The ketogenic diet – which involves eating lots of fat, moderate protein and very few if any carbohydrates – can force the body to become a fat-burning machine as it relies on fat and protein as its main source of fuel.
This guide takes a comprehensive look at the ketogenic diet, including how it can promote weight loss, foods to eat and foods to avoid, as well as potential side effects, which include light-headedness, insomnia, and constipation.
We’ve obtained input from three top experts for this subject, including a Registered Dietitian and Certified Diabetes Educator, a Certified Holistic Clinical Nutritionist, and an exercise physiologist who has worked with members of the Olympic team.
All of our experts agree that before you try the ketogenic diet, it’s important to talk to your medical advisor, first; and throughout the process, you should have an expert on hand to make sure you’re doing it correctly to avoid any potential negative outcomes.
- What Is a Ketogenic Diet?
- Foods to Eat on a Ketogenic Diet
- Foods to Avoid on a Ketogenic Diet
- Sample Ketogenic Diet Meal Plan
- Different Types of Keto Diets
- How Does a Ketogenic Diet Help with Weight Loss?
- Scientific Studies of the Ketogenic Diet
- Ketogenic Diet Pros and Cons
- Ketogenic Diet Side Effects
- What to Do Before You Get Started with the Ketogenic Diet
The ketogenic diet is a way of eating that includes high fat (80%), high or moderate protein (15% to 20%) and low (5%) or no carbohydrates, explained Ruth Pupo Garcia, a Registered Dietitian and Certified Diabetes Educator at Adventist Health White Memorial in Los Angeles, California.
Eating this way forces the body to use fat as fuel instead of glucose – which is blood sugar.
“The high rate of fat burning causes ketones to be released and build up in the bloodstream,” Garcia said. “Every fat molecule breakdown leads to two ketone bodies being released. Often the diet includes lots of bacon, cheese, red meat and some vegetables. Usually, fruits and all starches are off limits.”
The ketogenic diet involves “drastically” reducing your carbohydrate intake and replacing that with healthy fats, meaning no trans-fat or processed fat, said Susan Pugach, a Certified Holistic Clinical Nutritionist and owner of The OC Nutritionist in Newport Beach, California.
Pugach is trained in holistic and functional nutrition and deals with the function of the body as a whole. She works with people to restore their health and return balance to their system – cardio, immune, digestive and thyroid – and uses a whole food nutrition program that incorporates organic food and herbal supplement therapy.
“Once I get them healthy, then I start to incorporate detoxification then move into a ketogenic diet lifestyle,” Pugach said. “People will come in and say they need to lose some weight, but when we dig down they’ve got a lot of things going on. Just doing the ketogenic diet doesn’t always work until you can get things like digestion healthy.”
Your body normally relies on carbohydrates for energy, said Dr. Annthea Fenwick, and owner of Achieving Fitness After 50 in Nevada City, California.
“It breaks them down into glucose, which is your main source of fuel,” Dr. Fenwick said. “When you stop eating carbs, your body turns to fat for energy in a process called ketosis. Fat becomes your fuel.”
Dr. Fenwick has more than 25 years of experience, ranging from basic fitness instruction to working with members of the U.S. Olympic team.
“I have consulted with hundreds of clients on healthy nutrition over the years and only recommend diets that have a proven scientific background,” Dr. Fenwick emphasized. “The keto diet is not one that I would recommend.”
Fat is not an efficient source of fuel, Dr. Fenwick noted.
“The body cannot convert fat to fuel as quickly it can convert carbs,” she said. “This can lead to problems with lack of energy during high-intensity exercise, in addition to numerous other issues with a lack of adequate fuel for the body’s needs.”
Eating fat is the focus of the keto diet.
“The desired ratio in the ketogenic diet is consuming 3 or 4 grams of fat for every 1 gram of carbohydrate and protein, which amounts to getting about 75-80 percent of your daily calories from fat,” explained Dr. Fenwick, adding that you can eat foods like butter, heavy whipping cream, mayonnaise, and oils.
“It’s all about that fat to carbs/protein ratio,” Dr. Fenwick noted. “Even though you’re not really counting calories, the meals have to be planned very carefully to adhere to the strict formula or you will not enter ketosis.”
Garcia said this diet is mostly comprised of meats, poultry, fish, and other proteins like cheese and eggs.
“Some ketogenic diets allow almonds and non-starchy vegetables, like kale and dark green leafy vegetables,” Garcia said.
Any carbohydrate foods, like bread, crackers, rice, and potatoes, should be avoided on the keto diet, Garcia advised.
“Most fruits are avoided as well,” Garcia said. “Avoiding fruit is not recommended by most nutrition experts, as fruits are considered a nutritious component of a healthy diet.”
Dr. Fenwick agreed that carbohydrates are avoided on the keto diet, including all breads, pastas, grains, sweets, and fruit.
“The ketogenic diet restricts the intake of carbs to as low as 2-4 percent of calories,” said Dr. Fenwick, adding that a standard diet provides about 45-55 percent of calories from carbs.
Dr. Fenwick provided the following sample menu for a person on the keto diet:
- Breakfast: eggs with olive oil and avocado on the side
- Lunch: leafy greens, salmon, nuts and olive oil
- Dinner: steak, greens, vegetables and oil
In other examples, Garcia said that breakfast may be bacon and eggs, lunch may be a salad with meat and mayonnaise, and a high-fat hamburger wrapped in lettuce can be dinner.
Pugach drinks a smoothie in the morning that she also recommends to her clients.
“I make it very high fat – I put an avocado in there and a protein powder that has a higher fat content,” she said.
Pugach noted a typical day on the keto diet might include bacon and eggs and avocado for breakfast; tuna salad with lots of greens, olive oil and feta cheese for lunch; and salmon with asparagus for dinner.
“There are vegetables higher in carbs and lower in carbs, so if you’re working with a nutritionist you can go through the different vegetables because a higher carb vegetable will take you out of ketosis, whereas a lower carb will keep you in,” Pugach explained.
She emphasized that a person taking on the keto diet should be extremely mindful.
“There are a lot of people who are not working with anyone, and they’re eating trans fats, processed fats, frozen foods, processed foods,” Pugach said. “They’re not looking to see if it’s healthy fat; and while they may be losing weight, their body isn’t healthy.”
Typically, the keto diet mostly includes higher protein and fats, Garcia said.
According to Dr. Fenwick, the ketogenic diet only varies in recommendations of how much carbs to eat per day.
“The shift from using circulating glucose to breaking down stored fat, as a source of energy, usually happens over two to four days of eating fewer than 20 to 50 grams of carbohydrates per day,” Dr. Fenwick explained. “This is a highly individualized process, and some people need a more restricted diet to start producing enough ketones.”
Pugach noted that this way of eating “is a lifestyle – not really a diet.”
“Even vegans and vegetarians can do it – but it would be harder because there’s so much animal meat and protein in order to get fat,” Pugach said. “But you’ve got things like avocado and coconut and coconut oil. There’s a lot of coconut and avocado in the diet.”
Dr. Fenwick believes a vegetarian would have a difficult time following a ketogenic diet, since non-meat protein sources, like soy and tofu, do contain trace amounts of carbohydrates.
Pugach said there’s the standard keto diet, which consists of low carbohydrates, moderate protein, and very high fat.
“I like to up the carbs with leafy greens,” Pugach noted.
Some people doing the keto diet will follow it for five days on – and two days off – with high healthy carbs, like sweet potatoes, cauliflower, and zucchini.
“Then there’s the targeted keto diet which allows you to add carbs if you’re going to be working out,” Pugach said. “Then there’s the high-protein for people that need more protein.”
People on this keto diet might go out of ketosis at times because it’s not necessarily easy.
“I advise anybody wanting to do it to work with a nutritionist because it can be complicated, and you need somebody to guide you and make sure you’re doing it right,” Pugach emphasized. “There’s a lot involved until you know what you’re doing.”
Pugach added that “we need healthy fats in the body; but those people that probably shouldn’t do it are people like elite athletes that need their carbs, or people that want to gain weight.”
Initially, you will mostly lose water weight, Dr. Fenwick said.
“We store six grams of water for every gram of carbohydrate, so when our carb stores are depleted, we drop a ton of water weight,” she explained.
In the absence of the body’s favorite fuel, which is sugar in the form of glucose, the body will turn to fat burning for energy, Garcia said.
“Burning fat will initiate weight loss,” Garcia noted. “Also, some studies suggest that since the ketogenic diet involves high fat intake, people felt more satiety and ate smaller portions of food overall, decreasing calorie intake.”
According to Dr. Fenwick, the limited number of research studies that have been attempted on the ketogenic diet did not show significantly better results than most other diets.
“Many of the individuals in the research studies were unable to complete the study due to the restrictive nature of the diet and the inability to maintain it for any duration that could prove valuable for data,” said Dr. Fenwick, adding that according to US News & World Report, keto ranked last on a list of 40 evaluated diets, tied only with the wildly unpopular Dukan diet.
Garcia noted a study conducted in 2004 by the National Institutes of Health about the long-term effects of a ketogenic diet in obese patients that showed the beneficial effects of a long-term ketogenic diet.
According to this study, it significantly reduced the body weight and body mass index of the patients – furthermore, it decreased the level of triglycerides, LDL cholesterol and blood glucose, and increased the level of HDL cholesterol.
There is evidence showing that a ketogenic diet reduces seizures in children, “sometimes as effectively as medication,” Dr. Fenwick noted.
According to Garcia, some experts use the ketogenic diet to alleviate seizures for persons living with epilepsy; and other benefits include rapid weight loss and a decrease in blood sugar levels.
Because the ketogenic diet reduces carbohydrates, it puts the body into a metabolic state called ketosis, “and your body becomes incredibly efficient for burning fat for fuel or energy,” Pugach said.
“Either we burn sugar or burn fat,” Pugach explained. “We’re used to burning sugar – we’ve been taught when you need energy, you have sugar. But this is a whole change of thought. It’s something that’s been around many years; it teaches the body to burn fat for fuel and basically it reduces the insulin levels that the body needs as a fat storing hormone. Those levels drop greatly, which turns the body into a fat burning machine.”
As far as the cons are concerned, Garcia said it’s difficult to follow long-term.
“People tend to regain weight lost, and high saturated fat intake may lead to plaque accumulation in the arteries,” Garcia said.
According to Dr. Fenwick, the keto diet has not been cleared for those experiencing kidney or liver conditions, as it could potentially affect or worsen their symptoms.
“Potential risks include kidney stones, several vitamin and mineral deficiencies, decreased bone mineral density, and gastrointestinal distress,” Dr. Fenwick warned. “While some fats can be healthy, following a high-fat diet can lead to an increased risk of bad cholesterol and heart disease.”
According to Dr. Fenwick, the following six side effects could potentially occur by incorporating the ketogenic diet:
1. Keto Flu
Keto dieters experience symptoms that can mimic the flu as their bodies adjust to the diet. The symptoms include fatigue, dizziness, lightheadedness, poor sleep, difficulty with exercise, and constipation, all resulting from extreme restriction of carbohydrates.
These symptoms usually subside after the body adjusts to relying on fat for fuel, but it can take anywhere from a few days to a few weeks.
2. It Can Throw Your Body’s Hydration Levels Out of Whack
Electrolyte imbalances can also be common if people are not aware of their hydration needs. It’s very important to stay hydrated on the keto diet, especially in the beginning.
When you limit carbohydrates, your body produces less insulin, and glycogen stores (how carbs are stored) in the muscles and liver are depleted.
For every 1 gram of glycogen that’s depleted, you lose about 3 grams of water. This causes the kidneys to flush out more water, and along with it, electrolytes your body needs like magnesium, calcium, sodium, and potassium.
Imbalanced electrolytes can lead to muscle cramps, irregular heartbeat, fatigue, cognitive distortions, and lack of body temperature control. Some people may also be more likely to get kidney stones while on keto due to inadequate hydration.
3. The Initial Weight Loss Is Mostly Water
Your kidneys flushing out all that water is the reason people see dramatic scale drop in the beginning of their keto diet.
That means you’re not actually losing fat in the first couple of weeks, but instead losing water that will come back in the form of glycogen when you start eating carbs again.
4. You Could Have Some Seriously Funky Breath
When your body goes into ketosis, it's common for the breath to start smelling like acetone because of the breakdown of acetoacetic acid. Acetone is the chemical that gives nail polish its unpleasant smell.
5. The Brain Fog You’ve Heard About Is a Real Thing
Your brain and eyes run exclusively on glucose, a simple form of carbohydrates. When you cut out carbs, your body has to “make” carbs from breaking down other parts of the body.
You will experience slower cognition, memory loss, headaches, and confusion. Those who suffer from depression and anxiety may have higher levels when not eating carbs on a regular basis.
Due to this fact, the keto diet might not be the best choice for those with mental health issues.
6. You’ll Eat Fewer Fruits and Vegetables
The ketogenic diet significantly limits your intake of fruits and vegetables and the body needs the fiber, vitamins, minerals, and other compounds in fruits and veggies.
Fiber is protective against many gastrointestinal cancers; it is also a big factor in fullness and weight loss. Constipation is very common on low-fiber diets like the keto diet.
Garcia added that most people feel light-headed or dizzy and weak during the first few days.
“People living with diabetes may have very low blood sugar levels,” Garcia said. “Constipation and insomnia also has been a reported problem as the body accumulates ketones.”
Symptoms of the keto flu occur because our bodies aren’t used to burning fat, they’re used to burning sugar, Pugach noted.
“It’s basically the water and mineral balance; your body is changing, and when you change the mineral balance you can add more salt or take a mineral supplement,” she said. “You can’t get all your nutrients from fat, so taking supplements helps a lot.”
If you still want to try the keto diet, it’s important to talk to your doctor first about your body’s nutrient needs, your cholesterol levels, and your risk of heart disease, Dr. Fenwick advised.
“A diet that has this dramatic an effect on the inner workings of your body, it’s best to equip yourself with advice of a professional,” Dr. Fenwick recommended.
People should be advised that the ketogenic diet is considered extreme, Garcia emphasized.
“Fat intake should include healthy fats, such as avocado and olive oil instead of bacon and saturated fats that can contribute to heart disease,” Garcia said. “Overall due to rapid [weight] regain, the ketogenic diet is not encouraging as a healthy lifestyle.”
For anyone starting a diet, they should ask themselves, “Can I eat like this long term?”
“If not, the chance of regaining weight is probable,” Garcia warned. “A healthy balanced diet is the best for overall health.”
People can consult with a Registered Nutritionist/Dietitian for a balanced diet that can aid in weight loss and is sustainable long-term, Garcia advised.
“If you are considering a ketogenic diet, work with an expert that can tailor the diet to be the healthiest possible,” she said.
Before you decide to take on a diet like this, consider your health, first, Dr. Fenwick emphasized.
“Diets come and go. This is not a new thing. Keto is just the latest,” Dr. Fenwick said. “A healthy dietary intake does not have to be so complicated. Balance and moderation are not bad words. You do not need to remove food groups or drastically restrict your food choices to have a healthy diet that allows you to lose weight.”
Since this diet doesn’t provide all the essential nutrients, you’d need to take supplements and multi-vitamins, Dr. Fenwick added.
“You’d also need to make sure your digestive system works properly because the diet is very low in fiber,” she recommended.
“The ketogenic diet is a very restrictive plan that most people can’t stick with and shouldn’t try for weight loss. People with heart disease or diabetes should especially stay away.”
» For Further Reading:
- Mediterranean Diet: A Comprehensive Beginner’s Guide
- Intermittent Fasting for Women: A Comprehensive Guide
- Cancer Prevention Diet: 7 Foods That Can Lower the Risk of Cancer