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Fasting diets explained: what are they and do they work?
Written byLiz Connor
Reviewed byClementine Vaughan RD
Weight loss has become just as much about when you eat as what you eat. But is the latest wellness fad something you should try?
Summary
1What is fasting?
It might be a trend you've seen recently doing the rounds on your TikTok feed but it's actually been around for ages. Fasting is...
2What are the three main types of fasting diets?
You might have seen a few different fasting approaches cropping up online. There are a handful to get to know, one...
3What does science say about fasting?
Studies on fasting are limited, there’s some promising research to back its potential health benefits but there’s always some...
With celebs like Jennifer Aniston, Chris Hemsworth and Halle Berry swearing by it, and a slew of ‘what I eat in a day’ TikToks raving about the benefits, fasting has quickly become one of the most talked-about ways to lose weight.
But let’s clear one thing up straight away: you should never feel pressured to jump on a diet just because it’s trending on social media, or lose weight to fit a certain ‘bikini body’ ideal.
That said, if you're looking to shed for health or personal reasons, that’s totally valid and there are safe, dietitian-approved ways to go about it.
So, is skipping breakfast one of them? We asked Clementine Vaughan, a qualified dietitian, to demystify the trend once and for all…
What's fasting?
At its core, fasting is the act of intentionally going without food for a set period of time, usually anywhere from 12 to 24 hours.
Depending on the type of fast you’re undertaking, drinks like water, black coffee or herbal tea are often still allowed.
While it might be a trend you've seen recently doing the rounds on your TikTok feed, intermittent fasting has actually been around for ages, since as far back as the 5th century BC.1
It’s woven into nearly every major religion, from Buddhism and Christianity to Hinduism, Islam and Judaism.1 During Ramadan, for example, many Muslims fast from dawn until sunset each day for a month.
In recent years though, fasting has carved out a space in the wellness landscape, with leading experts like podcaster Dr Andrew Huberman advocating for its potential benefits, which we’ll dive into shortly…
The three main types of fasting diets
You might have seen a few different fasting approaches cropping up online. There are a handful to get to know.
- Intermittent fasting: this is the most widely known approach and comes in a few guises. There’s the 16:8 method, which involves fasting for 16 hours a day and eating within an eight-hour window.2 Or you may have heard of the 5:2 method, which means eating normally five days a week and restricting calories on two non-consecutive days2
- Alternate-day fasting: as the name might suggest, this type of diet involves alternating between days of normal eating and days of fasting. On fasting days, some people avoid food entirely, while others opt for a very limited calorie intake
- Time-restricted eating: this is a more structured daily routine where all eating happens within a set window of time, for example, between 10am and 6pm
What does science say about fasting?
Studies on fasting are limited. There’s some promising research to back its potential health benefits but there’s always some contradictions. Let’s break it down together.
Firstly, according to some research, when you skip brekkie in the morning, for example, it puts your body into a state of ketosis – the term to describe relying on body fat for energy rather than the usual glucose from your toast or porridge.
However, there’s a bit of contention in the literature, as other studies3 suggest that calorie-restrictive diets are more beneficial than time restriction when it comes to losing weight.
“There’s also some promising evidence4 around fasting insulin sensitivity in men with prediabetes, with those following an IF diet improving blood sugar control,” Clementine adds.
“One clinical trial looking at intermittent fasting also found positive shifts in cardiovascular markers, such as higher HDL (the ‘good’ cholesterol), lower LDL and reduced triglycerides,” she adds.
Plus, much of the fasting research so far has been done in animals and many studies look at more extreme 5:2 diets, rather than more accessible fasts.
Added to this, researchers traditionally haven’t taken into account the impact fasting can have on female hormones, although this topic is now finally coming to the fore.
Recent studies5 have suggested that fasting may have a detrimental effect on the delicate hormonal balance of oestrogen and progesterone, leading to potential issues with conceiving naturally. However, more research is needed for any definitive conclusions.
Intermittent fasting might have less to do with fasting itself and more to do with how it quietly reshapes our eating habits. When people shorten their eating window, say to eight or 10 hours, they often end up eating less overall. This isn’t always because they’re consciously cutting calories, but simply because there's less time to snack, graze or reach for that “just one more” after dinner treat.
It’s a subtle shift that can lead to a big impact. Without strict calorie counting or meal plans, intermittent fasting naturally nudges some people toward lower calorie intake, making it a sneaky but effective strategy for weight management.
Who should (and shouldn’t) consider fasting?
“While fasting is safe for most people, it really isn’t suitable for everyone,” warns Clementine.
“Pregnant or breastfeeding women, people at risk of certain conditions like hypoglycemia and type 1 diabetes, and growing children and teenagers shouldn’t be following time restrictive eating patterns.”6
“Likewise, anyone with a history of eating disorders7 needs to be really careful about limiting food intake for long periods of time, as food rules can be triggering and it can be dangerous if it's taken too far,” she adds.
The bottom line? “Always speak to your GP before starting a fasting diet to check if it's suitable for you,” says Clementine, “and consider scaling back if you feel dizzy, irritable or unwell.” Plus, it’s always worth speaking to a healthcare professional as these can be symptoms of something else.
Three fasting myths busted
- Fasting boosts your metabolism
“There isn’t specific evidence to say that you can burn more calories or speed up your metabolism with intermittent fasting,” Clementine clarifies. What it does do is encourage that metabolic switching action we mentioned earlier – helping your body shift between using carbohydrates to your body’s stored fat for energy. - You can eat anything you want and still lose weight
Just because you’re fasting for part of the day doesn’t mean you can gorge on junk foods and expect to feel your best. As Clementine explains, “You can’t fast a bad diet”. During your eating windows, focus on nourishing meals packed with protein, complex carbs and plenty of fruits and veggies to support your health. - Fasting causes you to lose muscle
There’s a whispered fear in gym circles that fasting will automatically lead to muscle loss. But Clementine says the truth is, “If you eat adequate protein in your open window and continue with resistance exercises, like progressively lifting heavier weights, there’s no reason you should lose muscle while fasting.”
The final say
When it comes to shedding pounds, the science is clear: being in a calorie deficit is what really moves the needle. That means burning more energy than you take in – not relying on trends or fads, but focusing on the basics.
If weight loss is your goal, you're better off making small, manageable changes that stick. Even something as simple as cutting 100 calories a day, that’s just one less biscuit or a smaller handful of nuts, can lead to sustainable results over time.
Want to know how many calories you should be aiming for? The NHS calorie guidelines are a great place to start.
Up next: is bone broth worth the buzz? We asked an expert…
This article provides informational advice and is not a substitute for medical care. Curated by experts for accuracy, we take great care to ensure the information is up-to-date and relevant. However, you should always consult your GP or healthcare professional before using supplements or alternative products, particularly if you have medical conditions or are under supervision.
1. Brogi S, Tabanelli R, Puca S, Calderone V. Intermittent Fasting: Myths, Fakes and Truth on This Dietary Regimen Approach. Foods [Internet]. 2024 Jan 1;13(13):1960. Available from: https://www.mdpi.com/2304-8158/13/13/1960
2. Patterson RE, Laughlin GA, LaCroix AZ, Hartman SJ, Natarajan L, Senger CM, et al. Intermittent Fasting and Human Metabolic Health. Journal of the Academy of Nutrition and Dietetics [Internet]. 2015 Aug;115(8):1203–12. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516560/
3. O’Connor SG, Boyd P, Bailey CP, Shams-White MM, Agurs-Collins T, Hall K, et al. Perspective: Time-Restricted Eating Compared with Caloric Restriction: Potential Facilitators and Barriers of Long-Term Weight Loss Maintenance. Advances in Nutrition. 2021 Jan 19;12(2).
4. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism [Internet]. 2018 Jun;27(6):1212-1221.e3. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5990470/
5. Mao L, Liu A, Zhang X. Effects of Intermittent Fasting on Female Reproductive Function: A Review of Animal and Human Studies. Current Nutrition Reports. 2024 Sep 25;
6. News in Health. To Fast or Not to Fast [Internet]. NIH News in Health. 2019. Available from: https://newsinhealth.nih.gov/2019/12/fast-or-not-fast
7. Hanlan ME, Griffith J, Patel N, Jaser SS. Eating Disorders and Disordered Eating in Type 1 Diabetes: Prevalence, Screening, and Treatment Options. Current Diabetes Reports [Internet]. 2013 Sep 12;13(6):909–16. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4002640/
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