The ketogenic diet, often called the keto diet, is one that is very high in fat, very low in carbohydrates, and low to moderate in protein. It typically supplies 75 to 90 percent of calories from fat, versus a more usual intake of 20 to 35 percent. It is intended to force the body to burn fat for energy rather than glucose—a state known as ketosis. Though many use the diet in order to accelerate weight loss and curb appetite, a growing body of research and anecdotal evidence points to psychological and cognitive benefits too, as well as the possibility for increased longevity and improved physical health.
Although ketogenic diets have been enjoying a wave of popularity in recent years, primarily for weight reduction, they have been in use therapeutically for a century, under carefully controlled conditions, to treat epileptic seizures in children. It’s not clear how a ketogenic diet reduces the hyperexcitability of nerve cells that leads to seizures. But there is mounting evidence that ketogenic diets, used judiciously for short periods of time, can benefit several systems of the body, especially the brain.
The diet gets its name from ketone bodies, which are metabolites of fat produced in the liver as it breaks down stored body fat for fuel use by the rest of the body. A distinguishing feature of ketone utilization is that insulin is not required to escort ketones into cells, as it is with glucose. Forcing the body to burn fat as fuel gives insulin time off. That restores the body’s sensitivity to insulin, resulting in lowered levels of both insulin and glucose in the blood. That has distinct metabolic benefits for the body—not just lowering weight but also cutting the risk of diabetes and other metabolic disorders and boosting brain function.
The keto diet generally consists of low-carb vegetables, meat and seafood, dairy, eggs, oils, nuts and seeds, and certain high-fat, low-carb fruits such as avocados or berries. Such foods are combined in various amounts so as to provide approximately 75 to 90 percent of their calories from fat.
Keto dieters should avoid sugars, grains, starchy vegetables (such as potatoes or beets), fruits that are high in carbs or sugar (such as bananas or pears), and most processed foods. Some keto dieters avoid beans and legumes, as they are generally high in carbs, while others eat certain varieties in small amounts.
Consuming a high-fat diet (as in the keto diet) is one way to trigger ketosis, but it is not the only way. Fasting can do it too, as can general carbohydrate restriction. Bouts of extreme exercise and excess alcohol intake can also result in ketosis.
Intermittent fasting refers to the practice of alternating periods of food consumption with periods of fasting, where either no calories or much fewer calories than normal are consumed. Some plans alternate between fast and non-fast days; others restrict eating to specific time periods each day, with the rest of the time dedicated to fasting.
Glucose is the brain’s preferred source of fuel because it’s fast. But glucose is inefficient; it doesn’t burn clean. Debris accumulates in the mitochondria, the fuel furnaces of cells. And glucose generates free radicals of oxygen, which progressively damage cells. Oxidative stress also leads to the buildup of toxic proteins that are the hallmark of Alzheimer’s disease. Over time, the processes that power brain cells lose efficiency, setting the stage for cognitive decline. The same processes may contribute to other psychiatric disorders, such as depression.
During periods of ketosis, by contrast, glucose is not available and brain cells must call on fat as their back-up fuel; as a result, they get a more efficient source of energy. Oxidative damage to brain cells is curbed. Turning to ketones intermittently also resets the machinery of glucose utilization and insulin sensitivity, and it stimulates cell-renewal processes. Memory and mental processing speed are maintained.
There is some evidence that the keto diet can help with depressive behaviors; however, many of these studies have been conducted in animals. Anecdotal evidence points to some mood-boosting effects in humans, but these claims have yet to be fully fleshed out in research.
Evidence is mixed. Some research reports that high-fat diets improve sleep quality; others, however, point to the opposite conclusion. Other studies find that keto diets reduce daytime sleepiness or increase time spent in REM sleep; some, however, focused specifically on children with epilepsy and may not be generalizable to other populations.
Some experts believe that the best way to retain cognitive capacity into old age is through periodic ketogenesis—but not necessarily triggered by the keto diet. Instead, evidence supports intermittent fasting, especially that which confines food consumption to an 8- to 12-hour window during the day and restricts intake, especially of carbohydrates, for 12+ hours overnight.
The many similarities between epileptic seizures and bipolar disorder make ketogenic diets of experimental interest in treating bipolar disorder. However, like depression, evidence is limited, and is primarily limited to small case studies.
While ketogenic diets can come with many benefits, there are potential risks and unpleasant side effects for keto dieters. For instance, ketogenic diets tend to be high in saturated fat and are known to increase levels of “bad” (LDL) cholesterol, which is associated with cardiovascular disease. They are also often deficient in micronutrients, including vitamins, and in fiber. Ketogenic diets may impede healthy gut function, and they can put a burden on the kidneys. Anyone looking to embark on either a short- or long-term keto diet should discuss their plans with their doctor; for short periods, however, they have generally been proven safe.
It can be. While nutritional ketosis is generally healthy in the short term, excessively high levels of ketone bodies can lead to ketoacidosis, a life-threatening condition. It most often occurs in untreated or undertreated Type 1 diabetes, from a lack of insulin, but it can also result from extreme keto dieting.
Some children living with epileptic seizures maintain a ketogenic diet for years. However, long-term keto diets have not been well-studied in non-epileptic populations. Keto may come with health risks, such as ketoacidosis or vitamin deficiencies; it may also be difficult to maintain. Some experts advise only sticking to its strictest guidelines for 30 to 90 days.
Ketones triggered by the ketogenic diet confer a distinctive smell to one’s breath. This smell may be described as sweet, fruity, or ammonia-like (similar to nail polish remover). While many people find this side effect strange or unpleasant, it is a sign that the body is in ketosis.