Keto and low-carb diets: why cutting carbohydrates is not always better for your health

What happens when you cut carbohydrates?

Carbohydrates are broken down into glucose, which supplies energy to the body. Some glucose is stored as glycogen in the liver and muscles.

A ketogenic diet restricts carbohydrates enough for the liver to increase its production of ketones from fat. These ketones provide an alternative fuel for several tissues, including the brain. However, being in ketosis does not mean the body has stopped needing glucose. Some glucose is still required, including by red blood cells and for part of the brain’s energy needs. [1, 2]

Gluconeogenesis: how the body makes glucose

When less carbohydrate is available, the body maintains blood glucose partly through gluconeogenesis, meaning “making new glucose”. This happens mainly in the liver, with the kidneys also contributing.

The body can make glucose from lactate, produced during normal metabolism and recycled; glycerol, released when triglycerides are broken down; and glucogenic amino acids, the building blocks of protein.

Amino acids can come from dietary protein or from the breakdown of the body’s own proteins, including muscle. Muscle protein is first broken down into amino acids; the liver can then convert some of those amino acids into glucose. Gluconeogenesis itself is the glucose-making process, rather than the process that breaks down muscle. [3]

Does keto make the body break down muscle?

Muscle can contribute amino acids for glucose production, particularly during fasting or inadequate food intake. However, it would be inaccurate to say that everyone following a ketogenic diet continuously sacrifices muscle to fuel their brain.

As the body adapts to carbohydrate restriction, the brain uses more ketones. This reduces its glucose requirement and helps conserve body protein. Glucose production can also use dietary amino acids, glycerol and recycled lactate, so it does not rely entirely on muscle. Muscle preservation depends on factors such as adequate energy and protein intake, physical activity and the person’s health. [2–4]

The heart also needs a separate explanation. It is metabolically flexible and can use fatty acids, glucose, lactate and ketones. It does not depend solely on glucose made from muscle. [2]

The misleading claim is therefore that carbohydrates are “unnecessary” because the body no longer needs glucose. The body continues to need glucose and can manufacture it when carbohydrate intake is low. This adaptation does not, by itself, prove that avoiding carbohydrate foods is healthier.

Why restrictive keto diets can be a concern

They may raise LDL cholesterol

Some versions of keto replace carbohydrates with large amounts of butter, cream, coconut oil, fatty meat and cheese. This can substantially increase saturated fat intake.

In the Keto-Med trial, adults with prediabetes or type 2 diabetes followed both a ketogenic diet and a Mediterranean-style diet. Blood glucose control improved similarly on both, but LDL cholesterol increased on keto and decreased on the Mediterranean diet. Triglycerides improved more on keto, illustrating why one favourable result does not tell the whole health story. The trial was relatively small and short, so it cannot establish long-term heart outcomes. [5]

They can remove valuable sources of fibre and nutrients

Wholegrains, legumes and fruit provide much more than carbohydrate. Restricting these foods can make it harder to obtain enough fibre and a range of vitamins and minerals.

In the Keto-Med trial, the ketogenic diet provided less fibre and lower intakes of several nutrients, including folate, vitamin C and magnesium, than the Mediterranean-style diet. Removing nutritious foods therefore involves trade-offs, even when blood glucose or weight improves. [5]

They are not clearly superior for lasting weight loss

Low-carbohydrate diets can produce weight loss, but they do not consistently outperform more balanced approaches over time.

A Cochrane review found probably little or no difference in weight loss between low-carbohydrate and balanced-carbohydrate weight-reducing diets over follow-up periods of up to two years. This does not mean nobody benefits from reducing carbohydrates; it means severe restriction is not a requirement for successful weight management. [6]

They can be difficult to sustain

A diet needs to work beyond its first few weeks. Strict carbohydrate limits can complicate family meals, social occasions and eating away from home.

Participants in the Stanford Keto-Med study generally found the Mediterranean approach easier to maintain. A less restrictive pattern can offer many of the same benefits while allowing a wider variety of foods. [5]

Are there situations where keto is useful?

Yes. Ketogenic diets have an established medical role in treating certain forms of epilepsy. Some people with type 2 diabetes may also benefit from a supervised reduction in carbohydrate intake.

However, therapeutic use requires individual assessment. People taking glucose-lowering medicines may need medication changes, and ketogenic diets require particular caution with SGLT2 inhibitors because of ketoacidosis risk. Nutritional ketosis and diabetic ketoacidosis are different conditions, but this medication-related risk should not be overlooked. [1]

A more balanced approach to carbohydrates

For many people, improving carbohydrate quality offers a practical starting point: reduce sugary drinks and heavily refined foods while retaining suitable portions of fruit, legumes, wholegrains and vegetables.

A Mediterranean-style pattern combines these foods with olive oil, nuts, fish and other protein sources. It provides flexibility and aligns more closely with heart-health guidance than a strict ketogenic diet. Carbohydrate amounts can still be adjusted to suit individual needs, preferences and blood glucose management. [7]

The body’s ability to make glucose is a remarkable adaptation. It is not evidence that carbohydrate-rich foods lack value. A healthy eating pattern should support adequate nutrition, muscle preservation and long-term wellbeing—and remain manageable in everyday life.