How the ketogenic diet works

How the ketogenic diet works

To understand the ketogenic diet, you need a quick primer on how the human body gets energy. We are fueled primarily by glucose, or blood sugar, much of which we derive from carbohydrates in foods like bread, fruit, potatoes, and sweets.
If glucose levels in the blood drop to really low levels, we’d pass out and die. But, interestingly, the body can’t store much glucose — only enough to last a couple of days. So if we forgo eating carbs for a few days, we need other ways to keep going. One of those is a process called ketogenesis.
In ketogenesis, our livers start to break down fat into a usable energy source called ketone bodies, or ketones for short. “Organs like the brain that normally rely primarily on glucose for fuel can begin to use a substantial amount of ketones,” said Kevin Hall, a National Institutes of Health senior investigator who has studied the ketogenic diet. So ketones can stand in for glucose as fuel for the body when there’s a glucose shortage. “It’s an amazing physiological adaption to starvation that allows tissues like the brain to survive,” Hall added.
Once ketogenesis kicks in and ketone levels are elevated, the body is in a state called “ketosis,” where you’re burning stored fat. There are a few ways to get into ketosis. One is through fasting: When you stop eating altogether for an extended period of time, the body will ramp up fat burning for fuel and decrease its use of glucose (which is part of the reason people can survive for as long as 73 days without food).


Another way to get into ketosis is by eating less than 20 to 50 grams of carbs — or a slice or two of bread — per day. So people on a ketogenic diet get 5 percent of their calories from carbohydrates, about 15 percent from protein, and 80 percent from fat. Note that that’s a much lower ratio of protein and a lot more fat than you’d get on other low-carb diets, but it’s this ratio that will force the body to derive much of its energy from ketones. If you eat too much protein, or too many carbs, your body will be thrown out of ketosis.
In practice, that means subsisting mainly on meats, eggs, cheese, fish, nuts, butter, oils, and vegetables — and carefully avoiding sugar, bread and other grains, beans, and even fruit. Again, if this sounds familiar, it’s because it’s not that different from the Atkins diet, among the most famous very low-carb diets that promise to get your body burning fat. (Atkins, who reportedly said ketosis is “as delightful as sunshine and sex,” promised to help people “stay thin forever,” the same way the now popular Keto Reset Diet book promises to “burn fat forever.”)
While the evidence behind ketogenic diets for diabetes is still preliminary and the evidence for weight loss isn’t all that convincing (more on that next), the evidence of using the diet to treat epilepsy is extremely robust. The idea of treating people with epilepsy with the keto diet came about in the 1920s, when researchers observed that people who fasted experienced fewer seizures. (Researchers still aren’t sure why the diet can work for epilepsy, but a few mechanisms have been proposed, including making neurons more resilient during seizures.)
Today, studies have shown that children and adults whose epilepsy doesn’t respond to medications seem to experience a pretty large reduction in seizures when following a ketogenic diet. That doesn’t, however, mean that the diet works for other conditions.

The theory behind very low-carb diets is that they help people burn extra calories and fat — and lose more weight

Advocates of ketogenic diets for weight loss claim that ketogenesis can lead to a “metabolic advantage” that helps burn 10 times more fat and an extra 400 to 600 calories per day — the same as a vigorous session of physical activity. The main scientific model that’s used to explain that advantage is the ”carbohydrate-insulin hypothesis,” which has been promoted by experts like Harvard professor David Ludwig, Obesity Code author Jason Fung, journalist Gary Taubes, and pediatric endocrinologist Robert Lustig, among others.
Eating carbs drives up insulin production, the hypothesis suggests, stirring hunger and causing the body to hold on to fat and suppress calorie burn. But when you replace carbs with fat, you subdue hunger, boost calorie burn, and melt away fat. With fewer carbs, your body also doesn’t produce as much insulin — and that increases the rate of ketogenesis and decreases the body’s need for glucose.
This might sound great, but what’s often lost in all the boosterism is that this is still just a hypothesis. And most studies of ketogenic and other very low-carb diets suggest they don’t actually outperform others in the long run when it comes to weight loss.

Keto diets don’t seem to help people lose extra weight in the long run

The keto concept has been catnip for many dieters, perhaps because of the failure of the low-fat experiment of the 1980s and ’90s to help people lose weight, and the fact that food companies increasingly tell us to be suspicious of carbs and eat more protein.
When you look at head-to-head studies comparing low-carb diets to other kinds of diets, weight loss on a very low-carb diet can be a little more dramatic in the short term, but by the one-year mark, all diets perform equally miserably.
This seminal randomized trial, published in JAMA in 2007, involved 300 women and measured their weight loss on the Atkins diet compared to the Zone, Learn, and Ornish diets. The researchers found that while women on Atkins shed a few more pounds, the weight loss on low-carb diets was “likely to be at least as large as for any other dietary pattern” and “the magnitude of weight loss [on Atkins] was modest, with a mean 12-month weight loss of only 4.7 kg.” In other words, long-term weight loss on Atkins wasn’t meaningfully different from the other diets.

“The graph makes it look like the Atkins group did better but the numbers are clinically unimportant and the weight is coming back on faster,” explained the study’s lead researcher, Christopher Gardner, a professor of nutrition at Stanford.
Other big studies comparing popular diets of different macronutrient compositions, like the one I mentioned above, consistently suggest that the very low-carb approach isn’t a sustainable solution for weight loss. A review of the research on weight loss for different types of diets, published in the Lancet in 2015, found that people on low-carb diets lost 1 kilogram of additional weight after one year compared to people on low-fat diets — again, a marginal difference.
Still, in the short term, low-carb diets like keto can sometimes help people lose more weight because they cause rapid water loss, which gives people the impression they’ve lost fat. “This happens because low-carbohydrate diets deplete stored glycogen, and glycogen binds large amounts of water,” explained obesity researcher Stephan Guyenet.
Another reason very low-carb diets seem to help with weight loss initially is that there’s some evidence they’re effective for appetite control. “Most people actually eat fewer calories than they would on most other diets,” said Guyenet, adding, “The evidence supporting this isn’t great right now, but that seems to be where it’s going.”
But again, these benefits seem to disappear in the long run on average, probably because very low-carb diets — like many other fad diets — are hard to stick to. In our food environment, it’s extremely difficult to avoid eating foods like bread, cookies, or pasta for months on end. As Guyenet wrote on his blog:

The more extreme a diet, the harder it is to adhere to, and the ketogenic diet is extreme. “But wait”, you say, “I’ve been on the ketogenic diet for five years and it’s easy!” That may be true, but randomized controlled trials don’t lie. The average person can’t even stick with the diet for six months, as judged by urinary ketone levels. The minority of people who find it easy, get good results, and stick with it are the ones who write about it on the Internet.

Even people who adhere to a very low-carb diet in the short term don’t necessarily reap the benefits proponents claim they will — like the increased calorie burn and fat loss. That’s what the NIH’s Kevin Hall found in another study he designed that’s considered the most rigorous scientific test of ketogenic diets for weight loss.
For the study, he confined 17 overweight and obese patients for two months to a hospital, where researchers measured their every movement and controlled what they were eating. (Diet researchers called this study the “gold standard” since it was an extremely well-controlled experiment, with all food provided, and it used the best technologies for measuring energy expenditure and body composition.)
In the first month of the study, participants were put on a baseline diet, which was designed to be similar to what they reported they were eating outside the hospital, including lots of sugary carbohydrates. For the second month, the participants got the same amount of calories and protein as they did in the first month of the study, but ramped up the amount of fat in their food and ate far fewer carbs.