ketogenic diet in the scientific literature it appears that adherence to the ketogenic diet is not all that much worse than other dietary frameworks7, 8, 9.
There are several aspects that can make it difficult to follow. The first is the “adaptation phase”. Many people report the “low carb flu” where their body is transitioning from relying on carbohydrates and fatty acids for energy to a more ketone-based metabolism, this often causes lethargy and sometimes even flu like symptoms in some individuals.
The second is that to stay in ketosis people must keep their dietary protein and carbohydrate intake to fairly low levels, changing food choices to this extreme can be difficult for some people. On the other side, the lack of calorie counting and ability to eat ad-libitum and have success for large numbers of people makes this diet an attractive choice.
Mainstream Belief Behind Diet
The mainstream belief behind the ketogenic diet is that it hacks an interesting aspect of human physiology and allows us to live on very low carbohydrate intakes for long periods of time.
This is believed to allow us to maintain lower levels of insulin, which may help increase fat loss.
Scientific Studies and Interpretation of Data
Of all the diets out there, perhaps only the mediterranean diet has more scientific studies conducted on it. A pubmed search for the term “ketogenic diet” returns 1913 references, indicating substantial interest in the topic.
There have been several main areas of investigation in to the ketogenic diet and its benefit on weight loss, epilepsy, cancer, and most recently human performance.
While there is a long history of ketogenic diets on fat loss as it has been proposed that the suppression of insulin signaling by removing dietary carbohydrates will improve fat loss
For decades these studies did not provide robust enough control to remove key cofounders like total calorie intake. Only recently have we seen studies conducted with a high enough level of control to provide enough hard data to answer this question.
One study fed people either a restrictive carbohydrate diet or a restrictive fat diet for 6 days after 5 days on a baseline diet1. This study found that decreasing carbohydrate intake increased fatty acid oxidation and decreased carbohydrate oxidation, an expected finding as the body usually adapts to the fuel it is given.
Interestingly, reducing fat intake led to a slightly greater loss in body fat than the group that restricted carbohydrates. The main conclusion from this study was that an increase in fatty acid oxidation due to eating more fat doesn’t necessarily result in more fat loss, in fact it may be the opposite. So the idea of eat fat to burn fat is likely not the best approach to fat loss.
The second study is one of the most tightly controlled and well-designed dietary studies ever done in the nutrition world, especially in regards to the ketogenic diet2. In this study overweight or obese men were admitted to metabolic wards (they essentially were locked in a testing facility), can consumed either a normal calorie restricted diet or a ketogenic diet.
When all was said and done, the participants lost more body fat in 15 days (about 1 pound) on the higher carbohydrate diet than while on the ketogenic diet (about 0.5 pounds).
These two studies show that there doesn’t appear to be some secret metabolic advantage to the ketogenic diet. Now this does not mean it can’t be an effective tool as there are a plethora of studies showing that the ketogenic diet is highly successful as a fat loss tool, mainly by being a way to easily control calorie intake.
One of the more hotly debated areas surrounding the ketogenic diet is the impact it has on performance. There have been several studies done on the topic and going through them in a bit of detail will help us highlight the benefits and pitfalls of adopting this diet with regards to performance.
In one study looking at how ketogenic diets effect moderate intensity running, athletes lost weight, including lean mass, with no change on their VO2 max3. However, they had a reduced ability to recover.
In a different study looking at the effect of a ketogenic diet on cycling performance the ketogenic diet improved body composition and also improved relative VO2 max. There is a huge caveat to this study, namely the fact that VO2=ml/kg/min, so if you reduce body mass you reduce the number in the denominator thus you increase the relative value4.
This is actually proved by the study itself as their max effort work decreased and the breakdown of the variables in the VO2 equation show that the improvement in relative VO2 was indeed from their lower body weight.
What exactly does this mean? Ketogenic diets likely help with weight loss through calorie restriction but often times lower maximal effort capacity in metabolically demanding activities.
The same theme of weight loss, typically due to spontaneous reduction in caloric intake and no improvement in performance has been seen in several more studies5,6.
Another key piece of information is that muscle biopsies from studies showed that the keto diet reduced muscle glycogen to almost half of normal. That alone is enough to indicate high-intensity performance might be impaired.