Why eating less carbs burns just as much fat as fasting

Telegraph

22-05-2025

Intermittent fasting is now one of the most-followed dietary patterns in the developed world. According to a 2022 survey, about 10 per cent of American adults practise intermittent fasting, and while the same stats for Britain don't exist, we all know someone who now skips breakfast to maintain their eight hour 'eating window' each day.
It's a fact that pleases nutritionists, doctors and diabetes experts alike. Keeping our eating to a tight timeframe 'can help your body shift from burning food as fuel to burning fat, helping with weight management or weight loss, regulating your blood sugar levels and giving you more energy throughout the day,' says nutritionist Rhian Stephenson, founder of the supplement brand Artah.
But there is an obvious downside to time-restricted eating. The hours spent waiting for the eating window to open are often spent feeling ravenous, miserable and totally drained, and the early dinners that are necessary on this diet – as eating typically stops at 8pm sharp – can become anti-social. What if there was a way to get all of the health benefits of intermittent fasting, and at the same time enjoy both a filling breakfast and a late evening meal?
New research suggests that this could be possible after all – if you're willing to cut carbohydrates out of some of your meals. A study from the University of Surrey, published in April, suggests that 'by restricting carbohydrate intake, without restricting calorie consumption, people can potentially reap all the same benefits that are associated with short-term fasting,' says Dr Adam Collins, an associate professor of nutrition who led the research.
A recent study led by Dr Collins found that by reducing the amount of pasta, potatoes of bread you eat and following a low-carb diet, it can bring all the benefits that we've come to associate with fasting. Most importantly, a switch to burning fat rather than food for fuel, which can lower blood sugar and inflammation and potentially make us less prone to heart disease and diabetes in the long run
'Cutting down on carbs for a few days each week could therefore be a more sustainable but equally effective way for people to manage and improve their metabolic health,' he adds. 'That could take the form of having a 'carb window' instead of a food window, or something similar to the 5:2 diet, which is where people eat normally five days a week, and on the remaining two they eat one meal or five to six hundred calories.'
How it works
Dr Collins' study followed the same group of 32 people, all of whom were overweight according to their BMI, across three different days (with five 'washout' days in between).
On day one, the participants ate enough calories to meet their daily needs, ranging from 1,800 to 3,000 based on their weight and gender. On day two, they ate a low-carb, low-calorie diet: women in the study were restricted to around 550 calories, while men ate around 650 calories, and both groups ate 50g of carbohydrates (roughly the amount of carbs in a small portion of cooked pasta).
On the third day, the group could eat as many calories as they needed, but had to keep their carbohydrate consumption to 50g a day. 'Both low-carb days led to improvements in the participants' metabolic health markers, including a shift into a fat-burning state, and how well they could process a high-fat meal after we observed them,' Dr Collins says. 'Essentially, all the benefits you usually get from fasting were still observed when people just ate fewer carbs.'
The Surrey University team were 'tease out the effects of carb restriction from those of calorie restriction,' explains Dr Collins. Crucially, the study didn't look at weight loss – 'you can't measure that after just one day,' he points out. But in the real world, 'when people eat fewer carbohydrates, they take on fewer calories,' leading to a loss in weight, primarily in the form of fat.
This is important to understanding metabolic health as a whole, not only for those who want to lose weight. 'Restricting your carb intake for a few days each week means that you avoid over-fuelling your body and will only ever be burning food as fuel. Being able to switch into fat-burning mode is crucial to avoiding health complications as you age, like diabetes, heart disease and weight gain,' Dr Collins says.
Though you'll only get visibly slimmer if you eat fewer calories than you burn, fasting and carb-restricted eating in any amount 'could both potentially burn some of the fat that develops inside of your organs when you consistently eat more carbohydrates than you burn,' a huge health boost as this kind of fat has been linked to increased risk of heart disease, stroke, and Type 2 diabetes.
As Naveed Sattar, a professor of metabolic medicine from the University of Glasgow points out, 'this study is small, and you'd need a big, long-term study to see exactly what the effects of this type of eating would be'. But 'it's very exciting, and potentially could do a lot to inform the dietary advice we give to people at risk of diabetes and heart disease in future'.
This study only involved people who are over a healthy weight range for their height, 'but we'd expect to see similar impacts in people who are a healthy weight too,' says Dr Collins, 'though the less metabolically healthy someone is, the more exaggerated the effect will be'. This is good news if you're attempting to turn your health around quickly.
Intermittent fasting vs low-carb diets
For all the benefits that come with intermittent fasting there are also drawbacks, and more serious ones than raging hunger. 'When people practise intermittent fasting consistently for weeks or months, they often put themselves at risk of nutrient deficiencies,' says Stephenson. 'The fewer calories you eat every day, the harder it is for you to get all of the nutrients you need – and keeping a small eating window doesn't mean that you will eat healthily.'
There are also groups of people who don't get on well with fasting for reasons other than hunger. 'I don't recommend fasting to perimenopausal or menopausal women, because fasting can disrupt your hormones and put more strain on your body at a time when it is already under a lot of stress,' she says. A carb-cycling diet could give these women 'some of the same benefits of fasting without the drawbacks,' Stephenson adds.
And while fasting can be good for all of us, not everyone wants – or needs – to lose weight. 'Weight loss is a product of eating fewer calories than you expend, so if you eat enough or more than enough calories while limiting your carb consumption, you won't lose weight,' Stephenson explains. This could make a few low-carb days 'a particularly good option for people who have lost weight and are looking for a way to maintain it,' says Dr Collins. 'That's very difficult to do, and currently we don't have much to offer people in that position.'
That said, restricting carbohydrates could come with its own risks. 'There's some evidence to suggest that low-carbohydrate diets can be bad for your heart, where you replace the carbs primarily with fat,' says Prof Sattar. 'There could be other dangers that we wouldn't be able to identify outside of a large-scale randomised clinical trial.'
For this reason, Stephenson suggests cutting your daily carbohydrate intake to no less than 100g, unless you're working with a nutritionist or doctor. 'It's also worth considering counting 'net carbs' rather than total carbs, by subtracting the grams of fibre in your food from the total carb count on the packet,' she adds. This is because your body doesn't use all of the carbohydrates you consume. Net carbs represent the amount of carb that can impact your blood sugar.
One day on a low-carb, fat-burning diet
Eating a low-carb diet for one or two days each week can be a lot easier than you might think. Here are two plans put together by Rhian Stephenson, so that you can eat well while you give it a go.
Option 1
50g of carbohydrates, 1,800 calories
'This is similar to what participants ate in terms of carbohydrates in the study, and is concentrated within one meal to mimic a 'window' style of eating,' says Stephenson. 'It might also lead to weight loss if you follow this plan regularly, depending on your current weight and how much you exercise.'
Breakfast: Spinach and egg scramble
2g net carbs
Ingredients
150g chicken breast cooked with olive oil, lemon, salt, pepper and herbs
200g lentils cooked with half an onion and olive oil
150g cherry tomatoes
1 kiwi
Option 2
Under 100g of carbohydrates, 1800 calories
'This is a more realistic version of a low-carb day that would better support your health if followed regularly, as it involves more carbohydrate intake that is spread throughout the day,' says Stephenson. 'Depending on your weight and how much you move, eating with this plan regularly might also lead to weight loss, but you could reduce some portion sizes or skip the snack to create a calorie deficit if you want to lose weight.'
Breakfast: Overnight oats
34g net carbs
Ingredients
45g of jumbo oats
1 tbsp peanut butter
200ml unsweetened soy milk
1 tbsp walnuts
100g raspberries
Sprinkle of cinnamon
1 boiled egg
Lunch: Tuna and chickpea salad
14g net carbs
Ingredients
Large green salad with mixed veg, cucumber, tomatoes and red pepper
1 can tuna in olive oil
50g chickpeas
Oil and vinegar to dress
Dinner: Chicken stir fry
12g net carbs
Ingredients
Half a head of cauliflower (to make cauliflower rice)
One head of pak choi
150g chicken
100g mushrooms
35g sugar snap peas
Sauce with tamari, sesame oil, vinegar, garlic and ginger
Snack: Yogurt and fruit
10g net carb
Ingredients
100g Greek yogurt
80g low-sugar fruit (eg Cantaloupe)
2 tbsp sunflower seeds