What is the keto diet, and is it a safe way to lose weight?

Last Updated : 03 July 2025

Key takeaways

• Ketogenic diets are not a long-term solution to weight loss. While it may result in short-term weight loss, it’s mostly due to calorie reduction and water weight loss.
• The claim that carbohydrates, which the keto diet almost entirely restricts, lead to weight gain because they increase insulin levels is not strongly supported by scientific research. Eating more calories than the body needs leads to fat storage and weight gain over time, regardless of the nutrient source.
• The keto diet can cause deficiencies in fibre, vitamins and minerals, and increase the intake of saturated fat and cholesterol excessively. In turn, this is associated with increased risks of certain types of cancer, type 2 diabetes, cardiovascular disease, and other health risks.
• The European Food Safety Authority (EFSA) recommends adults should get around 45-60% of their daily calories from carbohydrates, of which as little as possible added and free sugars and at least 25 grams of dietary fibre, to reduce the risk of non-communicable disease and manage weight.

The ketogenic diet, often shortened to “keto”, has been promoted as an effective method that can help with weight loss, hunger control, and overall health. Some even claim that the diet can be used to treat cancer. The ketogenic diet is very low on carbohydrates, high in fat (up to 90% of total energy intake), and moderate in protein.¹ Carbohydrates come mainly from non-starchy vegetables, nuts, and seeds. Grains (e.g., bread and cereals), legumes, dairy, refined sugar, most fruits, and starchy vegetables are excluded.²

Severely lowering carbohydrate intake forces the body to switch from using glucose (from carbohydrates) to burning fat for fuel. This process is called ketosis, where the liver produces chemicals called ketones to supply energy. But is it really a healthy and effective long-term solution? We’ll start with the facts, then look at the myth, and finally explain why the myth doesn’t hold up.

Fact: healthy, long-term weight loss is best achieved through sustainable, balanced diets, not carb-cutting extremes.

Low-carbohydrate diets come with serious risks: a review of nearly 500,000 people in Europe, the US, and Japan found that people who followed a low-carb diet over a decade had a 22% higher risk of early death, 35% higher risk of heart-related death, and an 8% higher risk of cancer death compared to those following higher-carb diets.³ This association was mainly due to their food choices that were used to replace those calories (e.g., more red meat and butter, and a lack of plant foods rich in polyphenols and fibre).

While many dietary guidelines recommend limiting our intake of carbohydrates from sugar-sweetened beverages and other foods rich in free sugars, there is no need to cut out carbohydrates in the form of whole grains, fruits, vegetables, and legumes.⁴

Diets low in whole grains, are the leading dietary risk factor for ill-health, disability, and early death in Europe.⁵ Ketogenic diets exclude wholegrains almost entirely. Eating more whole grains lowers our risk of developing non-communicable diseases, such as colorectal cancer, cardiovascular disease, and type 2 diabetes. Eating as little as 50 grams of whole grains a day can make a huge difference: 25% lower risk of developing type 2 diabetes, 20% reduced risk of dying from heart disease, 12% reduction in cancer mortality, and a 15% decrease in total mortality.⁶

Keto diets are also often deficient in dietary fibre, as they significantly limit foods such as fruits, vegetables, whole grains, and legumes which are key sources of fibre. Fibre plays an important role in weight management by promoting fullness and satiety, among many other benefits^.7-11

The European Food Safety Authority (EFSA) suggests that carbohydrates should make up 45-60% of total energy intake for adults and children.¹² Ketogenic diets typically limit carbs to around 20-50 g per day¹ (to put it in context, one banana contains around 20 g of carbohydrates¹³), which is far lower than the recommended 225-300 g per day for a 2,000-kcal diet or 281-375 g for a 2,500-kcal diet. Including a variety of carbohydrate-containing foods in a diet ensures a diverse range of nutrients and maximizes health benefits, something a keto diet inherently restricts.

Myth: the keto diet is healthy and effective for long-term weight loss.

The ketogenic diet was originally developed in the 1920s to help children with epilepsy. It works in that medical context to reduce the frequency of seizures.¹⁴ But recently, it’s been repackaged as a trendy weight-loss tool and a supposed fix for chronic diseases and weight loss.

Followers of the keto diet claim that carbohydrates cause insulin levels to rise, insulin promotes fat storage and prevents fat breakdown independent of calorie intake, thus people gain weight and become hungrier. It sounds convincing. But this so-called carbohydrate-insulin model isn’t supported by science, for several reasons:¹⁵

• Fat storage can occur without dietary carbohydrates and insulin secretion is influenced by several factors beyond dietary carbohydrate intake.
• Blocking the breakdown of fat in fat cells does not really impact energy intake, resting energy expenditure, or overall body composition.
• Epidemiological data do not show a clear link between carbohydrate intake and differences in body weight and does not support the idea that dietary carbohydrates are the main driver of the obesity epidemic.
• Diet interventions have found that low-glycaemic load diets don’t generally result in significantly more weight loss compared to higher-glycaemic-load diets (diets characterised by foods with a large amount of carbohydrates per portion and which have a high glycaemic index). Low-glycaemic-load diets have also been found difficult to sustain.
• The most effective approved medications for treating obesity acutely increase insulin secretion.

Eliminating food groups unnecessarily from the diet can lead to nutrient deficiencies and create a negative relationship with food.¹ We often hear about deficiencies related to vegan and vegetarian diets because they’re restrictive, but very little is said about the deficiencies linked to cutting out nutritious carbohydrate-containing foods (such as vegetables, fruits, and whole grains) like on the keto diet. By following a keto diet, you’ll be at much greater risk of nutrient deficiencies such as vitamins B and C, selenium, and magnesium, which could lead to fatigue, muscle weakness, mood changes, and ulcers.¹⁶

Additionally, a tightly controlled study found that just four weeks on a ketogenic diet worsened key blood markers linked to heart disease, including LDL cholesterol and Apolipoprotein-B, in healthy women.¹⁷ Ketogenic diets may excessively increase saturated fat intake by displacing nutritious foods like whole grains, fruits, legumes, and certain vegetables – foods rich in fibre, vitamins, minerals, and protective compounds linked to heart and microbiome health – in favour of foods such as red (and processed) meat. High intakes of saturated fats from these foods can elevate cholesterol levels and is linked to increased risks of heart disease and colorectal cancer.^18,19

However, it’s worth noting that very low-calorie ketogenic diets (<50 g/day of carbohydrates, 1-1.5 g of protein/kg of ideal body weight, 15-30 g of fat/day, and a daily intake of about 500-800 calories) may offer effective treatment for individuals with obesity who need immediate and substantial weight loss.²⁰ In this case, the diet should be tailored to their needs, potential contraindications should be assessed, and medical surveillance is recommended. Personalization remains key to ensuring safety, avoid nutrient deficiencies, and other health complications.

Fallacy: this myth relies on “cherry-picking” science and confusing short-term effects with long-term health.

A fallacy of the keto diet is that it relies on oversimplified biology. Yes, insulin helps store fat. But that’s only part of the picture. Insulin’s main job is to move glucose from the blood into cells, where it’s used for energy or stored for later. It also signals the body to pause fat breakdown when plenty of energy is available, which is a normal, healthy process. Keto advocates often misinterpret this by claiming that any rise in insulin from eating carbohydrates automatically leads to fat gain, regardless of how many calories are consumed. However, research has demonstrated that when calorie and protein intakes are kept consistent across diets, low-carb approaches don’t result in greater fat loss than diets with higher carbohydrate contents.^21,22

The reason why some people may experience initial weight loss when cutting out carbohydrates is that they also lose water weight.¹ Among other mechanisms, restricting carbohydrates depletes glycogen stores. Each gram of glycogen is stored with approximately 3 grams of water. In turn, this glycogen with bound water will be excreted in urine, resulting in weight loss. However, this is not the same as fat loss and after a few weeks weight loss will plateau. The effect will also be reversed once the diet is stopped. People following keto diets may also inadvertently eat fewer calories (because they cut out several food groups) or adopt other healthy lifestyle changes contributing to weight loss. In fact, benefits on body weight, blood pressure, and cholesterol levels are generally not seen after twelve months of following the diet.²³

Another overlooked factor of proponents of the keto diet is the failure to distinguish between refined carbs (e.g., sweets, white breads, sugar-sweetened beverages) and nutritious and beneficial sources such as fruit, whole grains, and legumes – all of which are associated with either a neutral effect on weight or even contributing to weight loss.^24-26