You can combine keto with intermittent fasting, and some people find it easier because keto meals keep them full for longer. But there’s no good evidence that the combination works better than either one alone. In trials, intermittent fasting hasn’t beaten ordinary calorie cutting for weight loss: a 12-month trial in the New England Journal of Medicine found no significant difference between time-restricted eating plus calorie restriction and calorie restriction alone. Fasting isn’t safe for everyone, especially people taking diabetes medicines, people who are pregnant, and anyone with a history of an eating disorder.
What intermittent fasting is
Intermittent fasting means limiting when you eat rather than, or as well as, what you eat. Harvard’s Nutrition Source describes the common types:
| Method | How it works |
|---|---|
| Time-restricted eating (such as 16:8) | Eating within a set window each day, for example 8 hours, and fasting for the other 16 |
| 5:2 diet | Eating normally 5 days a week and about 400 to 500 calories on 2 days |
| Alternate-day fasting | Alternating normal days with days of one small meal, about a quarter of normal intake |
Most people who combine fasting with keto use time-restricted eating, often skipping breakfast or eating two meals a day.
What the research shows
The evidence for intermittent fasting is less impressive than its popularity suggests:
- In the TREAT trial, 116 adults with overweight were assigned to eat within an 8-hour window or eat three meals a day for 12 weeks. Weight loss wasn’t significantly different between the groups, and the time-restricted group lost slightly more lean mass.
- In the 12-month NEJM trial of 139 adults with obesity, both groups cut calories, and adding an 8-hour eating window didn’t lead to significantly more weight loss, fat loss or improvement in blood pressure or blood sugar.
- Harvard’s review notes a one-year trial in which alternate-day fasting wasn’t better than daily calorie cutting, raised LDL cholesterol, and had a higher dropout rate.
The common message: intermittent fasting helps some people eat less, but it isn’t magic. Total intake and what you can keep doing long term matter most.
Keto and intermittent fasting have rarely been tested together, so claims that the combination “speeds up ketosis” or helps you lose more fat are mostly based on theory and personal reports.
Why some people like combining them
- Less hunger. Many people find keto meals filling, which can make a longer overnight gap feel easier.
- Simpler days. Fewer meals means less planning and cooking.
- Fewer evening snacks. A fixed eating window can stop late-night grazing, which is where many extra calories come from.
If those things are true for you, the combination may suit you. If fasting leaves you very hungry, irritable or bingeing later, it’s not a good fit.
Who should not fast
Harvard notes that intermittent fasting isn’t appropriate for people whose medicines require regular meals, such as some people with diabetes, or for people with a history of eating disorders such as anorexia or bulimia. Be especially careful, and talk to your doctor first, if you:
- Take insulin or sulfonylureas such as gliclazide or glipizide, which can cause low blood sugar if you skip meals. Our article on keto and type 2 diabetes explains how these medicines interact with low-carb eating.
- Take an SGLT2 inhibitor, because the UK medicines regulator lists restricted food intake as a risk factor for ketoacidosis with these drugs.
- Are pregnant, breastfeeding or trying to conceive.
- Are under 18, or an older adult at risk of losing muscle.
- Have a history of disordered eating. Strict rules about when you can eat can trigger old patterns. An eating disorder is serious but treatable, and if you need someone to talk to now, findahelpline.com lists free helplines in many countries.
- Take medicines that must be taken with food, or have low blood pressure, gout or kidney disease.
How to try it safely
If your doctor agrees and you want to try:
- Get used to keto first. Give yourself 2 to 4 weeks for any keto flu to settle before adding fasting. Our guide to keto flu explains what to expect.
- Start gently. A 12-hour overnight gap, such as 7pm to 7am, is a reasonable first step. Move to 14:10 or 16:8 only if it feels easy.
- Eat enough protein in your meals, so you don’t lose muscle. Our guide to keto macros has protein targets.
- Keep vegetables in every meal for fiber, potassium and magnesium.
- Drink plenty of water during the fast. Black coffee and plain tea are fine. Our article on coffee on keto covers what you can add.
- Watch your electrolytes. Early keto and fasting both increase salt and water loss. Our guide to keto electrolytes explains how to replace them safely.
- Stop if you feel unwell, dizzy, shaky or faint, and eat something.
- Keep up strength exercise, which helps protect muscle while you lose weight.
A sample 16:8 keto day
| Time | What to eat |
|---|---|
| 7am | Water, black coffee or tea |
| 12pm | Chicken and avocado salad with olive oil dressing, and a handful of nuts |
| 4pm | Greek yogurt with a few berries, or celery with nut butter |
| 7.30pm | Baked salmon with roasted broccoli and cauliflower mash |
| 8pm to 12pm next day | Fasting: water, black coffee or plain tea |
Common questions
Will fasting get me into ketosis faster?
A long overnight fast does lower insulin and raise ketone levels, so it may get you into ketosis a little sooner at the start. Once you’re eating very few carbs, ketosis continues whether or not you fast.
Does coffee with cream break a fast?
Strictly, anything with calories breaks a fast. A splash of cream has very few carbs and won’t stop ketosis, but it does add calories.
Is one meal a day okay on keto?
It’s hard to eat enough protein, fiber and nutrients in one meal, and very long gaps make hypos and dizziness more likely. Most people do better with two or three meals.
What if fasting makes me binge?
That’s a sign fasting isn’t right for you. Regular meals with plenty of protein and vegetables are a better approach.
Safety note: Keto isn’t suitable for everyone. If you have diabetes, kidney or liver disease, take regular medication, or are pregnant or breastfeeding, talk to your doctor before cutting carbs this far (why it matters).
This article is general information, not medical advice. Talk to your doctor or a registered dietitian before making big changes to how you eat, especially if you have a health condition, take regular medication, or are pregnant or breastfeeding.
