Can you do keto and intermittent fasting together?
Yes for most adults, unless you’re pregnant or breastfeeding, take diabetes medicines or have had an eating disorder. In one 3-month trial, doing both lost most: 5 kg on average. But one cholesterol line rose.
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Can you do keto and intermittent fasting together?
Yes. Keto is a very low-carb diet that makes ketones, a fuel your liver makes from fat. Intermittent fasting means you only eat in set hours.
The two overlap. Your body uses ketones for energy typically during fasting, or when you eat fewer carbs.
The most common fasting plan is 16:8. You eat all your food within 8 hours, such as 9am to 5pm, and fast for the other 16.
Some people should not try one or both. Check this list first.
| If you | What the advice says |
|---|---|
| Are pregnant or breastfeeding | Fasting is not suitable. |
| Have had an eating disorder | Fasting is not suitable. It could trigger one. |
| Are underweight | Fasting only with a health professional’s support. |
| Are a child or teenager | Fasting is not for teenagers. Low carb can affect a child’s growth. |
| Have type 1 diabetes | Diabetes UK does not recommend low carb. |
| Take insulin or other diabetes medicines | Speak to your diabetes team first. If your medicine can cause low blood sugar (hypos), cutting carbs may raise that risk. |
| Take a diabetes tablet whose name ends in -gliflozin, such as dapagliflozin | Low carb or keto raises the risk of a dangerous build-up of ketones (DKA). |
Sources: Dietitians Australia; Diabetes UK: low carb; Diabetes UK: SGLT2
Diabetes UK says to speak to your diabetes team before you try keto. If you take a gliflozin, you might need to stop it first.
Never stop or change a prescribed medicine without your prescriber.
Does keto with intermittent fasting help you lose more weight?
In one trial, yes. Researchers in China put 169 adults with metabolic syndrome (a mix of extra belly fat, high blood pressure, high blood sugar and unhealthy blood fats) on one of three plans for 3 months. Nobody was asked to count calories.
Source: He et al, 2022
One catch. The trial’s low carb meant under 130 g of carbs a day. That’s less strict than keto, which trials set at 50 g or less.
Neither half is magic on its own. A review of 99 trials found fasting works about as well as a normal calorie cut.
And keto didn’t speed up fat loss when calories stayed the same, in a small study of 17 men. So pick the version you can keep up.
Aim to lose 0.5 to 1 kg a week, the NHS pace. See fasting for weight loss to compare plans.
Does keto fasting raise cholesterol?
It can. That line is your LDL cholesterol, often called bad cholesterol. In the trial, LDL rose on average in both low-carb groups, while fasting alone improved blood fats.
Keto trials of a year or more found the same on average: a rise of 0.12 mmol/L compared with low-fat diets.
Saturated fat is the part to watch. The NHS says it can raise your cholesterol. Men should eat no more than 30 g a day, and women no more than 20 g.
So build keto meals on olive oil, nuts, avocado and oily fish, not butter, cream and fatty meat. Staying on it for months? Ask your GP about a cholesterol test, and see cholesterol results on keto.
How to do keto and intermittent fasting together
| Food plan | Carbs a day | Who says |
|---|---|---|
| Keto | 50 g or less | How trials define it |
| Low carb | Under 130 g | Diabetes UK |
| One slice of bread or an apple | About 15 to 20 g | Diabetes UK |
Sources: Bueno et al, 2013; Diabetes UK
Low carb shouldn’t be no carb. Start gently, then tighten.
Six steps for your first keto fasting week
- Cut out night-time snacks first, then move to an 8-hour window, such as 9am to 5pm.
- In your fasting hours, stick to drinks such as water, coffee, tea or sugar-free squash.
- Cut sugary drinks, cakes, biscuits, chips, white bread and fruit juice first.
- Get your carbs from vegetables, pulses, nuts and whole fruit.
- Cook with olive or rapeseed oil instead of butter, ghee or lard.
- Build up to 30 g of fibre a day to help prevent constipation.
Sources: Dietitians Australia; Diabetes UK; Diabetes UK: low carb; NHS: fats; NHS: fibre
Fasting still means thinking about what you eat in your window. For ideas, see high-protein low-carb meals and low-carb snacks. New to fasting? Start with the 16:8 guide.
Side effects of keto and fasting
Hunger is the main side effect of fasting. You may also feel weak, and find it harder to think.
Low carb can cause constipation or bad breath, which is usually temporary.
Sleep may suffer at first. In a 4-week trial of 68 young adults with overweight or obesity, keto eaten in set hours made sleep worse.
In the 3-month trial, a few people had constipation, dizziness, trouble sleeping, dry mouth or hair loss. None were serious.
If side effects keep disrupting your day, stop and get advice from a health professional.
When to speak to a GP
Book an appointment, or speak to your diabetes team, if:
- You take insulin or other diabetes medicines and want to try keto or fasting. Your team can adjust your medicines.
- Side effects of fasting keep disrupting your daily life.
- Your BMI is overweight or obese and you want help to lose weight, have diabetes, high blood pressure or high cholesterol, or think you may have an eating disorder.
Call 999 or go to A&E if you take a gliflozin and get any signs of DKA. Do the same if you have diabetes, have DKA signs and don’t know your ketone level, or if you have DKA signs and haven’t been diagnosed with diabetes.
DKA signs include feeling thirsty, peeing more, stomach pain, diarrhoea, feeling or being sick, and breathing more deeply than usual.
Others are fruity-smelling breath, like pear drops, feeling tired, sleepy or confused, and blurred vision. Do not drive to A&E.