Intermittent fasting (IF) has been in the spotlight for the past few years. These diets are often promoted as boosting health, weight loss, and healthy ageing.
But as with all diets, we need to take a look at the scientific evidence to see whether IF is actually good for us.
What is Intermittent Fasting?
IF diets focus on when you eat, rather than what you eat. This involves alternating periods of fasting and eating.
During fasting times, little to no calories are allowed, and the main intake is usually from calorie-free drinks (like: water, black tea, black coffee, and diet drinks). But on non-fasting days there are often no rules or restrictions in place.
There are two main types of IF:
- Alternate day fasting: This involves fasting on a few days per week. For example, on the 5:2 diet you consume less than 600 calories on 2 days per week, and consume your usual intake on the remaining 5 days of the week. The 6:1 diet (also known as the ‘eat-stop-eat-diet’) is similar but involves a 24 hour fast one day per week, and your usual intake on the remaining 6 days.
- Time restricted feeding: This involves limiting your intake to a daily ‘eating window’ every day. The most popular example of this is the 16:8 diet – this includes an 8 hour eating window with a 16 hour fast. Other variations include limiting your intake to a 4 – 12 hour eating window.
Impact on Ageing
There is some evidence that IF might trigger the body to boost cell repair and recycle damaged parts of cells (which is known as autophagy) (1, 2, 3, 4). IF may also reduce inflammation and the number of free radicals; which are molecules that can cause harm in the body (5). But we don’t have strong evidence that this occurs in humans, as a lot of this research has been carried out in animals or petri-dishes (6).
These processes may also reduce the risk of age-related cognitive diseases such as Alzheimer’s disease (7, 8). But much of the research in this area suggests a beneficial effect from calorie restriction – which can be achieved from a lower calorie diet, rather than IF specifically (9).
Overall, we need more research to look into calorie restriction, IF and ageing.
Impact on Weight
Because IF usually leads to an overall reduction in calorie intake, it can help some people to lose weight.
Multiple studies have found that IF leads to the same amount of weight loss as a traditional diet (A.K.A. ‘daily calorie restriction’) (10, 11, 12).
It isn’t clear which type of IF is most beneficial for weight loss, as most studies have looked at alternate day fasting. However, the available studies have reported roughly 2 – 4% weight loss in participants who reduced their daily eating window to 8 – 11 hours per day (13, 14).
But some studies have also found that weight is quickly regained if IF is not continued (15).
The evidence is mixed about whether it is easier to stick to IF or traditional calorie reduction in the long term (16, 17). This most likely depends on the each individual, as different approaches work for different people.
So IF works well for some people as a weight-loss method. But others prefer the more moderate approach of consistently having a reduced calorie intake each day. More evidence is emerging about diets like 16:8 IF, but there isn’t a lot of evidence to back this up yet.
Impact on Heart Health
IF has been linked with a similar improvement in heart health as daily calorie reduction, due to improvements in cholesterol and blood pressure levels (10, 18, 19, 20).
However, one trial found that LDL-cholesterol (‘bad cholesterol’) levels increased after a year on an alternate daily fasting diet, which didn’t occur in the daily calorie restriction group (10).
It is important to note that current studies have looked at cholesterol and blood pressure levels, rather than heart disease itself. And most of these have examined alternate day fasting, rather than time restricted feeding. Therefore more research is needed before we conclude that it can reduce the risk of heart disease.
Impact on Diabetes Risk
Insulin resistance occurs when our cells can’t use insulin properly, which can subsequently lead to high blood glucose and an increased risk of diabetes. Losing weight, especially around the middle of the body, and reducing calorie intake have been seen to reduce insulin resistance and the risk of diabetes (21, 22).
Human studies have found that alternate day fasting can reduce insulin resistance as effectively as traditional calorie restriction (23). But there is no strong evidence that IF is any better than calorie restriction in this regard.
IF diets have also been seen to improve levels of glucose and insulin in the blood to the same degree as daily calorie restriction (10).
A recent study investigated the effect of early time restricted feeding (an eating window from 9am – 3pm) on men at high risk of developing diabetes (24). Interestingly, this study found numerous health benefits related to this approach, which were unrelated to weight loss, this included: improved insulin sensitivity, improved pancreatic function (β cell responsiveness) and reduced blood pressure. So syncing up the timing of IF in line with our circadian rhythm may lead to even greater health improvements.
Overall, research is emerging which suggests that IF may be beneficial in reducing the risk of diabetes. Therefore this approach may work well for some people, whereas others may prefer to stick to general healthy eating advice along with daily calorie reduction.
Impact on Cancer
Some studies have found that restricting calorie intake, while still providing enough nutrients for our body, may reduce the risk of cancer (25). So IF may help some people achieve a lower calorie intake.
Some studies suggest that autophagy (recycling damaged parts of cells) triggered by IF might reduce the growth of certain types of tumours (26, 27, 28). However, most of this research has been carried out in animals. Furthermore, autophagy has also been seen in increase tumour growth in some cases (26).
So there definitely isn’t enough evidence to say that following an IF diet reduces cancer risk.
It would also be unsafe for most people who have cancer to follow and IF diet. This is because weight loss and difficulty eating is common among cancer patients, but consuming a regular intake of high energy, high protein food and drink can help to boost strength and health during cancer treatment. For more information about diet during cancer, here are some tips from Cancer Research UK.
Are There Any Risks?
Prolonged fasting for 24 hours or more has been seen to cause serious medical harm in some cases. This includes: kidney problems, heart problems and even death (29, 30).
IF is usually less dangerous than prolonged fasting, but there are still some significant risks to consider.
First of all, it might be harmful for those who need a regular intake of energy and nutrients. For example: somebody who is very unwell, somebody with a poor appetite, pregnant women, a young child or some somebody with diabetes who is prone to low blood sugars.
IF may also be damaging for somebody who has an eating disorder, or a history of one. This is because it is a rigid eating framework, which could become obsessive. Furthermore, prolonged fasting has been linked with bulimia and binge-eating in teenage girls (31).
For some people, fasting can also lead to (32, 33):
- Reduced energy levels
- Irritability and low mood
- Poor sleep
Anecdotally, women tend to report more issues related to fasting than men. Not much research has investigated this specifically, but some studies in rats have found that fasting can cause females to undergo hormonal changes, negatively affect reproduction, and increase stress levels (34). But male rats were not negatively impacted in this way. Some human studies have also found that men respond better than women to IF in term of metabolic health (35, 36).
Overall, we need stronger evidence before we can say whether IF is safe and healthy in the long-term (6).
Take Home Message
IF is way to reduce calorie intake by restricting the times at which you can eat, rather than restricting what you eat. Most studies have looked at alternate day fasting (like the 5:2 diet), but time restricted feeding (like the 16:8 diet) is becoming more popular, and more researching is emerging related to this.
IF has been seen to boost weight loss, improve heart health and reduce the risk of diabetes. But in many studies this isn’t seen to be any better than a daily reduction in calorie intake, with the exception of early time-restricted feeding (only eating between 9am – 3pm). So it depends on each individual as to whether IF or daily calorie restriction works best for them.
IF may also promote healthy aging, but we don’t have reliable human evidence to support this at present. There is also no strong evidence that IF reduces the risk of cancer.
This is an interesting and promising approach, but it is important to be aware that we don’t have much evidence about the long-term effects of IF. There are also possible risks to be aware of, and IF should be avoided if you are: pregnant, breastfeeding, have a low weight, a history of an eating disorder or certain medical conditions. If in doubt you can seek support from a Registered Dietitian.
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Feature image: Debby Hudson on Unsplash