Does fasting work? The case for and against, by expert nutritionists

Hailed as the easy way to lose weight quickly, is fasting everything it’s cracked up to be – or just another diet fad?

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Since the 5:2 diet stormed on to the scene in 2012, fasting has become a hot topic. Defined as ‘The willing abstinence or reduction from some or all food, drink, or both, for a period of time,’ it’s said to aid quick weight loss and offer a multitude of health benefits. 

From 16-hour fasting to completely avoiding food for one day each week, there’s a range of different fasting styles. But does fasting actually work?

If you’re not sure whether it’s for you, or you’re wondering how on earth you’re meant to go without food for so long, our experts share their opposing opinions on this growing trend... 

Does fasting work? The case for…

Nutritionist and author Rob Hobson reckons there are many pluses to fasting.

‘Although the research is lacking a little in terms of the health benefits of fasting, many people have found it useful as a way of dieting that doesn’t involve rules or foods that you have to omit,’ he says. 

‘Plus, our bodies are more than equipped to go without food for periods of time. Think about how we used to survive when we had to catch our own food, which some cultures around the world still do.’

He continues: ‘The research we DO have highlights benefits that occur in the body when it’s deprived of food. These include low levels of blood insulin (which promotes fat burning), increase in human growth hormone (which boosts fat burning and muscle gain) and the promotion of cellular repair.’

There’s also good news when it comes to diabetes. ‘Some of the strongest research for fasting is around the potential to reduce insulin resistance and lower the risk of type 2 diabetes,’ Rob says. ‘Other potential benefits include reducing oxidative stress and inflammation in the body (both of which increase ageing), lowering heart disease risk factors and protecting brain health.’

However, he adds: ’It’s important to still eat nourishing calories – eating anything you want on non-fasting days won’t get you anywhere if you are doing it for weight loss.’ 

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Does fasting work? The case against…

On the other side of the argument, nutritionist Alix Woods says fasting can interfere with our bodily functions.

‘I don’t believe in fasting,’ she says. ‘Without food and/or water, the body lacks all the vital micronutrients (vitamins and minerals), antioxidants and hydration for cell survival and energy.’

She warns that the effects of fasting, saying: ‘Symptoms of fatigue, dizziness, constipation and dehydration can occur when you fast and, for some, it may exacerbate feelings of stress and disrupt sleep. It’s best avoided by hypoglycaemic (low blood sugar) sufferers as the dips in blood sugar could be severe and lead to loss of consciousness.

‘Fasting may also interfere with the normal functioning of the body and lead to inflammation, causing flare-ups like gout and gallstones.’

She’s not convinced when it comes to using fasting for weight loss, either. ‘Fasting for quick weight loss isn’t an effective or healthy way to lose weight as metabolism slows down and essential water and nutrients are lost. A slow metabolic rate hampers healthy weight loss in the future. 

‘Yes, there are studies to say that it’s an effective way to lose weight. However, there are studies that reveal fasting results in quick fluid loss, and once you start eating normally again the weight returns rapidly. Other research shows that fasting leads to cravings for starchier, higher-calorie carbohydrates, resulting in bingeing, so the fasting was effectively thwarted.’

She has concerns around mental wellbeing, too. ‘With regards to mental health, fasting has been found to worsen emotional and psychological issues for people with eating disorders and it may even be a trigger for binge eating and/or anorexia.’

Her advice for anyone looking to lose weight? ‘Follow a healthy whole-food diet with a balance of proteins, carbohydrates and fats; three main meals a day with two to three snacks. I use a ‘lunch’ plate and the palm of my hand to illustrate portion size and encourage cooking from scratch.’ 

3 fasting diets that may work for you

Still keen to discover if fasting works for you? Here are the three main fast diets to consider…

The 5:2 

This diet involves eating normally (and healthily!) for five days of the week and limiting food intake to 500 or 600 calories (depending on gender) for the remaining two. If you have a very active social life, 5:2 is a good option as you can work around fasting days. With exercise, try to plan your training for non-fasting days. 

The 16:8 

You’ll be fasting for 14-16 hours each day, leaving an eating window of eight to 10 hours, so your evening meal is likely to be a little earlier and breakfast later. If you enjoy evening socialising, it can be tricky as you’ll probably be eating later in the night. If you exercise first thing, Rob recommends eating a carb-rich meal the night before to fuel your morning workout. 

The 6:1 (Eat-Stop-Eat) 

A full 24-hour fast every one or two weeks – it’s not for the faint-hearted! The start of your fast may be fine but most people become ravenously hungry towards the end of the day, leading to mood swings, dizziness and an inability to concentrate – so it’s definitely not something you want to be doing on a work or a training day.