Eat Fat to Burn Fat? The Skinny on the Ketogenic Diet

Last week I went to a birthday party. Andrew, the birthday boy, shared that he’d been on the ketogenic diet for 3 months. He’d lost 10 pounds and felt great. The party food was fatty and delicious–we dined on jalapeno poppers with cheddar cheese and bacon, made our own beef and pork tacos with shredded cheese, bell peppers, and onion in lettuce wraps, and at the end celebrated with a low-sugar peanut butter cake covered in whipped cream and dark chocolate. Chips and tortillas were available for the non-keto eaters, and a wonderful time was had by all.


This week I am pleased to introduce the second guest article from the group at Don’t Eat the Pseudoscience. Ari Kenney arrived at the University of Georgia after I had departed. We are connected, however, by the stories about me, for good or ill. I hope that you will be as impressed with her discussion of the ketogenic diet as I am.


The ketogenic or “keto” diet has skyrocketed in popularity over the past few years, reaching the highest interest in the summer of 2018. It is heavy in proteins and fats, encouraging users to consume meats, eggs, cheeses, fish, nuts, butter, oils, seeds, and fibrous vegetables. The goal of the keto diet is to bring carbohydrates down to less than 5% of a person’s daily calories by eliminating most grains, fruit, starchy vegetables, legumes, and sweets and replacing those calories with fat.


This isn’t the first time we’ve heard the hype about low-carb diets. The Atkins diet, which became popular back in the 70’s, recommended consuming fewer carbohydrates and replacing those calories with high protein and fat. The ketogenic diet takes it one step further by advocating for even further carbohydrate reduction and increased fat, which shifts the body’s metabolism into burning fat.

Carbohydrates are the easiest to metabolize and thus the body’s first choice for energy. During metabolism, carbs are broken down into glucose molecules which are either immediately used to fuel our tissues or converted into glycogen, a storage form of glucose in the liver and muscles. Excess glucose is stored in the body as fat. In contrast, fats are broken down into smaller molecules known as ketones and fatty acids, which are then transformed into components which can be used as energy.

While the Dietary Guidelines for Americans recommends that carbohydrates make up 45 to 65 percent of total daily calories, the ketogenic diet recommends consuming 5-10% carbohydrates, 20-25% protein, and 70-80% fat. “Going keto” or beginning a ketogenic diet means the body must switch to metabolism of fats as a supply of energy to accommodate the lack of carbs. This typically happens when one consumes only 20-50g of carbohydrates per day for two to four days. The body is flooded with ketones as it moves to a lipid metabolism, inducing a metabolic condition called ketosis or ketogenesis, which gives the diet its name.

The body experiences several changes as it shifts to accommodate the new form of calories. First, the glycogen stores in the body disappear. Glycogen is bound up in water, so dieters will find themselves needing to urinate frequently as their bodies deplete these stores. The loss of glycogen and glycogen-bound water results in immediate weight loss, which may be encouraging to dieters. In addition, the process of transitioning from a glucose metabolism to a lipid metabolism may cause symptoms that some have termed the “keto flu.”  The most common symptoms include headache, fatigue, nausea, dizziness, irritability, leg cramps, constipation, bad breath, and heart palpitations. Ketogenic diet patients reported fatigue, headache and diarrhea during their first week, peaking on day 4 and leveling off after day 7. These symptoms may be due to sugar withdrawals and electrolyte deficiency from water loss.

The ketogenic diet was originally developed in 1921 as a therapeutic diet for epileptic patients. Fasting was known to be effective in reducing seizures and doctors discovered that a very high fat, low carb diet could simulate the metabolic effects of fasting or starvation by forcing the body to use primarily fat as a fuel source. During fasting, the fatty acid breakdown from the body metabolizing fat stores produces certain ketone bodies which are able to cross the blood-brain barrier and provide anticonvulsant properties in the brain. While the exact mechanism by which this happens is unclear, some theories indicate that this change in fuel production could make brain cells more resilient in the face of metabolic demands during seizures or the presence of ketone bodies may favor the synthesis of a “calming” inhibitory neurotransmitter called GABA. Treating epilepsy with the ketogenic diet instead of anticonvulsants can relieve patients from medication with known negative cognitive and behavior effects.

Blocking glucose metabolism has been highly effective in reducing seizures in epileptic patients, and has also shown promise for promoting healthy blood sugar in type 2 diabetics in the short-term , although a review found no difference found after the first year of treatment. It can be challenging to discern dietary impact—one meta-analysis found there to be no difference in glycemic control between those on a low carb diet and those on a high carb diet. They suggested that the lack of effect may be because patients were unable to achieve the strict prescribed carbohydrate intake of as little as 20g.  The benefits from the ketogenic diet only come about when people go into ketosis and eating more than 20g a day prevents ketosis. The keto diet is also currently being investigated as a medical intervention for polycystic ovary syndrome, neurodegenerative diseases, and cancer as blocked glucose metabolism has been correlated with reduced brain and body inflammation in mice.

The current keto-hype, however, is about weight loss. The ketogenic diet claims to aid in weight loss in two ways. First, by forcing the body to burn fat, which takes more energy to digest and break down than carbohydrates. This is referred to as “increased energy expenditure”. Second, by reducing the consumption of calories through a feeling of fullness or reduced appetite due to the floating ketone bodies in the blood.

There are many promising studies that show weight loss from a ketogenic diet for up to one year.  One put participants on a strict ketogenic diet, even measuring ketones in blood/urine to validate ketogenesis. Over a 6 week period they observed mild weight loss.  Another with 132 severely obese patients found that participants on a carbohydrate-restricted diet lost more weight than those than on a calorie- and fat-restricted diet.

A third study had participants alternate between a ketogenic diet, a low-carb non-ketogenic diet, and a normal Mediterranean diet. Significant weight loss and reduction of body fat percentages were observed only during ketogenic periods compared to the two other diets. In addition, if the patients complied with the prescribed Mediterranean diet (which was relatively strict: 1800 kcal/day) during the maintenance period, no weight regain was observed at 12 months.

A big challenge in studying the ketogenic diet is poor adherence. To ensure compliance, one study confirmed 17 overweight volunteers to metabolic wards for 8 weeks. The first four weeks they ate a high-carbohydrate, high sugar diet and the second four they were switched to a carefully designed low-carbohydrate low-sugar diet. After switching to the keto diet participants immediately lost an average 3.5 pounds, which was attributed to body water loss. Over the 28 day ketogenic diet period the participants lost 4.8 pounds with 1 pound from body fat while consuming about 300 fewer calories per day. Overall, the ketogenic diet increased energy expenditure by ~100 calories/day after adjusting for body weight and composition, which was complicated by the loss of weight from water. Researchers concluded that the ketogenic diet contributed to weight loss and increases in energy expenditure that were near the limits of detection.

The initial data we have about the ketogenic diet shows positive results in weight loss, but we have less data on long-term effects of this diet with regard to losing weight and keeping it off, as well as the enduring effects on health.  In mice we have seen that a high-fat diet may cause initial weight loss and then over time rebound in weight but this has not been shown in humans. A review article published in the journal Diabetes suggests that we need additional data to draw definite conclusions about this diet, which would come from robustly controlled long-term studies (minimum of 2 years) in which carbohydrate, fat, energy and dietary fiber intake are carefully monitored along with changes in body weight.

ketooneWhile the ketogenic diet has been shown to help with weight loss, the drastic metabolic shift comes with potential negative physiological effects. One of the biggest concerns doctors cite about the ketogenic diet is the impact on liver and kidneys, as well as the small possibility of ketoacidosis which is when the blood becomes acidic due to a ketone build-up. Keto dieters are at an increased risk for developing kidney problems, potentially including kidney failure. Patients on the keto diet are typically urged to supplement with oral potassium citrate to reduce or delay the production of painful kidney stones, which occurred in 6.7% of one study’s participants. In addition, rodents put on the ketogenic diet have shown development of non-alcoholic fatty liver disease and insulin resistance, although we have not yet seen the same results in humans.

Another side-effect of the ketogenic diet is that physical activities may be more exhausting. Patients following the ketogenic diet experienced a negative impact on physical performance, including reduced endurance capacity, maximum work load and faster exhaustion. The reduction in peak power is likely due to lowered muscle glycogen stores from decreased carbohydrates. The study found that physical fitness was not impacted in a way that would impair daily life, but may be a matter of concern for competitive athletes.

One review of studies found that participants on low carb diets lost more weight compared to participants on low fat diets, but also increased their LDL cholesterol.  Increased LDL-cholesterol, or “bad cholesterol” is associated with a higher risk of cardiovascular disease. A separate meta-review concluded that low-carbohydrate, non-energy-restricted diets appear to be at least as effective as low-fat, energy-restricted diets in inducing weight loss for up to 1 year.  However, just like the previous review they noted unfavorable changes in LDL-cholesterol.

Some of the physiological repercussions of this diet may depend on what types of foods are being consumed instead of carbohydrates. A study tracking the long-term effects of dietary carbohydrates on mortality found that both high and low percentages of carbohydrate diets were associated with increased mortality, with the lowest risk observed at 50–55% carbohydrate intake. They observed that low-carb diets that replace carbohydrates with proteins and fat from animal sources were associated with higher risk of mortality compared to those that replace carbohydrates with proteins and fats from plant sources.

In a review of all current diets including Paleo, Vegan, Gluten-free, Whole30, and more, US News Health ranked the Keto diet 39th—dead last. Concerns include repercussions for cycling in and out of the diet, and that it can be especially risky for those with liver or kidney concerns, who should avoid it altogether. One major downside of the Keto diet is that it doesn’t specify the type of fats to replace carbs, despite decades of prior research indicating that animal-based proteins and saturated fats are linked to increased risk of cardiovascular disease. Another concern is that a diet containing 70% fat may be naturally lower in fruits and vegetables, which could lead to deficiency in vitamins and minerals and a decline in overall health.

In addition to the physiological concerns, there are also more practical ones. The Keto diet may be a challenge to maintain over time and may not work with everyone’s lifestyle.  To be a true Keto diet adherent, a diet must be <10% carbohydrates and >60% fat to induce and maintain the increased levels of circulating ketone bodies. To stay in a state of ketosis one must constantly maintain a certain percentage of carbohydrates, meaning that a slice of cake at a company party is enough to throw the metabolism back to burning carbs and storing fat.

A recent review paper summarizes the current state of affairs: “results regarding the impact of such diets on cardiovascular risk factors are controversial, both in animals and humans, but some improvements notably in obesity and type 2 diabetes have been described. Unfortunately, these effects seem to be limited in time, and more studies are therefore warranted to better assess the effects of a long-term ketogenic diet.”

So what should we say to the Andrews of the world, or to those who are considering making the leap? Ultimately there isn’t enough data for us to fully understand the long-term effects and it’s not clear at what point the benefits outweigh the risk, particularly to vulnerable populations or those with other health issues. While many dieters have seen drastic weight loss after the ketogenic diet, and there are many enthusiasts with and without medical credentials, the choice to make a big metabolic change should only be done after consulting with a doctor.

If you have chosen to make the leap, let us know about your experiences in the comments! Please invite me to all your fun keto parties and in the mean time I’ll be sure to eat enough donuts for both of us.

Ari Kenney is a food scientist with a BS from UC Davis and a MS from University of Georgia. Her master’s thesis was on the emotions of coffee drinking and she’s particularly interested in how people feel about their food. Doesn’t everybody have a lot of feelings about their food? She worked in product development with fruits and vegetables, as a technician in a flavor lab, and currently works as a sensory analyst at E&J Gallo Winery.

Next week: Making Sense out of Obesity and Diets to Prevent It

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