What are ketones? Ketosis explained scientifically

What are ketones?

Ketones are molecules produced in the body when fat is broken down and used as an energy source instead of carbohydrates. This happens mainly when the availability of glucose in the blood is low – for example during starvation, fasting, a low-carbohydrate diet, or in certain medical conditions such as type 1 diabetes. When the amount of ketones in the blood increases, this is called ketosis.

Ketones are produced in the liver through a process called ketogenesis, in which fatty acids are converted into three different ketone bodies: beta-hydroxybutyrate, acetoacetate, and acetone. These ketone bodies are then transported via the bloodstream to different organs, where they are used for energy – especially by the brain, which would otherwise rely on glucose. Ketones are therefore a natural and important alternative fuel for the body when carbohydrate levels are low.

Healthcare professionals often use special test strips or meters to detect ketones in blood or urine. During fasting or a low-carbohydrate diet, moderate ketosis is usually a normal state. In people with type 1 diabetes, however, high ketone levels together with insulin deficiency can be a sign of developing ketoacidosis, a serious condition. There are also other, less common medical reasons for ketones in the urine.

When does ketosis occur?

During fasting or long periods without food

‍After around 12–24 hours without food, the liver’s glycogen stores are often greatly reduced, which means the body starts producing more ketones from fat.

During a low-carbohydrate or ketogenic diet

‍On a diet very low in carbohydrates – under 50 grams per day – the body gradually shifts from mainly using glucose to using more fat and ketones for energy.

During prolonged endurance exercise

‍During longer, endurance-based exercise, muscle glycogen stores are used up. If you also eat few carbohydrates or train in a fasted state, the body may increase fat oxidation and ketone production. Short, explosive high-intensity exercise, by contrast, is fuelled mostly by glucose and does not on its own produce the same rise in ketones.

During insulin deficiency and diabetes

‍In people with type 1 diabetes – or severe insulin deficiency – ketones can be produced in large amounts if insulin becomes too low and blood glucose rises.

How do ketones affect the body?

Ketones affect the body on several levels – primarily through energy metabolism, hormones, the brain, and appetite regulation.

Metabolic flexibility

‍When blood glucose is low, many tissues can use ketones as fuel instead. The liver makes ketones from fat and they are transported via the bloodstream to, among other tissues, the brain and muscles, where they are used in cellular energy production. This allows the body to keep functioning even when carbohydrates are limited. The ability to switch between carbohydrates, fat, and ketones is called metabolic flexibility – an important part of adapting energy use to supply and situation.

An alternative energy source for the brain

‍The brain can use ketones – especially beta-hydroxybutyrate – as fuel. Studies show that ketogenic interventions are in some cases linked to improved cognitive function in certain people and in certain disease states.

Effects on appetite regulation

Research suggests that ketosis and a ketogenic diet can influence hunger and satiety hormones. For some people, ketosis is associated with reduced appetite, fewer cravings, and a lower spontaneous energy intake, which may support weight loss.

Effects on blood sugar and insulin

‍A ketogenic diet can lower blood glucose levels, reduce the need for insulin, and improve insulin resistance, particularly in people with overweight or type 2 diabetes. The effect is mainly linked to lower carbohydrate intake, reduced glucose production in the liver, and weight loss.

Fat breakdown

‍In ketosis, the body uses more fat as fuel. This can be both fat from the diet and fat stored in adipose tissue. Fat oxidation is sometimes confused with fat loss, but they are not the same. Fat oxidation only describes where energy is coming from. To reduce body weight and fat mass, you still need an energy deficit over time.

Is ketosis dangerous? Symptoms of ketoacidosis

For healthy people without diabetes, ketosis is generally a normal and harmless state. During fasting, a low-carbohydrate diet, or a ketogenic diet, the body shifts to using fat and ketones as fuel. This is sometimes called physiological ketosis and is part of the body’s normal energy adaptation.

What can be dangerous is ketoacidosis. This is an acute, serious condition in which ketone levels in the blood become very high while severe insulin deficiency causes the blood pH to fall (acidosis). This is mainly seen in type 1 diabetes, but can sometimes also occur in people with type 2 diabetes.

Symptoms of ketoacidosis may include:

• abdominal pain
• nausea and vomiting
• fatigue and feeling generally unwell
• confusion
• deep, rapid breathing
• sometimes breath that smells of acetone

If you have diabetes and experience these symptoms, especially together with high blood glucose and ketones in your blood or urine, you should always seek urgent medical attention. Ketoacidosis can require hospital treatment and can become life-threatening without treatment.

Blood ketones (mmol/L)

To confirm whether the body is in ketosis, ketone bodies need to be measured in blood, urine, or breath. The most common method is measuring ketones in the blood, reported in mmol/L.

• Below about 0.3 mmol/L – no or very low ketosis, typically corresponds to a “normal” mixed diet.
• About 0.5–3 mmol/L – mild to moderate ketosis, typical with a low-carbohydrate or ketogenic diet.
• About 3–7 mmol/L – deeper ketosis, can be seen during longer fasts or a very strict ketogenic diet in healthy individuals (physiological ketosis).

In people with diabetes: ketone levels above about 1.5–3 mmol/L should be taken seriously, particularly if blood glucose is also high. From around 3 mmol/L and above, together with low pH (acidosis), this indicates diabetic ketoacidosis, which is an acute condition.

What effects are reported during ketosis?

Research and clinical observations show that ketosis can have varying effects depending on the individual, dietary approach, and health status. Among the most studied effects are:

• improved blood sugar and insulin regulation in people with type 2 diabetes, especially in studies where a ketogenic or very low-carbohydrate diet is combined with weight loss
• reduced hunger and fewer cravings in some individuals
• reduced seizure frequency in people with treatment-resistant epilepsy

Studies also suggest that ketosis and ketogenic dietary patterns may have positive effects in metabolic diseases such as type 2 diabetes, obesity, and metabolic syndrome – at least in the short term. When the body produces ketones, fat use increases and insulin levels fall, which may contribute to more stable blood glucose and reduced strain on the pancreas. However, some improvements appear to be linked to weight loss and reduced energy intake – not solely to ketones themselves.

Ketosis is not a guarantee of weight loss, but it may help some people regulate their energy intake more easily. Effects vary between individuals, but research highlights ketosis as a possible component of a broader strategy to support metabolic health.

Is ketosis suitable for everyone?

Ketosis is a normal physiological state that the body can enter during fasting or a low-carbohydrate diet, but it is neither necessary nor optimal for everyone. Some people feel well on a low-carbohydrate or ketogenic diet, while others experience fatigue, headaches, digestive issues, or reduced exercise capacity.

In addition, a ketogenic diet is unsuitable – or should be used with great caution – in certain medical conditions, and it requires extra consideration during pregnancy, breastfeeding, and in some chronic illnesses. People with diabetes, especially those treated with insulin, or those with other serious conditions should always discuss major dietary changes with a doctor or other healthcare professionals.

Summary

Ketosis is a natural metabolic state in which the body produces ketones as an alternative fuel source to glucose. It can occur during fasting, starvation, a ketogenic diet, or when glycogen stores are depleted. Ketones can then be used as fuel by the brain and other organs, and research shows that ketosis can influence appetite regulation, insulin sensitivity, and blood sugar control.

At the same time, ketosis is not a goal in itself for everyone, and its effects vary between individuals. For most people, ketosis is primarily an example of the body’s ability to adapt and switch between different energy sources depending on the situation.