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What is a Ketogenic Diet?

I love to listen to podcasts. Lately I have been listening to a variety of podcasts about the ketogenic diet. It seems to be gaining momentum. Some tout its benefits for enhancing sports performance, and others for therapeutic reasons, longevity, diabetes, and weight loss. I have been well aware of the ketogenic diet as a dietary therapy for epilepsy, specifically for children. I remember learning about the ketogenic diet while obtaining my degree in nutrition and thinking, “wow, I am glad I don’t have to follow that diet.” It is restrictive and hard to follow, especially for a child. At this point the research on the ketogenic diet for health reasons and performance is interesting, but I recommend people proceed with caution.

The original ketogenic diet was designed as a 4:1 ratio of fat to protein and carbs. The diet consists of 80% of the daily energy intake from fat, 15% from protein, and 5% from carbohydrate.1 There are some variations and modifications that have been introduced including changing the ratio of macronutrients or no restriction on daily energy intake. The types of fats recommended on the ketogenic diet have also been modified to include more polyunsaturated fats from fish, oils, and nuts. Athletes may be able to consume higher amounts of carbohydrates and proteins and still maintain ketosis because they have a higher energy demand than non-athletes. To achieve ketosis the daily carbohydrate intake for most people is around 20 grams per day. To keep that in perspective, 20 grams of carbohydrate is equivalent to one apple, one small tortilla, ½ cup beans, or 1/3 cup of pasta. That is it! That is all the carbohydrates you can consume in one day.

So what is ketosis? For those people who do not have a background in biochemistry or biology this process is complicated so bare with me. The goal of the ketogenic diet is to induce ketosis. Ketosis is a normal metabolic process to fuel the body when carbohydrate stores run out. The body shifts from burning carbohydrates for fuel to burning fat. After a period of several days of fasting or a drastic reduction in dietary carbohydrate intake, the glucose (carbohydrate) reserves become depleted and no longer sufficient to support normal fat breakdown or to supply energy for the brain and central nervous system.1 For those that remember the Krebs cycle or TCA cycle in biology class, basically there is not enough glucose to provide oxaloacetate to the Krebs cycle. This (oxaloacetate) is an important metabolic intermediate for the Krebs cycle and also needed for the oxidation of fat. Fats (as free fatty acids) can’t cross the blood brain barrier and thus glucose (carbohydrate) is the main energy supply for the brain.

So how do we get energy to the brain during times of fasting or on low carbohydrate diets? This energy is supplied by ketone bodies, which are generated by a process called ketogenesis. This process occurs when acetyl-coA (an intermediate of fat breakdown) can no longer enter the Krebs cycle and ends up in the liver or mitochondria. The liver and mitochondria use the acetyl-coA to make ketone bodies (acetoacetate, 3-hydroxybuterate, and acetone). Ketone bodies are able to cross the blood brain barrier and thus able to supply the brain with energy. In a nut-shell, when carbohydrate supplies are low, the body breaks down fat to produce ketones, which supply energy to the muscles and brain.

Some people may read this and think that they have to go on a ketogenic diet to burn fat and otherwise they are always burning carbohydrate. This is not true. Our muscles and other tissue have no problem using fat for fuel, but because free fatty acids don’t cross the blood brain barrier the brain is not able to use fat for fuel, unless it is broken down into ketone bodies. In between meals we use a combination of fat and carbohydrate to fuel our bodies. The carbohydrate is coming from the liver, where it is stored. When we eat a meal high in carbohydrate our bodies switch to burning more carbohydrate than fat. Once the meal is digested and absorbed we switch back to using fat and carbohydrates for fuel. The ketogenic diet is different in that there is very little carbohydrate in the diet and thus the body switches over to burning predominantly fat for fuel. Keep in mind that the diet also consists of mostly fat. So yes, the body is burning fat, but that does not mean it is burning all your body fat stores for energy. It is using a combination of fat consumed in the diet and stored fats. In a regular, non-ketogenic diet the same principle applies, but the body is using both carbohydrates and fats for fuel. In between meal the body is using fat from the diet and stored fat.

A myth I hear often is that people on the ketogenic diet can eat as many calories as they want and still lose weight and body fat. The concept of energy in vs. energy out is still relevant here. If you are consuming more calories than your body needs you will gain weight. Period. It does not matter what diet you are on. Yes, some macronutrients might be utilized differently than others, but the law of thermodynamics still applies. This myth may exist because many people feel more satiated on a ketogenic diet. Fat is a highly satiating food and many people following this diet report eating a high fat meal and not feeling hungry for hours afterwards. They may eat 2-3 high fat meals during the day and feel full, but their overall calorie intake was equal or lesser than their calorie requirements for the day. Sorry folks, but there is not magic diet in which you can eat as many calories as you want and still lose weight.

I believe this provides some insight into the ketogenic diet. Basically, the goal of the diet is to switch over from using a combination of carbohydrates and fat as fuel to using predominantly fat or ketone bodies as fuel. Now you are probably wondering what benefits there might be to following this type of diet. Why would someone want to use ketones instead of glucose? Many people wan to know if it is a safe diet to follow. I am going to explore this question in future blog posts, so stay tuned.

1. Paoli A, Bianco A, and Grmaldi K. The Ketogenic Diet and Sport: A Possible Marriage? The American College of Sports Medicine. 2015; 43:153-162

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