Intermittent fasting (IF) has been in the spotlight for the past few years. These diets are often promoted as boosting health, facilitating weight loss, and encouraging healthy ageing.
However, as with all diets, we need to take a look at the scientific evidence to see whether IF is actually helpful for a healthy lifestyle.
What is intermittent fasting?
This involves alternating periods of fasting and eating.
There are a few different types of IF. These often focus more on when you eat, rather than what you eat. However, some versions of IF specify certain food or calorie restrictions within the feeding windows as well.
Popular types of IF include:
Time restricted feeding (TRF):
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. The timing of the feeding windows can also vary i.e. early or late TRF.
Alternate day fasting (ADF):
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 usually involves a 24 hour fast one day per week, and your usual intake on the remaining 6 days.
Fasting mimicking diet (FMD): This diet is low in carbohydrate and protein, but high in unsaturated fat, and usually only contains a third to half of a person’s normal calorie intake.
Impact on weight
As IF often results in a reduced calorie intake, it can lead to short-term weight loss.
However, multiple studies have found that IF results in the same amount of weight loss as a ‘traditional’ calorie controlled balanced diet. For example, a systematic review from 2020 found intermittent fasting diets led to 0.8%—13.0% weight loss, which was similar to ordinary calorie-restricted diets (1). There was also some loss of muscle mass and improvements in blood glucose control observed. It was noted that most studies in this area are small and also short in duration.
When discussing dietary approaches and associated weight loss, it’s important to consider long-term sustainability to maintain weight loss. Current evidence shows that it is really difficult for those in a bigger body to lose and maintain weight, and weight regain within a few years is very common (2, 3,4). Cycles of losing and regaining weight are also quite common, and this has been linked with health issues in itself (4). As a result, for many people focusing on weight loss, an IM approach may not be realistic or helpful, especially if they have a disordered relationship with food or their body.
Impact on heart health
There are some mixed findings when it comes to the impact of IF on the risk of heart disease.
Data related to Ramadan fasting has found a reduction in cholesterol and blood pressure in healthy study participants (5). This data also identified an increase in heart issues during the time of ‘breaking fast’ as compared with non-fasting days; but ongoing research is needed.
Other studies have found similar reductions in blood pressure and improvements in cholesterol from IF (6). However, some studies have found a reduction in ‘good’ HDL-cholesterol, and mixed findings on LDL cholesterol (6). There are a number of factors which may contribute to this, including the timing of fasting and how this relates to our body clock.
Skipping breakfast is associated with an increased risk of death from heart disease and all-causes (7).
While there may be a positive impact of IF on heart health, there are some contradictory findings and ongoing research is needed to investigate this.
Impact on diabetes risk
A number of studies have found that IF may be linked with improvements in insulin effectiveness and blood glucose control (5). Some studies have even found that patients with type 2 diabetes were able to stop using insulin while using IF under the supervision of their Diabetes Team* (8).
An interesting study from 2018 investigated the effect of early TRF (an eating window from 9am – 3pm vs a 12-hour feeding window) on men at high risk of developing diabetes found a reduction in diabetes risk that was unrelated to weight loss (9). This included: improved insulin sensitivity, improved pancreatic function and reduced blood pressure. This provided evidence that syncing up the timing of IF in line with the participant’s body clocks, by eating more in the earlier part of the day, may play an important role.
Similarly, skipping breakfast has been linked with an increased risk of diabetes and worsened blood glucose control (10, 11).
IF is looking promising when it comes to reducing the risk of type 2 diabetes, and early TRF may be particularly beneficial. Nevertheless, similar to all areas of IF research, ongoing longer human trials are needed.
Gut health
Research carried out in mice has found that FMDs were linked with reconstruction of gut microbiota and an improvement in inflammatory bowel disease (IBD) (12, 13).
In humans, Ramadan fasting has been linked with positive remodelling of the gut microbiome (14). However, this was seen to return to baseline once fasting was stopped.
Although there is some interesting research emerging in this area, we currently have much stronger research that supports the gut health benefits of consuming a varied diet, especially a variety of high-fibre foods from fruits and vegetables as well as wholegrain containing foods (15, 16 ,17).
Are there any risks with intermittent fasting?
Prolonged fasting for 24 hours or more has been associated with serious health complications with examples including kidney and heart problems and possibly death (18,19).
IF is usually less dangerous than prolonged fasting, but there are still some significant risks to consider. For example, IF might be harmful for those who need a regular intake of energy and nutrients. For example: somebody who is unwell, has a poor appetite, pregnant women or a young child.
Fasting for long periods may not be suitable for people living with diabetes who are at risk of low blood sugar levels, particularly for those who use insulin or certain diabetes medication. There can also be a risk of hyperglycemia (high blood glucose levels) if a large amount of food is consumed after a fast, which is not an uncommon response to extreme hunger.
Many athletes and those who train regularly also need a regular supply of nutrients to optimise their training gains. IF has been linked with decreased muscle mass; although there have been some mixed findings about this (20, 21).
IF could negatively affect immunity by decreasing white blood cells that play an important role in our immune response (22).
Another consideration is that the risk of gallstones has been seen to nearly double in women who fast for 14 hours as compared with those who fast for 8 hours overnight (23).
It’s important to highlight that fasting, including IF, can be a trigger, symptom or socially-acceptable indicator of disordered eating. Consequently, any type of fasting, including IF, is not safe or appropriate for anybody who is in recovery from an eating disorder or disordered eating. Fasting has also been found to be a predictor of an eating disorder, including binge eating disorder, bulimia nervosa and purging disorder (24,25,26).
For some people, fasting can also lead to unpleasant side effects like (27, 28,29):
Anxiety
Irritability and low mood
Poor sleep
Headaches
Reduced energy levels
Less movement
Importantly, the long-term impact of fasting is unknown in various groups. Therefore, ongoing human research is needed.
Take home message
IF continues to be an extremely popular diet and there is some interesting and promising research related to heart health and diabetes risk.
The research related to early time restricted feeding and the impact of our body clock is looking particularly positive. However, there are a lot of significant risks related to fasting, including IF, so this isn’t a suitable approach for many people.
There is still a lot we don’t know about the long-term impact of IF on humans, and for those with different medical conditions. It’s always a good idea to seek individual advice from registered health professionals if you are considering making a significant change to diet like this.
References:
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