Intermittent fasting vs keto

How they differ

At a glance	Intermittent fasting	Keto
What changes	When you eat: a daily window or fasting days	What you eat: very few carbohydrates
Food rules	None, though quality matters	Usually no more than 50 g of carbs a day 2
Ketones	Rise as a fast goes on; Ketosis typically begins around 24 hours 3	Raised most of the day
Weight evidence	Similar to daily calorie cutting 1	Slightly more than a low-fat diet over a year 2
Watch out for	Protein in a short window, hunger waves	Fibre, LDL cholesterol, eating out

What the evidence shows

For intermittent fasting, a network meta-analysis of 99 trials found that daily eating windows led to about the same weight loss as cutting calories every day ¹. For keto, a meta-analysis of 13 randomized trials lasting a year or more found that people on a very-low-carbohydrate ketogenic diet lost about 0.9 kg more than people on a low-fat diet. Their triglycerides fell and HDL cholesterol rose, but LDL cholesterol rose a little too ². Neither is magic: both mostly help you eat less, in different ways.

Can you combine them?

Yes, and many people do. A low-carb diet leaves less glycogen in your liver, and the metabolic switch to fat and ketones comes earlier when there's less glycogen to use up ³, so a fast may feel different on keto. The evidence for combining them is still thin.

We haven't found a good trial of strict keto combined with 16:8 among the studies behind Fasting Tracker's guides. If you combine them, keep protein up to protect your muscle ⁵, choose non-starchy vegetables, nuts and seeds for fibre ⁶, and remember that bulletproof coffee still ends a fast.

Ketosis is not ketoacidosis

MYTH

Making ketones while fasting or on keto is the same as diabetic ketoacidosis.

FACT

Ketones naturally rise during fasting and exercise, and on a very-low-carb diet. In healthy people, insulin keeps ketone production in check, so levels stay moderate. Ketoacidosis is a different, dangerous state that happens when the body has too little insulin, mainly in type 1 diabetes ⁷.

Losing salt and water is common in the first days of either, as insulin falls and your kidneys let go of more sodium ^8,9. A pinch of salt in water often helps with headaches, unless your doctor has told you to limit salt ¹⁰.

Who should be careful

• Anyone with diabetes, especially type 1 or on insulin or other medication that lowers blood sugar: ketones can climb to dangerous levels and doses often need changing ^7,11.
• People with kidney disease, high LDL cholesterol or heart disease: get advice and check your numbers.
• Anyone who is pregnant or breastfeeding, under 18, or has a history of eating disorders.
• Get urgent help for nausea and vomiting, stomach pain, very fast breathing or confusion, especially if you have diabetes.

Fasting Tracker shows when you may reach Ketosis during a fast, around 24 hours, with a reminder when a new stage begins. See what happens after 24 hours of fasting.

Sources

1. Semnani-Azad Z, Khan TA, Chiavaroli L, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025;389:e082007. doi.org/10.1136/bmj-2024-082007
2. Bueno NB, de Melo IS, de Oliveira SL, et al. 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):1178-87. doi.org/10.1017/S0007114513000548
3. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi.org/10.1002/oby.22065
4. Li L, Li R, Tian Q, et al. Effects of healthy low-carbohydrate diet and time-restricted eating on weight and gut microbiome in adults with overweight or obesity: Feeding RCT. Cell Rep Med. 2024;5(11):101801. doi.org/10.1016/j.xcrm.2024.101801
5. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376-384. doi.org/10.1136/bjsports-2017-097608
6. Reynolds A, Mann J, Cummings J, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393(10170):434-445. doi.org/10.1016/S0140-6736(18)31809-9
7. Laffel L. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes. Diabetes Metab Res Rev. 1999;15(6):412-26. doi.org/10.1002/(sici)1520-7560(199911/12)15:6<412::aid-dmrr72>3.0.co;2-8
8. DeFronzo RA, Cooke CR, Andres R, et al. The effect of insulin on renal handling of sodium, potassium, calcium, and phosphate in man. J Clin Invest. 1975;55(4):845-55. doi.org/10.1172/JCI107996
9. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
10. He FJ, Li J, Macgregor GA. Effect of longer term modest salt reduction on blood pressure: Cochrane systematic review and meta-analysis of randomised trials. BMJ. 2013;346:f1325. doi.org/10.1136/bmj.f1325
11. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873