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Is intermittent fasting healthy?
Intermittent fasting is becoming a popular way to lose weight, with recipe collections, diet plans and apps dedicated to it. Research suggests that cycling between periods of fasting and eating may be able to help people manage their weight and health - and may even help to reduce the risk of diseases such as type 2 diabetes. But how does it work and is it safe?
What is intermittent fasting?
Intermittent fasting is an eating plan that switches between fasting and eating on a regular schedule. While traditional diets specify what you should eat, intermittent fasting focuses on when you should eat instead. This may mean fasting for a certain number of hours each day.
"There are many variations of intermittent fasting but in each version food intake is restricted for a period of time and then you can eat in a non-restricted way for the remaining period of time," says Chloe Hall, dietitian and spokesperson for the British Dietetic Association.
What's the 5:2 diet?
A popular version of intermittent fasting is the 5:2 diet. Dieters are recommended to consume a normal number of calories for five days a week (about 2,000 for women and 2,500 for men), but to eat just 25% of their usual calorie intake for two, non-consecutive days. This means restricting calories to around 500 for women and 600 for men. Crucially, there are no restrictions on the types of food you can eat.
Another regime, called the 16/8 method, involves restricting your eating to an eight-hour period before fasting for the 16 hours in between.
Intermittent fasting benefits
Fasting has long been associated with religious rituals, and some researchers argue that our bodies have evolved to be able to go without food for many hours or even days. When humans were hunter gatherers, going for long periods without eating was common. In recent years, however, the main benefits of intermittent fasting hae been in its capacity to manage weight.
Intermittent fasting for weight loss
Today, there is a growing body of evidence suggesting that specific types of intermittent fasting diets may be beneficial for certain people, including those with obesity and those at high risk of developing type 2 diabetes.
After hours without food, the body uses up its sugar stores and begins to burn body fat. This is sometimes referred to as metabolic switching. Intermittent fasting extends the period when your body has burned through the calories consumed during your last meal and begins spending fat.
Intermittent fasting to reduce cholesterol
"Some studies have shown that, as well as weight or fat loss, intermittent fasting may help improve cholesterol levels," says Hall.
Intermittent fasting for obesity
One study found that intermittent fasting increased insulin sensitivity and reduced obesity. A separate 2019 paper found that eating during a six-hour period and fasting for 18 hours can trigger a metabolic switch. This resulted in increased stress resistance, increased longevity and a decreased incidence of diseases, including obesity.
Intermittent fasting for type 2 diabetes
Some studies of humans have suggested that intermittent fasting can have health benefits for those with - or at risk of - developing type 2 diabetes, such as weight loss and lowering insulin requirements.
However, for people with type 2 diabetes, it's important to consult a medical professional before embarking on intermittent fasting. While studies suggest that for many people with type 2 diabetes, intermittent fasting can help improve blood sugar control and reduce insulin resistance, it can lead to potentially dangerous hypoglycaemia (low blood sugar) in people taking certain medicines to treat diabetes, including sulfonylureas and insulin.
It is also important to note that most research has consisted of small, short-term studies. Therefore, less is known about the long-term impact of intermittent fasting.
Downsides of intermittent fasting
There are also downsides to intermittent fasting. Not all fasting regimes are beneficial and for some, it may lead to headaches, problems concentrating, irritability and feeling faint and tired. It can also be easy to overeat during non-fasting periods too. This is because the hormones linked to appetite and hunger can go into overdrive when you are deprived of food.
It may also be more beneficial to concentrate on eating a balanced diet with plenty of fibre, fruit and vegetables, rather than fasting or stopping eating. "One diet is not right for everyone and the diet may not be sustainable," says Hall. "There is a lack of long-term studies at present and smaller healthy changes may be more sustainable than a restricted diet."
Additionally, recent research has suggested that the time in which you eat may be more important than the time spent fasting. Earlier this year, researchers at Northwestern University discovered those who started eating before 8:30 am had lower blood sugar levels and less insulin resistance, which could reduce the risk of developing type 2 diabetes.
It's also important to note that intermittent fasting can lead to an unhealthy focus on diet and eating habits too. It can be easy for diet regimes to become all-encompassing, which can lead to disordered eating and anxiety. "For those who have a history of eating disorders, any restrictive diet may be triggering for an eating disorder recurrence," says Hall.
How to prepare for intermittent fasting
Speak to your doctor
If you want to try intermittent fasting, it is important to consult your GP first. They will be able to provide you with advice to ensure you are safe and healthy. If you're underweight, have a history of eating disorders or are pregnant or breastfeeding, fasting is not advisable.
"Anyone with a long-term health condition, especially diabetes, should discuss this with their health professional before starting any sort of diet," says Hall.
Keep track of how you feel
It is important to bear in mind that intermittent fasting may have different side effects on different people. If you start experiencing anxiety, mood changes, headaches, nausea, changes to your periods, or other symptoms, speak to your doctor.
If you have been fasting for more than a few hours, break the fast by eating a small snack such as a few crackers. Tackling a full meal straightaway could lead to indigestion and stomach problems.
Focus on healthy foods
It is crucial to follow a healthy, balanced diet and eating pattern, and you should aim to eat five portions of fruit and vegetables every day. Include plenty of fibre and whole grains in your diet and drink at least six to eight glasses of fluids every 24 hours . "On 'unrestricted' days try to eat normally with regular meals and a healthy balanced diet rather than eating vast quantities of junk food," says Hall.