How the most popular diets work

For a person planning to lose weight, the world of diets looks like chaos. Thankfully, there are as many diets as their names indicate.

This article briefly discusses popular weight-loss diets and what science says about them. Diets are specified not by name but by type. The names may differ, but any can be attributed to one of the groups described below.

Very low-calorie diets

This approach has been used in medical practice for several decades to treat obesity. The goal is to quickly lose the first kilograms (on average, from 1 to 2.5 per week) and preserve the muscles as much as possible.

With extreme calorie restriction, muscle loss accounts for up to 25% of total weight loss [ 1 ]. But when you combine a diet with strength training and enough protein in your diet, you can save muscle, studies have shown.

On this diet, the calorie content of food is no more than 800 kcal/day. At the same time, there is a lot of protein in food – 70-100 g / day and more. Carbohydrates range from 30 to 80 grams, while fats are limited to 15 grams. If weight loss occurs in the clinic, the food is replaced with special cocktails and bars enriched with all the necessary vitamins and minerals.

For people with clinical obesity, this diet is a potentially powerful tool that helps to lose a few pounds. This is beneficial when obesity is a health hazard and motivates people to continue losing weight when they see quick results [ 2 ]. And in type 2 diabetes, it helps to improve glucose metabolism faster. But a very low-calorie diet works well in the short term, at the beginning of weight loss, and has no benefits in long-term weight loss (a year or more).

How long can you stay on this diet? In clinical practice, doctors use 8-12 weeks, which usually results in a loss of 1.5-2.5 kg per week. After that, you should move on to a less strict calorie restriction.

It is not recommended to use such diets on your own. If the excess weight is small, it will not bring additional benefits. If you are obese, you should consult a doctor.

Low-fat diets

This is a diet in which fat accounts for 15-30% of all calories received [ 3 ]. So, with a 1400-calorie diet, that’s about 20-40 grams of fat per day. Although doctors have been recommending obese patients reduce dietary fat since the 1950s, the official US dietary guidelines only included this item in the 1980s, when enough data had accumulated on the benefits of reducing fats.

Fats are the highest calorie macronutrient at nine calories per gram of fat, so cutting them down is an easy way to make your meals less caloric and eat less.

How the most popular diets work

In a controlled study, scientists gave people food that looked the same and tasted the same but had different amounts of fat. The higher fat diet resulted in weight gain and/or less weight loss [ 45 ].

However, in the long term, “fat-free” diets do not lead to greater weight loss than conventional calorie restrictions [ 678 ]. The detected difference is often less than a kilogram and has no practical significance.

Low carb diets

Scientists still have no idea exactly what amount of carbohydrates makes a diet low-carbohydrate. The Institute of Medicine’s Food and Nutrition Board has established a “normal” carbohydrate intake range for adults as 45-65% of total calories. Therefore, anything lower can be considered low-carb.

In the scientific literature, a low-calorie diet is considered a diet in which carbohydrates account for less than 40% of energy [ 910 ]. If we talk about the number of carbohydrates in grams, this is everything that is less than 200 grams per day [ 9 ]. Some experts give figures of 50-150 grams of carbohydrates per day.

In any case, meta-analyses (analyses of the accumulated research on the topic) have compared the effect of weight loss between low-fat and low-carbohydrate diets and found no difference. The results were similar, and no metabolic benefits of a low-carbohydrate diet were found in healthy individuals [ 1112 ]. A low-carb diet only works if it allows you to restrict calories.

How the most popular diets work

There are several explanations for why it is considered especially effective.

The first is simplicity. The simpler the nutrition rules, the better and longer a person keeps to a diet and loses weight [ 13141516 ]. Therefore, “eat fewer carbohydrates” is one of the simple and understandable ways to reduce calorie intake. A person removes cupcakes, donuts, chips, white bread, and pastries from food – everything that brings a lot of calories (many of these foods are high in fat at the same time) but does not saturate well, forcing you to eat more during the day.

The second is the loss of glycogen and water on a low-carb diet, which reduces up to 3 kg on the scale, though not at the expense of fat.

The third point, which does not apply to carbohydrates but is always associated with low-carbohydrate diets, is an increase in protein in food.

Diets high in protein

A high-protein diet is considered one in which the proportion of protein is 30-60% of the total calories [ 17 ]. If measured in grams, this is 1.2–1.6 g/kg of body weight [ 18 ]. At the same time, a two-fold excess of the official norm for a sedentary person (1.6 g/kg versus 0.8 g/kg) turned out to be more effective for preserving muscles and reducing the amount of fat [ 1932 ].

It was recently found that when combined with diet and high-intensity training (interval sprints and strength training), an even higher protein content (2.4 g / kg) was even more effective for muscle growth and fat burning.

Several recent meta-analyses [ 21222324 ] support the benefits of a high protein diet for weight loss and muscle preservation on a diet.

How the most popular diets work

Another systematic review found that 2.3-3.1 grams of protein per kg of dry body weight can be recommended for lean people who are on a diet and engage in strength training.

Protein plays a big role in weight loss nutrition. Of all the macronutrients, it is energetically the most expensive: the body spends 20-30% of the calories received on digestion and assimilation (for carbohydrates, this is 5-10%, for fats – only 0-3%). In addition, protein is the most satisfying of all macronutrients and is better at helping control appetite and manage hunger. So low-carb and low-fat diets are effective in that there is usually more protein in the diet.

Keto diets

The keto diet is a diet in which carbohydrates are reduced to 50 grams daily or less. Without glucose, the body switches to an alternative fuel – the ketone body, which it produces from dietary fats and body fats if a person is in a calorie deficit.

It is believed that cutting out carbohydrates can speed up fat oxidation and fat burning because insulin does not interfere with the process. The benefits of cutting out carbs here go beyond just cutting calories.

How the most popular diets work

It is also believed that the production and use of ketone bodies create a unique metabolic state in the body, which should create the best conditions for fat loss. In support of this idea, a recent meta-analysis found that the ketogenic diet suppresses appetite better than a conventional low-calorie diet.

But despite a large number of studies on carbohydrates and their effect on weight loss, a few controlled the total amount of protein and calories for the purity of the experiment. And those who controlled showed no benefit of the keto diet for weight loss over conventional calorie restriction [ 25262728 ].

Intermittent fasting

Intermittent fasting is a temporary refusal to eat, which helps to reduce the average calorie content per week. Most common options :

Alternating days

This is the most studied option. It is allowed to eat at 12 hours and fast for 36 hours. Or eat for 24 hours and fast for the next 24 hours. Or eat for 24 hours, and eat very little for the next 24 hours (no more than 500 kcal in total). There is no full compensation for calories during “full” days, so a person is still in a calorie deficit and loses weight.

Fasting several days a week

A person does not eat or severely restrict calories two days a week. While this is effective for weight loss (by creating a total weekly calorie deficit), the scientists found no difference in weight loss between a group of people who fasted several days a week and a group who maintained a moderate calorie deficit every day for six months.

Food restrictions during the day

A fasting period of 16-20 hours is followed by eating for 4-8 hours daily. The most studied form is Ramadan, during which the believers fast from sunrise to sunset for about a month.

In general, intermittent fasting and a regular diet with a uniform calorie deficit are similar. Both lead to weight loss, but in the case of prolonged fasting, it is more likely to lose muscle along with fat.

How the most popular diets work


These are the main types of weight loss diets. They all work similarly, creating an energy deficit over time rather than through metabolic and hormonal benefits.

This is not to say that hormones are not involved – they affect appetite, eating behavior, and many other things. And the balance of calories affects the production of these hormones. These are related things.

However, weight loss always comes down to a calorie deficit in food. Neither approach has clear weight loss benefits. Overweight and obese people can choose any diet listed based on personal preferences.

A low-carb diet will be a nightmare if you are a vegetarian. A high carbohydrate diet is not the best choice if you suffer from insulin resistance. The same goes for any food intolerance.

Any good diet involves a calorie deficit and enough protein, does not throw you out of social life, allows you to control hunger, and is easy to follow and choose products.

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