Keto Diet: how it works, risks and who it suits

Key takeaways

• Over 12+ months, keto led to about 0.9 kg (2 lb) more weight loss than low-fat diets on average, with higher LDL cholesterol.^[1]
• In a review of 121 trials, low-carb and low-fat diets gave similar weight loss at six months.^[2]
• The Jardiance (SGLT2 inhibitor) label lists a ketogenic diet among the triggers of ketoacidosis: ask your prescriber first.^[6]
• Common early side effects are tiredness, headache, nausea and digestive upset; low blood sugar is a risk with glucose-lowering drugs.^[5]

In short: The keto (ketogenic) diet cuts carbohydrates so low, usually to 50 grams a day or less, that the body shifts to burning fat and making ketones for fuel. Over a year or more it leads to slightly more weight loss than a low-fat diet on average, about 1 kilogram (2 pounds) in a meta-analysis, but in head-to-head trials of healthy diets the difference is small. It can raise LDL cholesterol, has side effects early on, and needs medical guidance for anyone taking insulin, sulfonylureas or SGLT2 inhibitors.

This page explains how ketosis works, what to eat, what the research shows, common side effects, medicine cautions and who keto tends to suit. It is general information, not personal medical advice. For the full range of options, see our diets hub.

How ketosis works

Normally your body runs largely on glucose from carbohydrates. When carbohydrate intake drops very low, the liver turns more fat into ketone bodies, which the brain and muscles can use for energy. This state is called nutritional ketosis. In research, a “very-low-carbohydrate ketogenic diet” usually means no more than 50 grams of carbohydrate a day. Fat makes up most of the calories, with moderate protein.

Ketosis is not a fat-melting switch. Weight loss on keto still comes from eating fewer calories than you use. Some people find they eat less on keto because the food choices are narrower; others do not, as the appetite study below shows. Our guide to the calorie deficit explains the basics, and our low-carb diet guide covers less strict versions.

Foods on a keto diet

Keto meals are built around meat, fish, eggs, cheese, non-starchy vegetables, avocados, nuts, seeds and oils. Bread, pasta, rice, potatoes, beans, most fruit, sweets and sugary drinks are cut out or kept very small. The healthiest versions lean on fish, olive oil, nuts, avocados and plenty of leafy and non-starchy vegetables rather than processed meats and butter, which matters because of the effect on cholesterol described below.

What the evidence shows

Long-term weight loss. A meta-analysis in the British Journal of Nutrition (2013) pooled 13 randomized trials with at least 12 months of follow-up comparing very-low-carbohydrate ketogenic diets with low-fat diets. People on keto lost slightly more weight, on average 0.91 kg (about 2 pounds) more. Triglycerides and diastolic blood pressure fell more and HDL (“good”) cholesterol rose, but LDL (“bad”) cholesterol also rose.

Low-carb vs low-fat in larger reviews. A 2020 network meta-analysis in The BMJ, covering 121 trials and nearly 22,000 people, found that low-carbohydrate and low-fat diets produced similar weight loss at six months (about 4.6 kg and 4.4 kg compared with a usual diet). The DIETFITS trial reached a similar conclusion at 12 months. In short, keto is one effective option, not a uniquely powerful one.

Low-carbohydrate diets4.6 kg

Low-fat diets4.4 kg

Average weight loss vs a usual diet at 6 months

Item	Value
Low-carbohydrate diets	4.6 kg
Low-fat diets	4.4 kg

Pooled estimates across trials; the two approaches gave similar results.

Source: Ge L et al., The BMJ 2020 (network meta-analysis of 121 trials) (checked on October 6, 2026)

Appetite. A tightly controlled NIH study in 2021 let 20 adults eat as much as they wanted of either a plant-based low-fat diet or an animal-based ketogenic diet, two weeks each. On the low-fat diet they ate about 690 fewer calories a day, the opposite of what the “carbs drive hunger” idea predicts. One participant left the study because of low blood sugar on the ketogenic diet.

Side effects

A 2026 review in the Annals of Medicine lists the most common side effects as short-lived “keto flu” symptoms such as tiredness, headache and nausea, plus digestive upset, needing to pass urine more often and low blood sugar. Because keto cuts many high-fiber foods, non-starchy vegetables, nuts and seeds help keep fiber up. The same review lists conditions where keto calls for particular caution or is not recommended, including pancreatitis, advanced liver or kidney disease, familial high cholesterol, gallbladder disease, pregnancy and breastfeeding, and some rare inherited metabolic disorders.

Medicine cautions: SGLT2 inhibitors and insulin

SGLT2 inhibitors. This class of diabetes and heart medicines includes empagliflozin (Jardiance). The FDA label for Jardiance lists a ketogenic diet, reduced calorie intake, missed or reduced insulin doses, illness, surgery, dehydration and alcohol abuse among the conditions that can trigger ketoacidosis, a dangerous build-up of acid in the blood that can occur even when blood sugar is not very high. Anyone taking an SGLT2 inhibitor should talk to their prescriber before starting keto.

Insulin and sulfonylureas. Cutting carbohydrates lowers blood sugar, so medicines that push blood sugar down can cause hypoglycemia. The Annals of Medicine review flags insulin and sulfonylureas, along with other glucose-lowering drugs, blood pressure medicines and diuretics, as needing particular care on keto. Doses often need review before and during the change, which is a job for your care team, never for guesswork.

If you take a GLP-1 medicine, the same review lists GLP-1 receptor agonists among the drugs to discuss; our guide to eating on a GLP-1 explains why protein, fiber and fluids matter while on these medicines.

Who keto tends to suit, and who it does not

Keto tends to suit people who like meat, fish, eggs and vegetables, do not miss bread and pasta much, and prefer a clear rule (“almost no starch or sugar”) to counting. It suits people less well if they eat mostly plant-based, have family meals built around rice, bread or beans, have high LDL cholesterol, or have any of the conditions above. Many people find a moderate low-carb approach or a Mediterranean diet easier to keep for years.

Difficulty, sustainability and cost

Keto is one of the harder diets to follow: carbohydrates hide in many foods, and social meals and travel are tricky. Food costs can run higher if meals rely on meat, fish and cheese; eggs, canned fish, frozen vegetables and nuts keep it more affordable. Because LDL cholesterol can rise, it makes sense to have blood fats checked when you start and after a few months.

Alternatives

If you want fewer carbs without strict ketosis, a low-carb diet is easier to sustain. A high-protein diet offers similar fullness with more food choice. For a broader comparison, see the scorecard on our diets hub or try Find My Options, and use the macro calculator to see what a carbohydrate target means in grams.

The approach

Very low carbohydrate (usually 50 g a day or less), high fat and moderate protein, so the body relies on fat and ketones for fuel. Weight loss still depends on eating fewer calories than you use.

What the evidence shows

A meta-analysis of 13 randomized trials with 12+ months of follow-up found 0.91 kg more weight loss with very-low-carbohydrate ketogenic diets than low-fat diets, with lower triglycerides and higher HDL but also higher LDL. A 2020 BMJ network meta-analysis (121 trials) found similar 6-month weight loss with low-carbohydrate and low-fat diets.

Foods

Emphasized

• Fish, seafood, poultry, eggs and meat
• Non-starchy vegetables such as leafy greens, broccoli, zucchini and peppers
• Avocado, olive oil, nuts and seeds
• Cheese and plain full-fat yogurt in moderate amounts

Limited

• Bread, pasta, rice and other grains
• Potatoes and other starchy vegetables
• Beans and lentils
• Most fruit
• Sugar, sweets and sugary drinks
• Beer and sweet alcoholic drinks

Protein

Protein is moderate on classic keto, with a protein food at each meal. For weight loss the overall calorie balance matters most; the protein calculator gives a daily range.

Scorecard

Evidence

ModerateNot supportedHigh

Invasiveness

NoneNoneSurgical

Medical supervision

RecommendedNoneRequired

Convenience

IntensiveSimpleIntensive

Cost

$$$$$$$$

Maintenance

HighLowOngoing

No overall score: each dimension stands on its own. Hover a value for its definition.

Advantages and limitations

Advantages

• Slightly more long-term weight loss than low-fat diets on average (about 0.9 kg)
• Clear rule that some people find easier than counting
• Lower triglycerides and higher HDL cholesterol in trials
• Filling foods for people who enjoy protein and fat

Limitations

• LDL cholesterol rose in trials
• Early side effects ("keto flu") and digestive upset
• Hard to keep up with social meals, travel and family cooking
• Needs medical guidance with insulin, sulfonylureas and SGLT2 inhibitors
• Cuts out fiber-rich foods such as beans, whole grains and most fruit

Check with a clinician first if

• People taking SGLT2 inhibitors (ketoacidosis risk; the Jardiance label lists a ketogenic diet as a trigger)
• People taking insulin, sulfonylureas or other glucose-lowering, blood pressure or diuretic medicines
• People with pancreatitis, advanced liver or kidney disease, gallbladder disease or familial high cholesterol
• People who are pregnant or breastfeeding

A starter week

Day	Meals
Monday	Eggs scrambled with spinach; tuna salad on greens with olive oil; salmon, broccoli and a side salad.
Tuesday	Plain yogurt with a few berries and walnuts; leftover salmon over greens; chicken thighs with zucchini and peppers.
Wednesday	Vegetable omelet with cheese; chicken and avocado lettuce wraps; shrimp stir-fry with cabbage and mushrooms.
Thursday	Hard-boiled eggs and cucumber; leftover stir-fry; turkey burger (no bun) with a large salad.
Friday	Yogurt and pumpkin seeds; tuna and olive salad; baked cod, cauliflower mash and green beans.
Saturday	Eggs with sautéed mushrooms; Greek salad with chicken and feta; lean steak, asparagus and side salad.
Sunday	Cottage cheese and a few raspberries; leftover chicken salad; roast chicken with roasted non-starchy vegetables.

Questions to ask a professional

• Is a ketogenic diet safe with my medicines, especially for diabetes, blood pressure or heart conditions?
• Should my medicine doses or blood sugar monitoring change if I cut carbohydrates sharply?
• Should we check my cholesterol and kidney function before I start and after a few months?
• Would a moderate low-carb or another eating pattern suit my health better?
• Which side effects should make me stop or call you?

Frequently asked questions

How many carbs can you eat on keto?

Research on very-low-carbohydrate ketogenic diets usually defines them as 50 grams of carbohydrate a day or less. Less strict low-carb diets allow more. Our macro calculator shows what different targets look like in grams.

How much weight can you lose on keto?

It varies widely. Over a year or more, keto led to about 0.9 kg (2 pounds) more weight loss than low-fat diets on average. The size of your calorie deficit decides most of the result.

What is keto flu?

A cluster of short-lived symptoms, such as tiredness, headache and nausea, that some people feel in the first days of cutting carbohydrates. If symptoms are severe or you take diabetes medicines, contact your clinician.

Is keto safe if I take diabetes medicine?

It needs medical guidance. Insulin and sulfonylureas can cause low blood sugar when carbohydrates drop, and the FDA label for the SGLT2 inhibitor Jardiance lists a ketogenic diet among triggers of ketoacidosis. Talk to your prescriber before you start.

Does keto raise cholesterol?

In trials, LDL cholesterol rose on average on very-low-carbohydrate diets, while triglycerides fell and HDL rose. Choosing fish, olive oil, nuts and avocado over processed meat and butter, and having blood fats checked, is sensible.

References

1. 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. British Journal of Nutrition, 2013. doi:10.1017/S0007114513000548 · PMID 23651522 (accessed October 7, 2026) Meta-analysis
2. Ge L, Sadeghirad B, Ball GDC, et al.. Comparison of dietary macronutrient patterns of 14 popular named dietary programmes for weight and cardiovascular risk factor reduction in adults: systematic review and network meta-analysis of randomised trials. BMJ, 2020. doi:10.1136/bmj.m696 · PMID 32238384 (accessed October 7, 2026) Meta-analysis
3. Gardner CD, Trepanowski JF, Del Gobbo LC, et al.. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults (DIETFITS randomized clinical trial). JAMA, 2018. doi:10.1001/jama.2018.0245 · PMID 29466592 · NCT01826591 (accessed October 7, 2026) Randomized trial
4. Hall KD, Guo J, Courville AB, et al.. Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake. Nature Medicine, 2021. doi:10.1038/s41591-020-01209-1 · PMID 33479499 · NCT03878108 (accessed October 7, 2026) Randomized trial
5. Dyńka D, Rodzeń Ł, Rodzeń M, et al.. The ketogenic diet is not for everyone: contraindications, side effects, and drug interactions. Annals of Medicine, 2026. doi:10.1080/07853890.2025.2603016 · PMID 41486865 (accessed October 7, 2026) Review
6. JARDIANCE (empagliflozin) prescribing information, section 5.1 Ketoacidosis. U.S. Food and Drug Administration, 2023. (accessed October 6, 2026) Drug label

Facts checked on October 6, 2026

Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.