A review of multiple studies in the journal Nutrients found that ketogenic diets are connected to significant reductions in total cholesterol, increases in “good” HDL cholesterol levels, dips in triglycerides levels and decreases in “bad” LDL cholesterol; there are questions as to whether diets high in saturated fat negate these benefits. The same paper reports that a ketogenic may slightly reduce blood pressure, but science is still very scant on this point.
A systematic review in 2018 looked at sixteen studies on the ketogenic diet in adults. It concluded that the treatment was becoming more popular for that group of patients, that the efficacy in adults was similar to children, the side effects relatively mild. However, many patients gave up with the diet, for various reasons, and the quality of evidence inferior to studies on children. Health issues include high levels of low-density lipoprotein (LDL), high total cholesterol, and weight loss.[23]
When a person goes off the ketogenic diet and regains much of their original weight, it’s often not in the same proportions, says Kizer: Instead of regaining lean muscle, you’re likely to regain fat. “Now you’re back to your starting weight, but you no longer have the muscle mass to burn the calories that you did before,” she says. “That can have lasting effects on your resting metabolic rate, and on your weight long-term.”

Just this week, a 25,000-person study presented at the European Society of Cardiology Congress in Munich suggested that people on the lowest-carb diets had the highest risk of dying from cancer, cardiovascular conditions, and all other causes. Another study, published this month in the Lancet, also found that people who followed diets that were low in carbs and high in animal proteins had a higher risk of early death compared to those who consumed carbs in moderation. (The opposite was true, however, for low-carb dieters who opted for plant-based proteins over meat and dairy.)
Because the main tenet of the keto diet is counting and cutting carbs — a commonly used way to control blood sugar — this eating approach has become increasingly popular among people with type 2 diabetes who are looking to lower their A1C, which is the two- to three-month average measurement of blood sugar levels. Indeed, research suggests this diet may lead to fast weight loss and potentially lower blood sugar for people with the disease. (13)
Those issues can be part of what's known as the “keto flu,” Warren says. Other side effects of the keto diet, all of which are tied to carb withdrawal, can include lightheadedness, nausea, mental fog, cramps, and headaches, in addition to tiredness. Luckily, the keto flu doesn't usually last more than a week—which is coincidentally about when people start to see the number on the scale go down, says Warren.

For endurance athletes, the transition to a ketogenic diet may reduce recovery time after training, but for casual exercisers, the transition to the ketogenic diet may make sticking with your fitness routine a challenge at first. (10) If you feel your energy levels drop too much when starting the ketogenic diet, slow down your reduction of carbohydrates, making sure to do it over time rather than all at once.
You can have a completely smooth transition into ketosis, or…not. While your body is adapting to using ketones as your new fuel source, you may experience a range of uncomfortable short-term symptoms. These symptoms are referred to as “the keto flu.” Low-sodium levels are often to blame for symptoms keto flu, since the kidneys secrete more sodium when you’re in ketosis, says Volek. A few side effects:
Fairly recently, the diet was introduced as a weight-loss diet by an Italian professor of surgery, Dr. Gianfranco Cappello of Sapienza University in Rome. In his 2012 study, about 19,000 dieters received a high-fat liquid diet via a feeding tube inserted down the nose. The study showed an average weight loss of more than 20 pounds in participants, most of whom kept it off for at least a year. The researchers reported a few minor side effects, like fatigue.
The ketogenic diet — a high-fat and very low-carb eating plan — can be tough to start. After all, it’s likely a radical departure from the way you’re eating now (a typical standard American diet is high in carbohydrates and processed foods). But many people are trying the keto diet, which puts your body in a state of ketosis. That's what happens when your body’s carb-burning switch flips to a fat-burning one, a change that can cause weight loss and has even been credited with controlling diabetes. (1)
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