Keto diet: what happens in the body and what comparisons have shown

The keto diet promises a non-standard mechanism: not counting calories, but switching metabolism to a different fuel. The mechanism actually exists and is well-described. The question that comparative studies address is different — does this switch provide a result that differs from the result of standard calorie restriction.

What is ketosis

The brain's primary fuel is glucose, and its reserves are limited: liver glycogen is depleted in about a day without carbohydrates. When carbohydrates in the diet are severely restricted—usually below 20–50 g per day—the liver begins to produce ketone bodies from fatty acids: beta-hydroxybutyrate, acetoacetate, and acetone. The brain is capable of functioning on these, covering a significant portion of its energy needs with ketones.

This is a normal physiological mechanism, not a pathology. It is the same process that occurs during prolonged fasting—there is a related article on intermittent fasting, where the pauses are shorter and sustained ketosis is usually not reached.

It is important not to confuse nutritional ketosis with diabetic ketoacidosis: in the latter case, ketone concentrations are an order of magnitude higher and are accompanied by a disruption of the acid-base balance. These are different conditions.

Why the first week looks impressive

Losing three to four kilograms in a week is a common start on keto, and fat has almost nothing to do with it. Glycogen binds water in a ratio of approximately one to three: the depletion of reserves in the muscles and liver carries away liters of fluid. Additionally, sodium excretion changes, which increases water loss.

This explains the reverse effect when carbohydrates are reintroduced: the weight returns in a few days, also not as fat. The mechanics are the same as those analyzed in the article on calories in a kilogram of fat.

What comparisons with other diets have shown

The main question is whether there is an advantage in the long run. A systematic review and meta-analysis of 38 studies with 6,499 participants compared low-carbohydrate and low-fat diets: at 6–12 months, the difference in average weight loss was about 1.3 kg in favor of the low-carbohydrate diet; it also had an advantage regarding HDL cholesterol and triglycerides, while the low-fat diet had an advantage regarding LDL and total cholesterol.

One and a half kilograms over a year is a figure that suggests comparability rather than a clear advantage. The practical conclusion is simple: it is not the composition of macronutrients that matters, but which diet a person is able to sustain. For the mechanism that works regardless of the diet's name, see the article on calorie deficit, and for an overview of popular approaches, see the article on diets.

A separate meta-analysis on the effect of a keto diet on metabolic markers in overweight people describes the same dual picture: some markers improve, others do not.

A word on cholesterol

This is the area where the divergence of results is most noticeable, and it should not be oversimplified. Regarding LDL, the data are contradictory: some studies record a decrease, others an increase. In the comparison cited above, the low-fat diet proved superior specifically for LDL and total cholesterol.

The reason for these conflicting results is partly that "keto" is not a single diet. Keto based on fish, avocado, and olive oil differs radically in fat composition from keto based on bacon, cream, and cheese. What is known about different types of fats is analyzed in the article on healthy and unhealthy fats.

Practical conclusion: when switching to such a diet, it makes sense to have a lipid profile test done before and three months after, and in cases of hereditary lipid metabolism disorders, discuss this with a doctor in advance, rather than after the fact.

Side effects and warnings

• "Keto flu." Weakness, headache, and irritability in the first few days. These are related to the loss of fluid and electrolytes and usually pass within a week.
• Constipation. A direct consequence of the disappearance of grains, legumes, and fruits from the diet—that is, the main sources of fiber. There is a separate article on the role of fiber.
• Dietary restriction. Entire food groups are removed from the diet, and along with them, some vitamins and minerals.
• Difficulty of adherence. Carbohydrates are in almost everything, including vegetables and dairy products; one can fall out of ketosis unnoticed.
• Social limitations. Dining out and family gatherings become a challenge.

Note: a keto diet is contraindicated for a number of conditions, including certain metabolic disorders and diseases of the pancreas and liver. This is a case where the decision must be made with a doctor.

Who might this be suitable for

As a weight loss method, keto is one of the working options, no better or worse than others given an equal deficit. It may be convenient for those who find it easier to eliminate an entire category of food than to limit portions: the rule of "no carbohydrates" is more achievable for some people than "a few carbohydrates."

It is poorly suited for those who train at high intensity, for whom flexibility in eating out is important, and who do not tolerate monotony well. In any case, the weight returns when returning to the previous diet—just as after any other restriction, if eating habits have not changed.

Count it from a photo

Frequently asked questions

Is the keto diet more effective than standard calorie restriction?

Not significantly. In a meta-analysis of 38 studies with 6,499 participants, the difference in average weight loss at 6–12 months was about 1.3 kg in favor of a low-carbohydrate diet. This is more of a comparability than an advantage: what matters is which regimen a person is able to sustain.

Why does weight drop so quickly in the first week of keto?

Glycogen stores are being depleted, and glycogen binds water in a ratio of approximately one to three. Additionally, sodium excretion changes. The first three to four kilograms are mostly fluid, and they return when carbohydrates are reintroduced.

What happens to cholesterol on keto?

Data are mixed: some studies record a decrease in LDL, others an increase. Compared to a low-fat diet, the latter showed an advantage in terms of LDL and total cholesterol. Much depends on the types of fats included in the diet.

What is the "keto flu"?

Weakness, headache, and irritability during the first days of the transition. These are associated with the loss of fluid and electrolytes due to glycogen depletion and usually pass within a week.

How does nutritional ketosis differ from ketoacidosis?

In concentration and consequences. In nutritional ketosis, the level of ketone bodies is moderate and the acid-base balance is not disrupted; in diabetic ketoacidosis, concentrations are an order of magnitude higher and the condition requires emergency medical care.

Read next

• Types of diets — When comparing diets in studies, an inconvenient result emerges: the difference between them is smaller than the difference between the people who follow them and those who do not.
• Calorie deficit: how to calculate it and how to hold it — A calorie deficit is the one thing without which weight does not come down.
• 16/8 intermittent fasting: what happens during the fasting window — People usually say the same thing about 16/8: at some point, the body "switches to fat," and then "autophagy kicks in.

This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.

Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.