Stories of people who have reportedly lost a lot of weight on very-low carbohydrate ketogenic diets seem to be everywhere. Despite the recent hype, this isn’t a new kind of diet. Ketogenic diets have been used for years as a medically prescribed treatment for difficult cases of epilepsy, especially in children. The original Atkins diet included an initial ketogenic phase. But key questions remain: Can they help control diabetes? Are ketogenic diets more effective than other weight-loss diets? Are they safe?
Not your typical low-carb diets
Ketogenic diets are very low in carbs and typically very high in fat. Glucose (blood sugar) is normally the body’s main source of energy. When carb intake and the body’s limited carb stores are insufficient to maintain minimum blood glucose levels, the body starts burning fat for energy and converts certain amino acids from protein (first from recently consumed food, then if necessary from muscles) into glucose. Fat becomes the primary energy source and also fuels the amino-acid-to-glucose conversion.
The breakdown of fat (and to a lesser extent protein and its constituent amino acids) generates compounds called ketones, which can be used for energy by the body (including the brain), though not as efficiently as glucose. This metabolic state, known as ketosis, is an adaptation that occurs during starvation as well as in certain diseases such as uncontrolled diabetes and alcoholism. Ketosis also occurs within a matter of days after people cut way down on carbs, usually to between 20 and 50 grams a day. (To put that in perspective, a one-ounce slice of bread has about 14 grams of carbs.) The lower the carb intake, the faster ketosis develops.
Today’s ketogenic diets typically derive 5 to 10 percent of their calories from carbs, 70 to 75 percent of their calories from fat, and about 20 percent from protein. (In contrast, an average American diet gets 50 to 55 percent of calories from carbs, 30 to 35 percent from fat, and 15 to 20 percent from protein.) Ketogenic diets can include full-fat dairy, eggs, fish, poultry, meat, nuts, seeds, non-starchy vegetables, and fats such as plant oils and butter. Starchy vegetables, most fruits, grains, and sweets are eliminated. Calories are not restricted-basically you can eat as much low-carb food as you like, until you feel full.
Most, but not all, studies find that ketogenic diets help people lose more weight than other diets and help people with diabetes (type 1 or the more common type 2) control blood sugar better.
- An analysis of 13 clinical trials, published in the British Journal of Nutrition in 2013, found that people on ketogenic diets tend to lose more weight and keep more of it off than those on low-fat diets, and they end up with lower triglycerides and blood pressure and higher HDL (“good”) cholesterol. However, the diet also usually increases LDL (“bad”) cholesterol-not surprising, since it is usually high in saturated fat.
- In a study in Nutrition & Diabetes in late 2017, researchers from UC San Francisco and elsewhere compared a ketogenic diet with a calorie-restricted, moderate-carb diet in 34 overweight or obese people with prediabetes or type 2 diabetes. After a year, the ketogenic group lost almost 18 pounds compared to barely 4 pounds in the low-fat group; they also had a greater reduction in HbA1c (indicating better long-term glucose control), on average, and were more likely to be able to reduce or even stop their diabetes medication.
- In a study in Diabetes Therapy in 2018, 262 middle-aged people (mostly obese) with type 2 diabetes were enrolled in an intensive outpatient program involving a ketogenic diet. After one year, they had a substantial reduction in HbA1c and a 12 percent average weight loss, and most were able to reduce or eliminate their use of diabetes medications. The program was unusual in that participants received continual, individualized education and care via web-based monitoring and interaction with a medical team and health coach to keep them on track.
- In a study in Pediatrics in 2018, researchers analyzed an international survey of 273 people with type 1 diabetes who were following a very-low-carb diet. (About 40 percent of participants were parents completing the survey on behalf of their children with type 1 diabetes.) Overall, participants reported excellent blood sugar control, high levels of satisfaction, and low rates of adverse events. “These findings are without precedent among people with type 1 diabetes, revealing a novel approach to the prevention of long-term diabetes complications,” the authors concluded. This was just a survey, however, not a clinical trial, and the participants and the results may not be representative of the general population of people with the disease.
How do the diets promote weight loss?
The relationship between ketogenic diets and weight loss is still debated, but there are likely to be multiple mechanisms. By eliminating entire food groups, the diets encourage people to eat less (that’s how many popular diets work). In addition, the initial rapid weight loss on ketogenic diets comes from a diuretic effect, but this water weight always returns when fluid balance normalizes after ketosis ends.
It’s also theorized that ketogenic diets may work by affecting appetite-regulating hormones, or that ketones themselves act as an appetite suppressant. The fat and protein in the diets are also more satiating than carbs. A paper in the Journal of the American Medical Association in 2018 noted that people who go on ketogenic diets report greater satiety and fewer hunger pangs than people with similar calorie intakes on other diets, so they tend to eat less.
In addition, whereas people who lose weight, especially on low-fat diets, almost always experience a reduction in metabolic rate (as the body compensates for the loss of body fat and tries to conserve energy), this happens much less with ketogenic diets.
The most obvious hurdle with ketogenic diets is that extreme carb restriction makes it very challenging for most people to stick to them, especially long term.
Common adverse effects include fatigue, dizziness, constipation, and sleep problems, which tend to get better within a few weeks. Longer-term adverse effects would depend on the exact composition of the diet and how long it is followed.
Because ketogenic diets eliminate whole grains, most fruits, and many vegetables, they tend to be low in key nutrients, such as folate and potassium, along with fiber, all of which are essential for good health.
LDL cholesterol levels often rise on ketogenic diets, though focusing on foods high in unsaturated rather than saturated fat can help prevent this (more nuts and avocados, less meat and butter, for instance). Overall, the diet’s cardiovascular benefits (notably for triglycerides, HDL, blood sugar, and body weight) may outweigh any increase in LDL, especially for people with diabetes. In any case, research suggests that the increase in LDL is in large, non-dense particles; these are less harmful for cardiovascular health than small, dense particles, which decrease on very-low-carb, high-fat diets. Longer, larger clinical trials are needed to better assess these effects.
People with kidney disease (including chronic kidney disease) should steer clear of high-protein versions of the diet because the excess protein, in addition to the increased burden of handling ketones and the associated loss of body water, could worsen their condition. They should consult their doctors before trying a ketogenic diet.
What about ketone supplements?
There’s little published evidence on the effects of dietary supplements containing ketones or ketone precursors. A small study in Obesity in 2017 found that a ketone beverage increased blood levels of ketones and reduced hunger and an appetite-related hormone (ghrelin) for 90 minutes, compared to a placebo beverage. A small study in the Journal of Physiology in 2018 found that participants did better on a fasting glucose tolerance test after drinking a ketone beverage than a placebo. Both studies involved healthy young adults and tested the beverages only for a day, so the clinical significance of the findings is unknown.
BOTTOM LINE: Research on ketogenic diets is still in its infancy, and several major trials on it are underway. Thus far, there are no long-term data on safety and efficacy. The diets eliminate too many healthful foods to be advisable for the general population-and even for people with obesity or diabetes, results are likely to be variable. Genetics, degree of insulin resistance, and many other factors may play a role.
If you are trying to lose, say, 20 or 40 pounds, the diet may help you shed them quickly, but as with any diet, the weight will likely come back when you return to your old way of eating. Ideally, after you achieve your weight-loss goals, you should transition to a healthy way of eating that you can maintain long term, such as a Mediterranean-style diet, the anti-hypertension DASH eating plan, or any plant-based diet that’s low in added sugar and refined grains.
If you have diabetes or you are obese and need to lose 50 to 100 pounds or more, you should undertake a ketogenic diet only under medical supervision in order to do it effectively and safely. It can take some training; consulting a registered dietitian may help. When combined with diabetes medications, the diet may lower blood sugar excessively (hypoglycemia), so the drugs may have to be adjusted. Doses of blood pressure drugs may also need to be adjusted.