Intermittent fasting is a long-term diet plan where you abstain from food for a certain amount of days or hours every week and eat normally the rest of the time. It's the principle behind several popular diets, including the 5:2 and the 8:16. But does it work for weight loss? And is it safe?
The 5:2 diet works around the principle that you eat a healthy balanced diet five days a week and then you fast for the remaining two days. You don't have to choose consecutive days.
On fasting days the calorie allowance is 500 for women and 600 for men, with recommended foods being lean sources of protein, vegetables and juices.
The 8:16 plan works slightly differently in that you restrict your eating time to an eight-hour window - for example: from 9 am until 5 pm - and fast the rest of the time.
Does it really work for weight loss?
Can it help you lose weight? The short answer is yes.
"There is evidence that is providing encouraging weight loss results from fasting," says dietician Dr Carrie Ruxton. "Fasting causes weight loss, as it reduces calorie consumption and improves metabolic health."
In a recent study, 13 women followed a four-month intermittent fasting plan. This involved following a low-calorie diet for two days a week, with five remaining days of normal, healthy eating. Nine women in the group successfully lost 5% or more of their total body weight during this time.
Another piece of research found that those on the 5:2 diet lost weight more quickly than participants on a conventional diet where their daily calorie intake was restricted.
But a 2017 report concluded that the 5:2 diet helped people to lose weight over a six-month period but was no more effective than a conventional diet. However, many people do find intermittent fasting plans easier to stick to, which could make keeping the weight off in the long term more likely.
"Fasting diets are easier to follow as you can still eat what you like on certain days and don't have to give up favourite foods wholesale," points out Ruxton.
What about any other positive effects?
There is a limited supply of research on intermittent fasting and much of it is carried out across short-term, small studies. However, what has been found is promising in terms of overall health benefits.
Research from the University of Southern California, for instance, suggests that fasting boosts the immune system and puts it in a youthful, regenerative state, which can help us to live longer and fight off disease.
Other studies suggest positive health effects of intermittent fasting might include: increased mental focus and improved metabolic health - including better fat-burning, and protection against heart disease, age-related diseases such as dementia, some cancers, including breast cancer, and type 2 diabetes.
A recent study investigating Alzheimer's disease in mice found a link between intermittent fasting and a slower decline in cognitive function as we age, meaning that fasting could keep our brains healthier too.
Should more of us be fasting?
"Yes, for two reasons", says Ruxton. "Firstly, it’s an effective way of managing your weight and produces a moderate sustainable weight loss for those who are overweight.
"Secondly, there is evidence that constant snacking puts pressure on our metabolism, leading to an increased risk of type 2 diabetes. Having periods of 12-16 hours when you give your digestive system a rest can be beneficial for glucose and insulin levels."
It's important to know as much as possible about fasting before you try it, and to follow a plan to avoid harming yourself, cautions nutritionist Ondrej Matej.
"Many of us are overeating and so giving your digestion a break is a very good idea," he says. "Fasting has been shown to help with obesity and illnesses and can be a great way to help you improve your health, increase your energy levels and focus."
What are the drawbacks?
The most common side effects of fasting are difficulty sleeping, dehydration, lethargy, anxiety, mood swings and bad breath. Fitting your fasting days around normal life can be a challenge at first, as you may feel hungry and have less energy.
Fasting sensibly is the key, says Matej. "You'll be missing some of the water you take in from food, so always drink more water when you are fasting. And if you are fasting in the winter, you may want to turn your heating on a bit more."
"You have to time your fasting days so that they don't clash with social occasions," says Ruxton. "However, a way of doing this is to follow the 8:16 diet instead of the 5:2. This is where you keep your eating within an 8-hour window - following a healthy balanced diet - and then fast for 16 hours. The advantage is that no portion control is required."
However, you do need to be aware that a 'normal' diet during your non-fasting period means a healthy, balanced diet, not a licence to overindulge. There is also some concern that it doesn't teach people to rethink their overall eating habits - so when they stop intermittent fasting, they may revert to the unhealthy habits which caused them to gain weight in the first place.
Who shouldn't try fasting?
There are certain groups of people who should avoid fasting due to their dietary needs or medical history. These include:
- Children - or anyone who needs regular meals.
- Pregnant or lactating women.
- People with diabetes.
- Underweight or highly active people.
- Elderly people.
- Those with a history of eating disorders.
People taking medication that needs to be consumed with food should also be cautious about intermittent fasting, advises Ruxton. Ask your doctor if you are unsure.