The PM says fasting at the beginning of the week means he can “indulge” in sugary treats at other times.
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Rishi Sunak has said fasting once a week is “an important discipline for me” as part of a “balanced lifestyle”.
The prime minister does not eat anything for 36 hours at the start of each week, the Sunday Times reported.
Mr Sunak told the BBC’s health editor Hugh Pym: “I tend to try and do some fasting at the beginning of every week as part of a general balanced lifestyle but everyone will do this differently.”
He admitted he also has “a weakness for sugary things”.
“It means that I can then indulge myself in all the sugary treats I like for the rest of the week,” he added.
Mr Sunak has previously described himself as a “Coca-Cola addict”, revealing he prefers Mexican Coke because it is made with “cane sugar rather than high fructose corn syrup”.
Friends of the prime minister told the Sunday Times he does not eat from 17:00 on a Sunday until 05:00 on a Tuesday.
He is also a fan of exercise and is known to enjoy morning workouts on his Peloton bike.
Intermittent fasting is where you eat normally at certain times and then restrict or avoid food during other times.
There are different versions or patterns.
A popular one is the 5:2 diet where people eat a normal amount of food for five days straight and then have two days of restricted eating where far fewer calories are consumed.
Another system, called the 16/8 method, involves restricting your eating to an eight-hour period before fasting for the 16 hours in between.
Fasting allows the body to use stored body fat for energy, which can lead to weight loss.
Clare Thornton-Wood, a registered dietitian and spokesperson for the British Dietetic Association, said intermittent fasting could help people lose weight but “it needs to go hand-in-hand with eating a healthy, balanced diet for the rest of the time”.
“If you then make up those calories by eating more on other days and eat whatever you like for the rest of the week, you actually won’t lose weight overall,” she added.
Ms Thornton-Wood said people should always get medical advice before trying intermittent fasting as it is not advisable for groups including children, teenagers, anyone with a history of eating disorders, with diabetes or who is breastfeeding or pregnant.