One of the most popular – and life-changing – of Dr Michael Mosley’s legacies is the 5:2 diet, where you eat normally for five days and ‘fast’ (with just 600 calories a day) on two non-consecutive days a week.
The plan has been shown to help weight loss and reverse type 2 diabetes; a study last year even showed it was potentially more effective than diabetes medication, reported the journal JAMA Open.
But it’s no longer the only form of intermittent fasting – i.e. alternating between periods of fasting and eating – used to shift excess pounds.
There’s also time-restricted eating – or only eating for a certain window of hours during the day; alternate-day fasting – consuming up to 600 calories every other day; and 24-hour fasts once a week.
But intermittent fasting doesn’t work for everyone. And new research reveals why one group in particular may not benefit – people who are obese.
Researchers at the University of Tokyo found that when healthy mice were deprived of food, this quickly triggered molecular changes in metabolic pathways that helped their liver switch from storing energy to burning it. This helped the healthy mice lose weight.
But in the obese mice, the same metabolic changes didn’t occur, reported the journal Science Signaling. In other words, the obese mice would have found it harder to lose any weight by not eating.
The researchers said the findings suggested obesity introduces a kind of metabolic ‘jet lag’, disrupting when – not just how – key changes affecting metabolism occur. They will now investigate whether there are markers in the blood that could be used to identify people who experience this, and those who would or would not benefit from fasting for weight loss.
Intermittent fasting doesn’t work for everyone, and research reveals obese people in particular may not benefit
Roy Taylor, a professor of medicine and metabolism at Newcastle University, who developed a low-calorie shake and soup-based meal replacement diet for type 2 diabetes that’s used by the NHS
So what exactly does intermittent fasting do to the body?
‘During fasting, the liver normally adapts to being short of fuel by converting fat from the fat stores into molecules called ketones, which can eventually provide fuel for the brain,’ says Roy Taylor, a professor of medicine and metabolism at Newcastle University, an expert in this field, and who developed a low-calorie shake and soup-based meal replacement diet for type 2 diabetes that’s used by the NHS.
‘That’s the most important thing because keeping the brain alive is the aim of metabolism.’
How intermittent fasting can also lead to weight loss is widely debated: some experts believe it is purely down to a reduction in overall calorie intake, while others say it directly activates other mechanisms that lead to pounds being shed.
‘This is a debatable issue in the research community – but there is preliminary data suggesting that there may be processes that change energy metabolism in the body as a result of intermittent fasting,’ says Dr Maria Chondronikola, a research scientist in human nutritional physiology at the University of Cambridge.
‘So we don’t have a complete understanding yet.’
Professor Taylor, however, is convinced it’s our overall calorie intake that’s important. ‘It really is a matter of the total calorie intake that determines fat-burning. If you’re following a form of intermittent fasting, then your overall calorie intake will be less than eating normally,’ he adds.
The other hotly debated issue is which type of intermittent fasting works best.
How intermittent fasting can lead to weight loss is widely debated: some experts believe it is purely down to a reduction in overall calorie intake, while others say it directly activates other mechanisms that lead to pounds being shed
A recent review of 99 studies compared people on ad libitum diets (eating what you want, when you want) with those on calorie-controlled diets, alternate-day fasting, time-restricted eating or whole day fasting, over six months.
The researchers, from the University of Toronto, found alternate-day fasting was the only type to show a greater amount of weight loss – with an average extra weight loss of 3.6kg – and it lowered total cholesterol, triglycerides (harmful blood fats) and low-density lipoprotein (‘bad’ cholesterol), reported the BMJ last month.
However, for many people, losing 3kg is a ‘drop in the ocean’, says Professor Taylor.
‘If someone is 100kg then losing 3kg won’t make a huge difference to their health.’ Fasting alone is not going to cut it, he suggests. ‘In comparison, the NHS diabetes remission programme, which I developed, achieves over 10kg weight loss at 12 months.’
Yet fasting has benefits beyond weight loss that should be taken into account, argues Alex Ruani, a researcher in nutritional science education at University College London.
While the BMJ study suggests time-restricted eating isn’t as beneficial for weight loss as alternate-day fasting, it is particularly effective at reducing levels of inflammation, she says.
‘All eating is pro-inflammatory – the mere activation of your digestive system from food will trigger an increase of pro-inflammatory markers,’ she says.
This inflammatory response is normal and short-lived while we digest food. The issue is that when we eat more often, it leads to more constant, low-grade inflammation which, in turn, is linked with chronic diseases such as type 2 diabetes, non-alcoholic fatty liver disease, heart disease and certain cancers.
‘Also, unlike with time-restricted eating, with alternate-day fasting it may be hard for some people to control their appetite on non-fasting days and there can be compensatory eating which is hard to regulate,’ says Ms Ruani.
‘And appetite hormones like predictability and rhythm – so alternate-day fasting can potentially lead to a dysregulated appetite and the brain doing a poorer job at recognising start-and-stop eating cues.
‘In contrast, with time-restricted eating you develop a regular daily pattern if your eating window is the same every day,’ she explains. This predictability means your hunger hormones and appetite regulation are less compromised.’
And the community of bugs in our gut (the gut microbiome, which research increasingly shows is important for maintaining good health) also prefer this predictability, she adds. ‘Research suggests unpredictable eating rhythms can prompt adverse changes in our gut microbiome and in turn alter hunger signals, potentially increasing appetite and the chances of snacking.’
The weight loss resulting from time-restricted eating seems to be proportional to the duration of the eating window, with smaller eating windows leading to people losing more weight, says Maria Chondronikola.
But if you choose a small eating window then it’s important you don’t eat only in the evening when you’re inactive, adds Professor Taylor, otherwise ‘none of the food is going to be burned off and it will be shunted into storage’.
And note if you’re someone who doesn’t do well when you skip a meal, then it’s not for you, says Ms Ruani. ‘It could lead to headaches, lethargy, mood swings, weakness, dizziness, irritability and even cognitive impairment. It can also disturb sleep if you go to bed hungry.’
She recommends starting slowly, saying: ‘It’s not generally recommended to go from eating meals within a 14-hour period to suddenly cutting down to four or six hours.’
Professor Taylor adds: ‘It’s also important to remember that everyone is different, and some people do quite well on intermittent fasting while others do far better on cutting down calories. Try any form for a month to see if you lose what you expected (4kg in a month, for example) – if not, switch to a different approach.’
NOTE: Check with your doctor before making major changes to your diet, particularly if you take regular medication.
