A large review of research on fasting shows discouraging results.
Many people fast because they want to lose weight, while others believe it improves health. Researchers have now investigated whether that is true.(Stock photo: Shutterstock / NTB)
There are several ways to fast. Some people fast for several days in a row. Others follow the so-called 5:2 diet, eating normally five days a week and consuming very few calories on the remaining two. Another approach is time-restricted fasting, known as the 16:8 method, where all meals are eaten within an eight-hour window and fasting continues for the rest of the day.
Researchers at the Cochrane Institute have now investigated whether fasting helps with weight loss.
Cochrane is an international organisation that evaluates health research. They check the quality of studies and summarise what good research says about health interventions.
The researchers reviewed studies involving people who were overweight or obese and who followed intermittent fasting, diets based on standard dietary advice, or no diet at all.
They found 22 international studies with a total of 2,000 participants.
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The researchers compared weight loss outcomes and quality of life across the different approaches.
Fasting was no better than other diets or no diet
The researchers defined effective weight loss as a five per cent reduction in body weight. For someone weighing 90 kilograms, that would equal 4.5 kilograms.
The findings were discouraging for anyone who believes fasting is a miracle cure.
Participants lost just as much weight on diets based on standard dietary advice as they did with fasting.
There was also no difference in weight loss between those who fasted and those who followed no diet at all.
People who fasted only lost three per cent of their body weight. That is 2.7 kilograms for someone weighing 90 kilograms.
There was also no clear evidence that fasting is better for quality of life than other diets.
The researchers were not impressed with the quality of the studies they found. For example, participants were not asked how they felt about fasting, and none of the studies examined whether fasting reduced the risk of diabetes or other health conditions.
Norwegian study showed weight loss
Kamilla La Haganes studies exercise at NTNU. Three years ago, she conducted a study that included intermittent fasting with time-restricted eating.
131 women with obesity were divided into four groups. One group followed time-restricted eating for seven weeks, while another group exercised. A third group did both. The study also had a control group that continued their usual habits.
The results showed that those who combined fasting with high-intensity exercise lost the most weight.
The exercise group lost the least weight, just 1.5 kilograms over the seven weeks.
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The fasting group lost 2.1 kilograms.
Those who both fasted and exercised lost 3.1 kilograms.
However, weight loss alone was not the researchers’ main focus. They also measured blood sugar levels, abdominal fat, and muscle mass.
Exercise and fasting worked
"Those who combined fasting with exercise lost twice as much fat around their internal organs. Their blood sugar levels were also more stable over time, and they lost less muscle mass despite losing weight," says Haganes.
Kamilla La Haganes researches exercise at NTNU. She has conducted a study on fasting and exercise.(Photo: NTNU)
Her findings suggest that fasting paired with exercise can produce positive results. This does not align with the review from Cochrane.
But Haganes' study was not among the 22 studies they reviewed.
"Cochrane included studies with at least six months of follow-up. Our study ended after seven weeks," Haganes tells Science Norway.
She also points out that the Cochrane researchers included all types of fasting, while participants in the NTNU study followed time-restricted eating.
"As a result, the review cannot determine whether certain types of fasting are more effective than others," says Haganes.
She nevertheless believes that there is no single method that works for everyone.
What works for you over time?
"There are many different diets that lead to weight loss, but the best one is the one you can stick with over time. For some, this is time-restricted eating. It eliminates the need to count calories or weigh food and encourages people to pay closer attention to hunger and fullness cues," says Haganes.
For others, fasting doesn't work at all.
"Work, family, and social life can make it difficult to stick to a strict eating window," she says.
Time-restricted fasting, for example from 8 pm to 8 am, creates natural barriers to food intake.
"If you usually eat until bedtime or snack on a bag of crisps in front of the TV, time-restricted fasting can automatically eliminate those unhealthy eating habits," says Haganes.
Starting a diet – or participating in a study – can also make you more aware of your bad habits. This was seen in the NTNU study, where those who fasted began eating less even though the researchers did not instruct them to do so.
Haganes believes the best strategy combines regular exercise, fasting, and a diet rich in healthy foods. She highlights the Mediterranean diet, which includes a lot of fruit, vegetables, seeds, whole grains, and legumes.
"Exercise improves physical fitness and overall well-being. Fasting can help curb bad habits in the evening, and the Mediterranean diet has been shown in multiple studies to lower the risk of disease," she says.
