The short answer
- For most healthy adults, keto is safe in the short term, but it's hard to stick to and long-term safety data are limited.
- It can lead to slightly more weight loss than a low-fat diet, but it often raises LDL ("bad") cholesterol.
- It's a proven medical treatment for some people with epilepsy that doesn't respond to medication.
- It isn't recommended for some groups, including people taking certain diabetes medications, people who are pregnant or breastfeeding, children, and anyone with a history of disordered eating.
The ketogenic (keto) diet is one of the most popular ways to lose weight, but it's also one of the most restrictive. As a dietitian, here's what the research says about who it may help, the side effects to expect, and who should steer clear.
What is the keto diet?
Keto is a very low-carbohydrate, high-fat way of eating. Carbohydrate is usually kept under about 50 g a day, which means cutting back hard on bread, grains, legumes, starchy vegetables, fruit and sugar.1 With so little carbohydrate, your liver starts making ketones from fat to use as fuel. This is called nutritional ketosis.
Keto was first developed as a medical treatment for epilepsy. A Cochrane review found it can reduce seizures in children whose epilepsy doesn't respond to medication, but side effects such as gut symptoms and higher cholesterol were common.2 In this setting it's always run by a medical team.
Does keto work for weight loss?
It can, but the advantage is small. A meta-analysis of 13 trials lasting a year or more found people on a very low-carb keto diet lost about 0.9 kg more than those on a low-fat diet. Their triglycerides and HDL ("good") cholesterol improved, but their LDL cholesterol went up.1
A 2023 review that graded all the trial evidence found moderate-quality evidence that keto lowers body weight and HbA1c (a measure of long-term blood glucose). It also found high-quality evidence that keto raises LDL cholesterol by a clinically meaningful amount. Most trials lasted only about three months, so we don't know whether these changes affect heart attacks or other long-term outcomes.3
You may see keto promoted for many other conditions, from depression to cancer. These are being studied, but the evidence isn't strong enough to recommend keto for them.4
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Find a weight management practitioner →What are the side effects of keto?
The first few weeks are often the hardest. Many people get "keto flu" as their body adapts. In a study of online reports, the most common symptoms were headache, tiredness, nausea, dizziness, "brain fog" and gut discomfort. Symptoms were most common in the first week and usually eased after about four weeks.4
Other things to be aware of:
- Higher LDL cholesterol. This is the most consistent downside in the research.3 If you have high cholesterol or heart disease, talk to your practitioner before starting.
- Not enough fibre. Australian adults need about 25 g (women) to 30 g (men) of fibre a day.7 That's hard to reach on keto, because the foods it restricts (whole grains, legumes and fruit) are some of our best sources. Fibre feeds your gut bacteria and helps prevent constipation.
- Changes in thyroid hormones. In a small trial of 11 healthy adults, three weeks of keto lowered levels of the thyroid hormone T3. What this means for health isn't yet clear.5
- Hard to sustain. Cutting out whole food groups makes keto hard to stick to, and weight is often regained when people stop.
Who should not do the keto diet?
Diabetes Australia advises that low-carbohydrate eating isn't recommended for:6
- people taking SGLT2 inhibitors (a type of diabetes medication), because it may increase the risk of ketoacidosis;
- people who are pregnant or breastfeeding;
- children and teenagers under 18;
- anyone with a current or past history of disordered eating;
- people with kidney or liver failure, or who are at risk of malnutrition.
If you use insulin or medications such as sulfonylureas, cutting carbohydrate can cause low blood glucose (hypos). Don't start keto without your practitioner's help to adjust your medication.6 If you've had your gallbladder removed or have trouble digesting fatty foods, check with your practitioner first too.
Ketosis isn't the same as ketoacidosis
Nutritional ketosis on keto produces modest ketone levels. Ketoacidosis is a dangerous build-up of ketones that mostly happens in people with diabetes. If you have diabetes and feel very unwell, with nausea, vomiting, stomach pain or fast breathing, get urgent medical help or call 000.
Should you try keto?
There's no single best diet for weight loss. The best approach is one you can keep up, that gives you enough nutrients, and that suits your health. Lasting change usually comes from nutrition, movement, sleep and stress all working together.
If you're thinking about keto, or have tried many diets with little success, a dietitian or nutritionist can check whether it's safe for you and help you find an approach that fits your life.
Thinking about keto?
A HealthBank dietitian or nutritionist can tell you whether keto is safe for you and build a plan you can stick with.
Find a dietitian or nutritionist →Frequently asked questions
Is the keto diet safe?
For most healthy adults it's safe in the short term, but it often raises LDL cholesterol and long-term safety data are limited. Some groups, such as people taking SGLT2 inhibitors or who are pregnant, shouldn't follow it.
How much carbohydrate can you eat on keto?
Most keto diets keep carbohydrate under about 50 g a day. The exact amount needed to stay in ketosis varies from person to person.
How long does keto flu last?
Symptoms are most common in the first week and usually ease within about four weeks.
Is keto better than other diets for weight loss?
Long-term trials show only slightly more weight loss than low-fat diets, about 0.9 kg. The diet you can stick to matters more.
Can I do keto if I have diabetes?
Only with your practitioner's guidance. Keto can cause hypos if you use insulin or sulfonylureas, and it isn't recommended if you take SGLT2 inhibitors.
References
Research (PubMed)
- Bueno NB, de Melo IS, de Oliveira SL, da Rocha Ataide T. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. Br J Nutr. 2013;110(7):1178–1187. PubMed 23651522
- Martin-McGill KJ, Bresnahan R, Levy RG, Cooper PN. Ketogenic diets for drug-resistant epilepsy. Cochrane Database Syst Rev. 2020;6(6):CD001903. PubMed 32588435
- Patikorn C, Saidoung P, Pham T, et al. Effects of ketogenic diet on health outcomes: an umbrella review of meta-analyses of randomized clinical trials. BMC Med. 2023;21(1):196. PubMed 37231411
- Bostock ECS, Kirkby KC, Taylor BV, Hawrelak JA. Consumer reports of "keto flu" associated with the ketogenic diet. Front Nutr. 2020;7:20. PubMed 32232045
- Iacovides S, Maloney SK, Bhana S, Angamia Z, Meiring RM. Could the ketogenic diet induce a shift in thyroid function and support a metabolic advantage in healthy participants? A pilot randomized-controlled-crossover trial. PLoS One. 2022;17(6):e0269440. PubMed 35658056
Australian guidelines
- Diabetes Australia. Low carbohydrate eating for people with diabetes: position statement. 2018 (updated 2019). View source
- NHMRC. Nutrient reference values for Australia and New Zealand: dietary fibre. View source
This article is general information and isn't a substitute for personalised advice. Speak to your practitioner about your situation.
Robbie is a Sydney-based Accredited Practising Dietitian (APD) and Exercise Scientist with over 17 years of experience. He holds a Bachelor of Exercise Science (Nutrition) and a Master of Nutrition and Dietetics, and is the Director and Co-founder of HealthBank.