Intermittent fasting is a diet approach that involves periods of eating and fasting. It’s become increasingly popular in recent years, with studies suggesting benefits from weight loss and metabolic health to diabetes management and improved brain health.
That said, intermittent fasting can still be controversial and there are downsides to consider.
With that said, we’ll take a closer look at the potential pros and cons of intermittent fasting, according to research.
What is intermittent fasting?
Intermittent fasting, also called time-restricted eating, is a pattern of eating that involves extended periods of fasting, followed by periods of eating.
According to experts*, our ancestors would have eaten like this – not by choice, but due to food availability. However, as we no longer have to hunt for our food, we’re unlikely to fast unless we decide to.
The idea behind intermittent fasting is that it gives your body a break from digesting food, and naturally encourages people to consume fewer calories than if they were eating regularly.
There’s also evidence* that intermittent fasting may trigger a process called autophagy, where the body works to remove damaged cells. Research* suggests that autophagy may have several health benefits, including improved sleep, gut health, and a reduced risk of inflammation and disease. However, further research is needed.
There are different forms of intermittent fasting, which vary depending on how long you fast. Some are more extreme than others. We’ll cover some of the most common types of intermittent fasting below…
The 5:2 diet
The 5:2 diet involves eating only 500 to 600 calories for two days every week. On the other days, you eat a normal, balanced diet following your recommended calorie intake.
To learn more, check out our article: The 5:2 diet – what is it and does it really work?
16:8 method
The 16:8 method involves fasting for 16 hours each day and eating within an eight-hour window. For example, if you ate dinner at 7pm, you’d fast until 11am the following day.
Alternate-day fasting
Alternate-day fasting involves fasting every other day. Water and other calorie-free drinks like black tea or coffee are allowed on fasting days.
24-hour fast
Involves fasting for 24 hours, sometimes weekly or monthly.
What are the potential health benefits of intermittent fasting?
While research into the potential health benefits of intermittent fasting is ongoing, several studies have shown promising results. We’ll explore these below.
Note: Much of the research into the effects of intermittent fasting is fairly recent, and many studies have included small groups of people. For this reason, further research is needed to confirm its potential long-term benefits.
Intermittent fasting may help weight loss
Research into the potential benefits of intermittent fasting as an effective weight loss method has been promising, particularly for those who are overweight or obese.
By restricting when you can eat, intermittent fasting generally causes people to eat less food overall. And, unless you compensate by eating significantly more during non-fasting periods, this will naturally result in a lower calorie intake – leading to weight loss.
In this study, when obese people practised intermittent fasting for 12 weeks, they lost an average of 9% of their body weight – more than those who followed other weight loss methods. Another review of 27 studies found that those who did intermittent fasting lost between 0.8-13% of their baseline body weight.
Plus, because most intermittent fasting methods don’t restrict what you can eat during non-fasting periods, many people find it more appealing than other diet approaches.
Other research has found that intermittent fasting may promote weight loss by regulating appetite and enhancing hormone function. In this study, 40% of participants said that intermittent fasting trained their bodies to feel less hungry. And, in this study, intermittent fasting helped to lower insulin levels and increase the production of norepinephrine – two hormones involved in fat burning.
That said, further research is needed to confirm the long-term effectiveness of intermittent fasting as a sustainable weight loss tool.
Intermittent fasting may reduce the risk of type 2 diabetes
Research has found that intermittent fasting may reduce the risk of type 2 diabetes – and there are a few reasons why.
Being overweight is one of the main risk factors for developing type 2 diabetes. And, as discussed above, studies have found that intermittent fasting may be an effective tool for healthy weight loss.
Research has also linked intermittent fasting with improved insulin resistance – a key risk factor for type 2 diabetes. In this study, intermittent fasting was as effective as limiting calorie intake for reducing insulin resistance.
Another study involving men with a higher risk of diabetes analysed the effects of intermittent fasting with a six-hour daily eating window ending at 3pm. The results found that this method of intermittent fasting improved both insulin sensitivity and pancreatic function (including insulin production). Interestingly, these benefits were present whether or not the participants lost weight.
Researchers believe these effects may be due to intermittent fasting’s ability to sync food intake with the circadian rhythm (the body’s internal clock). According to experts, eating in harmony with your circadian rhythm helps the body process glucose, reducing your risk of diabetes.
Intermittent fasting may help to reduce inflammation
Inflammation is the body’s natural response to fighting injury and infection, and we need a certain amount to stay healthy. However, too much inflammation, or chronic inflammation, can be dangerous for health and contribute to disease.
Research suggests that intermittent fasting may help reduce markers of chronic inflammation throughout the body, lowering the risk of several diseases, including arthritis, heart disease, and inflammatory bowel disease.
Intermittent fasting may improve brain health
As a result of its impact on factors like inflammation and insulin resistance, there’s evidence that intermittent fasting may benefit brain health.
In this study, intermittent fasting was found to increase the growth of new nerve cells, supporting brain function. Other studies have associated intermittent fasting with a reduced risk of neurological conditions like stroke, Alzheimer’s disease, and Parkinson’s disease.
Intermittent fasting may reduce the risk of cancer
The results of several animal studies suggest that intermittent fasting may reduce the risk of cancer.
For example, the results of this study suggest that intermittent fasting may delay the onset of tumours.
Intermittent fasting may also have cancer-related benefits due to its promise as a tool for healthy weight loss, because obesity is a significant risk factor for various cancers.
Intermittent fasting may boost longevity
There’s evidence that intermittent fasting may boost longevity – potentially due to its ability to reduce oxidative stress (an imbalance of harmful cells in the body), optimise circadian rhythms, and reduce the risk of disease.
In this study, rats who followed an alternate-day fasting diet lived 80% longer than those with unrestricted access to food.
Is intermittent fasting safe and are there downsides?
According to experts*, current evidence suggests that intermittent fasting is safe for healthy adults. However, there are a few exceptions.
For example, Bupa* advises that intermittent fasting isn’t suitable if you’re pregnant, have type 1 diabetes, suffer from anxiety or depression, or have a history of disordered eating. As a result, it’s important to speak to your GP before trying intermittent fasting.
More generally, studies have also highlighted several potential downsides to intermittent fasting that are worth considering. We’ll explore these below.
Note: If you practise intermittent fasting, it’s important to make sure your diet is still healthy and balanced. The risk of developing nutrient deficiencies increases when eating fewer calories, so focusing on eating plenty of fruit, vegetables, lean proteins, whole grains, and healthy fats is important.
Intermittent fasting may negatively impact energy levels
Because intermittent fasting involves not eating – sometimes for long periods – it can negatively impact energy levels. This can be particularly true of more extreme intermittent fasting methods, such as alternate day fasting.
For example, this study linked intermittent fasting with fatigue, low energy, irritability, and difficulty concentrating. That said, other studies suggest that these symptoms are often only temporary and may improve over time as the body adapts.
Intermittent fasting can be difficult to maintain
In some cases, research suggests that intermittent fasting can be difficult to sustain.
In this study, 38% of people following an alternate-day fasting programme left the study within one year, compared to 29% of those following a calorie-restricted diet. For this reason, if you’re new to intermittent fasting, experts generally agree that it’s best to start with a less extreme variation – such as the 16:8 method.
Intermittent fasting may cause you to overeat
Due to its restrictions on when you can eat, some people find that intermittent fasting actually causes them to eat more. According to experts, this is because as humans, we have a strong biological push to overeat following periods of fasting because hunger hormones go into overdrive when you’re deprived of food.
It’s also human nature to want to reward yourself after completing something difficult, including fasting. As a result, there’s a risk of overindulging during non-fasting periods. Though, this won’t be the case for everyone.
Other research suggests that intermittent fasting may negatively affect metabolism. Studies suggest that time-restricted eating patterns may disrupt metabolism by misaligning the circadian rhythm (the body’s internal clock).
Intermittent fasting may increase the risk of disordered eating
The link between restrictive diets and eating disorders is well-known.
While most intermittent fasting methods don’t require people to eliminate certain foods from their diet, their time-based restrictions can still be harmful. As a result, research has linked intermittent fasting with an increased risk of disordered eating and potentially dangerous, compulsive behaviours.
For example, this study linked intermittent fasting with an increased risk of bulimia and binge eating.
Final thoughts…
Intermittent fasting has grown in popularity in recent years, with many claiming its various health benefits. While further research is needed, initial studies into its effects on everything from weight loss to diabetes management have been promising.
For further reading, head over to our diet and nutrition section. Here, you’ll find information on everything from healthy snack ideas to cutting down on alcohol.
What are your experiences of intermittent fasting? Do you have any tips that you’d like to share? We’d love to hear from you in the comments below.
