Is ketosis dangerous? When it can be risky and the downsides of the keto diet

Introduction to the ketogenic diet

Long before keto became a trend, back in the 1920s, a ketogenic diet was used to treat severe epilepsy. In more recent years, it’s mainly been talked about as a possible strategy for weight loss and better health.

A ketogenic diet means cutting carbohydrates right down, usually to around 20 to 50 g per day, and increasing the amount of fat you eat instead. The aim is to get the body into a state called ketosis, where fat is used as the main source of energy rather than carbohydrates.

Studies suggest a ketogenic diet may help improve blood sugar control and reduce insulin resistance for people with type 2 diabetes. It can also affect hunger and fullness hormones, which may make it easier for some people to lose weight. And because it often limits lots of high-calorie, sugary foods, overall energy intake can drop, which may support weight loss and metabolic health.

So keto can help some people, but it’s not for everyone, and in certain situations it may be clearly unsuitable. So when can a ketogenic diet and ketosis involve risks?

When can a ketogenic diet and ketosis be risky?

1. If you have type 1 diabetes

If you live with type 1 diabetes, high ketone levels can be dangerous because the body doesn’t produce its own insulin. On a ketogenic diet, your insulin needs often go down, and dosing can become harder to get right. If insulin doses become too low, ketone production can increase uncontrollably and lead to diabetic ketoacidosis (DKA). This is an acute, life-threatening condition where both ketones and glucose rise in the blood.

Insulin normally acts like a brake, reducing the liver’s glucose production and limiting ketone formation. Without that brake, levels can rise quickly.

Typical symptoms include fast, deep breathing, breath that smells like acetone, nausea, stomach pain, extreme tiredness, confusion, or reduced consciousness. If you have these symptoms, seek urgent medical help.

2. If you have insulin deficiency or reduced insulin production

Some people with LADA (latent autoimmune diabetes in adults) or severe pancreatic disease can have very reduced insulin production. The same principle largely applies as for type 1 diabetes. Without enough insulin, the body can start producing a lot of ketones. On a ketogenic diet, both carbohydrate intake and insulin doses are often lower, which leaves less margin for error and increases the risk of ketones rising out of control.

3. If you have certain inherited metabolic conditions

With some rare metabolic conditions, such as fatty acid oxidation disorders, the body’s ability to use fat as fuel doesn’t work properly. You may be able to eat a certain amount of fat based on the dietary advice you’ve been given, but the body can’t cope with periods where it has to rely on fat as its main fuel source. That’s why fasting and a ketogenic diet can be risky, and should be avoided or only used under strict medical supervision.

4. If you’re seriously unwell

If you’re very unwell or your general health is significantly affected, any major diet change should be done in discussion with your doctor and a dietitian, whether it’s keto or anything else.

5. During pregnancy and breastfeeding

There isn’t enough research on how a strict ketogenic diet affects pregnancy and breastfeeding. Because a fetus and baby rely on a steady supply of energy, including glucose, keto is generally not recommended during these periods. The developing brain is especially sensitive to low energy availability, so getting enough balanced nutrition is particularly important during pregnancy and breastfeeding.

6. If you have an eating disorder, or a history of one

Strict eating rules can trigger old behaviours or worsen current difficulties. For that reason, keto isn’t recommended if you have an eating disorder now, or have struggled with one in the past.

7. If you take certain medicines

This is especially important if you use SGLT2 inhibitors (for example Jardiance, Forxiga), which can increase the risk of an unusual form of ketoacidosis even when glucose levels are relatively normal. The risk can be higher alongside a low-carb or ketogenic diet.

Other medicines, such as diuretics (water tablets) and steroids, can affect fluid balance, electrolytes and glucose levels. These may need closer monitoring, and sometimes dose adjustments, if you’re eating a ketogenic diet or are in a more pronounced ketosis.

Downsides of a ketogenic diet

1. Keto can be hard to stick with long term

A ketogenic diet means keeping carbohydrates very low every day. For many people it becomes socially and practically difficult to maintain over time. Studies often show that people’s carbohydrate intake starts to creep up after a while, suggesting adherence can be low and the diet can be hard to keep going.

2. Transition symptoms, “keto flu”

When the body switches into ketosis, you might feel tired, get headaches, feel dizzy, irritable, notice heart palpitations, feel sick, or struggle to concentrate. This is sometimes called “keto flu”. It’s common and usually passes after a few days or weeks.

3. Risk of nutrient gaps

Many carbohydrate-containing foods, like fruit, pulses, root veg, wholegrains and other grains, are also rich in important nutrients. That means keto can increase the risk of missing out on certain nutrients if it isn’t planned carefully. In particular, fibre, some B vitamins, magnesium, potassium and certain antioxidants may be lower.

4. Effects on gut health

A higher fat intake and lower fibre intake can contribute to gut symptoms like constipation, gas, bloating or diarrhoea.

5. Higher saturated fat intake can affect blood lipids

Some people see their LDL levels rise (often called “bad cholesterol”) on a ketogenic diet, especially if a large share of fat comes from saturated fat in foods like processed meats, red meat, butter, cream and coconut oil. High LDL cholesterol over time can increase the risk of cardiovascular disease. That’s why, if you do eat keto, it’s particularly important to focus on healthier fat sources like avocado, nuts, seeds and oils.

6. Effects on mood and energy

Some people feel mentally sluggish, irritable, or notice mood swings during the transition into ketosis, especially in the first few weeks. This often improves once the body adapts, but it varies between individuals and can also be influenced by sleep, stress and overall energy intake.

7. Effects on exercise performance

High-intensity exercise, like intervals, heavier strength training or team sports, mainly uses glycogen (stored carbohydrate) as fuel. On a ketogenic diet, glycogen levels are lower, which can make it harder for many people to push during tough sessions and may reduce performance.

Alternatives to a ketogenic diet

If keto feels too strict or hard to stick with, there are other approaches that can also improve health and support weight loss. A lower-carb way of eating with a slightly higher carb intake can be a more flexible option, where you cut back on sugar and refined carbs without needing to go as low as strict keto.

Whatever approach you choose, the most important thing is finding a way of eating that fits your life and that you can stick with over time.

Ketogenic diet and modern weight loss medicines

If you eat a very high-fat diet, the risk of gastrointestinal side effects from GLP-1 medicines may increase. Because of that, we at Yazen don’t recommend following a ketogenic diet alongside medication treatment.

If you still choose to try keto and you’re able to do so without side effects, we recommend focusing on healthier fat sources, protein, and vegetables and berries.

At Yazen, we don’t promote any specific diet. We focus on a healthy way of eating and living based on current nutrition recommendations. At the same time, we want you to be able to make your own choices and eat in a way that you enjoy, and that works for you day to day.

Always speak to your Yazen team before changing your diet, so they can help you make any adjustments needed based on your individual circumstances.

Discover a new approach to weight loss with medical treatment.

Summary

Ketosis is a natural state the body can enter during fasting or very low carbohydrate intake. For generally healthy people, ketosis is usually not harmful. But with type 1 diabetes, marked insulin deficiency, certain inherited metabolic conditions, pregnancy, breastfeeding, severe illness, or in combination with certain medicines, a ketogenic diet and high ketone levels can carry increased medical risks.

A ketogenic diet may improve blood sugar control and support weight loss for some people, but it can be difficult to follow long term. It can also have downsides like nutrient gaps, gut symptoms, changes to blood lipids, reduced exercise performance, and sometimes changes in energy and mood.

At Yazen, we use the current evidence and tailor nutrition support and treatment to your medical needs, goals and everyday life. The aim is to find an approach that feels safe, doable and sustainable for you.‍

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