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Dominic D’Agostino, PhD, is an associate professor in the Department of Molecular Pharmacology at the University of South Florida and a research scientist at the Institute for Human and Machine Cognition. He received his doctorate degree in Neuroscience and Physiology from the University of Medicine and Dentistry of New Jersey.
The ketogenic diet (or keto diet, for short) is a very low-carbohydrate, high-fat diet that switches the body from burning glucose (sugar) as its primary fuel to burning fat and ketones, putting the body in a state of ketosis.
The keto diet is a popular weight-loss strategy, but the benefits extend far beyond this.
Many studies show that the keto diet has been shown to effectively lead to a reduction in hyperinsulinemia1, and improvement in insulin sensitivity1.
Other benefits include thinking more clearly, having more energy throughout the day, and feeling less hungry between meals.
The truth is, there’s a lot to unpack when it comes to keto—but rest assured, you’re in the right place.
Consider this your ultimate beginner’s guide to this low-carb way of eating.
At its simplest, the ketogenic diet is a diet that allows the body to transition into and sustain a state of nutritional ketosis. This state is where the body has flipped its metabolic switch from using glucose to using fats and ketones for fuel. 
In this way, a keto diet is essentially mimicking what happens to our bodies when we fast.
In fact, the origins of the ketogenic diet date back to 1921, when Russel Wilder, M.D., of the Mayo Clinic, developed it as a more sustainable alternative to fasting for treating epilepsy.
The diet is high in fat, moderate in protein, and very low in carbohydrates.
The suggested ratios of macronutrients on a keto diet can vary among individuals but typically fall within the ranges of:
What this looks like is no more than 50 grams of total (not net) carbohydrates, roughly 1 to 1.8 grams of protein per kilograms of lean body mass, and fat to make up the remaining calories (or eaten to satiety).
What the diet does is suppress insulin, just as fasting would. The only difference is that it’s the fat from the diet fueling ketone production, whereas during starvation, ketone production is fueled by stored body fat.
That said, if you are eating a ketogenic diet in a calorie deficit or practice some variation of fasting, you will be able to tap into those fat stores.
There is no shortage of successful weight loss stories with the keto diet.
Whether it be your neighbor, co-worker, or aunt, someone you know has probably lost weight on keto and told you all about it.
Countless studies support these results, showing the diet to be an effective weight loss strategy.
In fact, a group of researchers from Cambridge University reviewed 13 randomized controlled trials comparing very-low-carb high-fat ketogenic diets (VLCKD) to low-fat (LF) diets and found that individuals following VLCKD can achieve greater weight loss than those following LF diets, and better yet, keep the weight off long term.
Initially, weight loss can be rapid. This is because when carbohydrates are removed from the diet, you excrete more fluids than usual.
Every gram of glycogen (stored glucose) holds around three to four grams of water, so when we are burning through this, we are losing all that water, too. But don’t worry; fat loss comes next!
When it comes to weight loss, the keto diet shines in its ability to provide sustainable energy and suppress appetite.
In a state of ketosis, you are no longer unhealthfully tied to glucose, needing to frequently refuel throughout the day.
In contrast, in a state of ketosis you essentially have an infinite source of fuel coming from either the fat that you eat or the fat you need to burn.
The keto diet is known for its impressive appetite control and keeping your hunger at bay. This can translate to eating less without even knowing it.
In fact, a 2015 meta-analysis2 reviewing the effects of calorie-restricted ketogenic diets on weight loss and appetite concluded that ketogenic diets are associated with greater appetite suppression, despite eating in a calorie deficit and losing weight, compared to other types of calorie-restricted diets.
In other words, the hunger pangs associated with eating less and losing weight may be blunted when done so with a keto diet.
This is why people find keto (and low-carbohydrate diets in general) sustainable: because your appetite is finally no longer controlling you; you are controlling it.
Beyond appetite, keto can help you maintain muscle while losing fat.
The keto diet does a pretty good job of preserving muscle mass3 during weight loss, improving overall body composition, and preventing metabolic rate from plummeting.
Ketones themselves have a muscle-sparing effect4, and most people trying to lose weight wants to lose fat, not muscle. This means that losing weight while in ketosis can help prevent the breakdown of our hard-earned muscle, all while losing fat.
It’s also worth noting that protein, too, plays an important role in preserving muscle mass during weight loss, so it’s recommended not to restrict protein too much.
Appetite isn’t the only thing that trends with glucose and insulin levels…energy can, too. Being reliant on glucose for fuel may translate to fading energy levels between meals.
If you are one of those people who would do anything for that post-lunch nap, the ketogenic diet may help! Energy is less likely to fluctuate throughout the day when in ketosis, again due to constant access to a superior energy source.
Lifestyle choices that improve how sensitive we are to insulin can be powerful tools. Insulin resistance is tied in one way or another to practically all modern chronic diseases, and the ketogenic diet has been shown in several studies (like this one5 in the Annals of Internal Medicine and this one6 in Diabetes Therapy) to improve glucose control by, of course, lowering glucose exposure but also by improving our sensitivity to insulin.
The brain thrives on ketones, due to a wide array of mechanisms from energy metabolism to the signaling roles of ketones.
There are many neuroprotective properties of ketones7 such as enhanced brain energy metabolism, reduced neuroinflammation, and the calming effect ketones can have on the brain.These benefits are even more pronounced as we age and may offer protection against various age-related neurodegenerative diseases.
New science is constantly emerging on how ketones may be therapeutic against various other neurological disorders, such as anxiety, depression,8 and migraines9.    
Don’t be surprised if you’ve been following a keto diet and someone tells you your breath smells a little fruity or “off.” This is actually a sign that you are in ketosis.
In fact, breath acetone is actually a good predictor of ketosis10. When we are in ketosis, a portion of the ketones gets converted into the ketone acetone, which is excreted via our breath. This is nothing to be concerned about. 
Hair loss is not a common side effect of keto, but it can happen. However, it may not be related to keto at all and is more related to rapid weight loss.
You can prevent this by avoiding extreme calorie restriction of prolonged fasts, as these can be very stressful on the body when taken too far.
Another possible cause of hair loss is mineral and vitamin deficiencies11, such as a zinc deficiency, thus consuming foods such as shellfish, seafood, and meat can be helpful.
The keto diet is actually known to benefit risk factors for cardiovascular disease.
In a study of 83 obese individuals, a long-term keto diet significantly reduced triglycerides and LDL cholesterol and increased HDL cholesterol12. In other words, the keto diet improved all measured cardiovascular disease risk factors. 
The safety concerns with a keto diet are relatively minor, and easily preventable. However, that isn’t to say that the diet is appropriate for absolutely everyone. 
Women can be more sensitive to dietary changes because of the way these adjustments affect hormones related to reproduction. The effects on women’s hormones though are more likely related to calorie restriction than to keto itself.
Living in a chronic calorie deficit, paired with increased exercise and weight loss, can shift hormones so dramatically that it can affect menstruation. It is especially important for women to maintain proper calorie consumption and to discontinue the diet if menstruation stops.
It is also up for debate whether keto is suitable during pregnancy and breastfeeding. Despite low-carb diets being shown to benefit fertility13 and ketosis being a natural state many women enter during pregnancy14, we recommend consulting with a medical professional or a keto-savvy functional medicine doctor.
The pros of the keto diet certainly outweigh the cons, but that isn’t to say that the diet is for everyone.
Most of the side effects of keto are only short term and will likely resolve with time and proper keto-adaptation. But if there are too many signs suggesting that keto isn’t for you, it’s worth recognizing that many benefits can be derived from a less strict low-carb diet in which ketosis isn’t necessarily the goal.
You won’t be getting the boosted brainpower from ketones, but you’ll probably still experience improved blood sugar levels, metabolic flexibility, and better control of your appetite, too.
Since its introduction in 1921, the ketogenic diet has evolved into a number of different diets that can be used to achieve ketosis, so by definition they are all ketogenic diets.
Here are some of the most common meal plans on a keto diet:
The classical ketogenic diet is about 90% fat, 6% protein, and about 4% carbs, also referred to as a 4:1 keto diet, because it is four parts fat to one part carbs and protein (by weight). This was the first keto diet to be used therapeutically in the treatment of pediatric epilepsy. The classical keto diet is very strict and often difficult to follow.
The modified keto diet is much more liberal with protein and carbs than the classical keto diet and can contain anywhere from 65 to about 85% fat, ~15 to 35% protein, and  ~zero to 10% carbs, which is much more liberal with protein and carbs than the classical keto diet. This diet can make keto considerably more sustainable than the classical keto diet. 
The modified Atkins diet is a keto version of the traditional Atkins diet, adjusted to include a little more fat and less protein while still restricting carbohydrates. It works out to be ~55% fat, ~30% protein, and ~5% carbs. 
The MCT-based keto diet is essentially a keto diet that uses medium-chain triglycerides (MCTs) as the primary source of fat. MCTs are a ketogenic fat that can help make achieving ketosis easier while allowing for more liberal amounts of protein and carbohydrates in the diet.
A cyclical keto diet involves following a standard keto diet for most days and periodically introducing carbs (~50 to 100 grams/day) for one or two days out of the week, as carbohydrate “refeeds.” This isn’t a full-blown cheat day but rather increasing carbs slightly. Maybe you use these increased carb days strategically around training, or maybe having less strict days makes keto more of a sustainable lifestyle. Additionally, short-term “carb-ups” can help increase metabolic flexibility by training your body to go in and out of ketosis and also can be used strategically to support female hormones.
Keto paired with intermittent fasting can work very well together and create a nice symbiotic relationship. In essence, since the diet is so satiating, it becomes easier to go extended lengths of time without food making intermittent fasting easier, and since fasting is the surest way to enter ketosis, intermittent fasting can make entering and sustaining a state of ketosis easier. 
Keto can be followed with any dietary restrictions, including vegetarianism. In fact, a vegetarian keto diet is quite easy to follow, especially if you are still consuming eggs and dairy—two very ketogenic foods! There are also various sources of healthy plant-based fats that can be included on a keto diet and can make a vegetarian keto diet accessible. 
Here are the basic keto-friendly foods you want to focus on with a ketogenic diet to provide your body with ample fat, some protein, and minimal carbs (while still getting in plenty of fiber).
Avoiding these foods is key to keeping your carb count low enough to enter a state of ketosis. 
Ketosis is the metabolic state defined by an elevation of blood ketone bodies.Clinically, it is defined as an elevation of the primary ketone body, beta-hydroxybutyrate (BHB), in the blood at levels at or above 0.5 mM.
How long it takes to enter a state of ketosis can vary from person to person, but here is a typical timeline: 
Fuel source: Glucose from circulation and previously consumed carbohydrates
Fuel source: Glucose from liver glycogen (the storage form of glucose)
Fuel source: Glucose from amino acids and/or dietary protein (gluconeogenesis)
Fuel source: Ketones from stored body fat and/or dietary fat (ketogenesis)—this stage overlaps with gluconeogenesis as the body progressively shifts toward ketosis
Fuel source: Ketones from stored body fat and/or dietary fat (ketosis)—glucose utilization diminishes, and ketones and free fatty acids become the body’s major fuel source.
If you want to know if your body is truly in a state of nutritional ketosis, you have a few options for testing. That said, definitely do not stress over ketone readings—try to judge how the keto diet is working for you based on how you feel!
There really is no right or wrong way to start a ketogenic diet. You can ease into it, eliminating carbohydrates from the diet slowly or dive in headfirst and cut them out entirely.
People tend to overthink the ketogenic diet, especially if you are switching from a very carbohydrate-heavy diet. But you should get the hang of it pretty quickly by following these tips:
As you can see, the keto diet has legitimate health benefits and doesn’t have to be overly complicated or restrictive (there’s plenty of room for loads of fibrous veggies on your plate).
It also makes sense from an evolutionary perspective. But the truth is, everyone is different, and because of this, you may find you need to tweak your keto approach slightly after evaluating how you feel on a more conventional ketogenic diet.
For example, women may want to practice carb cycling once a month to support healthy hormone production and minimize side effects.
And, as always, if you have any concerns or are dealing with a specific health condition, talk to your doctor or a registered dietitian (ideally one who is well versed in using low-carb diets with their patients) before implementing a ketogenic diet.
Kristi Storoschuk is a PhD student in the Muscle Physiology Lab at Queen’s University in Ontario, Canada where she is studying the effects of fasted exercise on metabolic flexibility and blood sugar control. Outside her research Kristi works in science communication with a focus on ketogenic diets, exercise, and metabolic health. When she’s not writing and researching, you’ll find her in her gym doing CrossFit, traveling the world, or in the kitchen cooking healthy eats.
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