Intermittent fasting – The Food Medic

This article was written by one of The Food Medic team; registered dietitian – Maeve Hanan

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?

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:

  1. 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.  
  2. 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. 
  3. 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 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). IF may also reduce inflammation and the number of free radicals; which are molecules that can cause harm in the body (2). These processes may have a positive impact on longer term cognitive health, but ongoing research is needed to investigate this (3). 

But most of the research in this area has been carried out in animals or petri-dishes, rather than humans. However, a small human study from 2019 compared the impact of consuming the same amount of calories between 8am and 2pm (early TRF) or 8am and 8pm (4). The researchers found that the early TRF pattern was associated with an increase in brain-derived neurotrophic factor (BDNF;  a protein involved in brain repair and neural connections) as well as gene expression that indicated autophagy and healthy aging. There was an increase in LDL cholesterol and ketones in the early TRF group. 

There are also risks related to intermittent fasting and aging. For example, muscle mass play a really important role in healthy aging and lowering mortality risk, and intermittent fasting has been linked with reductions in muscle mass; although evidence is mixed in this area (5, 6, 7).   

Similarly, skipping breakfast has also been linked with increased mortality risk (8). But this is an association, not a ‘cause and effect link’. 

Impact on weight

Because IF usually leads to an overall reduction in calorie intake, it can help some people to lose weight, at least in the short-term (as most weight loss studies last less than 2 years, and can often last just a few weeks).

Multiple studies have found that IF leads to the same amount of weight loss as a traditional diet (A.K.A. ‘daily calorie restriction’). 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 (9). 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. 

The timing of feeding and fasting windows may play a role. For example,  skipping breakfast has been linked with a higher BMI (10). Similarly, early TFR may be more beneficial in terms of fat burning and managing appetite, but ongoing research is needed here (11). 

As always when discussing weight loss diets, it’s important to consider suitability and long-term sustainability. 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 (12, 13). Cycles of losing and regaining weight are also quite common, and this has been linked with health issues in itself (13). So for many people focusing on weight loss 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 (14). 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 (15). However, some studies have found a reduction in ‘good’ HDL-cholesterol, and mixed findings on LDL cholesterol (5, 15).  There are a number of factors which may contribute to this, including the timing of fasting and how this relates to our body clock.  

A Cochrane review from 2021 that examined IF for the prevention of  heart disease found no significant difference between IF and continuous energy restriction in improving heart disease risk factors (16). They also concluded that “Further research is needed to understand the safety and risk-benefit analysis of IF in specific patient groups (e.g. patients with diabetes or eating disorders) as well as the effect on longer-term outcomes such as all-cause mortality and [heart attacks]”.

Skipping breakfast is associated with an increased risk of death from heart disease and all-causes (17). 

So there may be a positive impact of IF on heart health, but 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, 18). 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 doctor* (18).   

*Please note: fasting carries additional risks for those with diabetes, especially those using insulin (as discussed later in this article), so individual advice and supervision should always be sought in this situation.

However, a study in rats from 2020 found an increased risk of pancreas damage and insulin resistance with alternate day fasting (19​​).  This contrasted to a study in mice which found restoration of pancreatic cells from an intermittent FMD (20). This obviously can’t be applied directly to humans, but may add to the evidence that suggests that different types of IF may have a different impact on diabetes risk. 

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 (21). This included: improved insulin sensitivity, improved pancreatic function (β cell responsiveness) 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 (22, 23). 

IF is looking promising when it comes to reducing the risk of type 2 diabetes, and early TRF may be particularly beneficial. But 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) (24, 25).  

In humans, Ramadan fasting has been linked with positive remodelling of the gut microbiome (26). However, this was seen to return to baseline once fasting was stopped. 

A systematic review from 2021 that included 6 eligible human studies also found that IF may enhance gut microbiota composition and diversity

(27). The authors of this review also concluded that “further research in larger clinical trials is warranted before practical recommendations for the public health can be made”

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 (28, 29, 30).   

Impact on cancer

Some studies suggest that autophagy (recycling damaged parts of cells) triggered by IF might reduce the growth of certain types of tumours (31, 32, 33). However, most of this research has been carried out in animals. Furthermore, autophagy has also been seen to increase tumour growth in some cases (31). 

A review from 2021 concluded that although some early studies suggest that prolonged fasting may be safe, and possibly even helpful, in some cancer patients, the impact of IF on human cancer risk and outcomes is unknown overall due to a lack of high-quality studies (34). They also highlighted that “because additional trials are needed to elucidate the risks and benefits of fasting for patients with cancer, the authors would not currently recommend patients undergoing active cancer treatment partake in IF outside the context of a clinical trial.”. 

There’s a high risk of malnutrition for those with cancer, so fasting carries the risk of worsening nutritional status in an already nutritionally-vulnerable group. Therefore fasting is not currently recommended during cancer treatment. 

For more information about diet during cancer, here are some tips from Cancer Research UK.

Are there any risks with intermittent fasting?

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 (35, 36).   

IF is usually less dangerous than prolonged fasting, but there are still some significant risks to consider.

As discussed above, 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 can also be risky in those with diabetes who are at risk of hypoglycemia (dangerously low blood glucose 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 (5,  37, 38).  

IF could negatively affect immunity by decreasing white blood cells that play an important role in our immune response (39).  

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 (40).  

Fasting can also be harmful for female hormone levels as these are sensitive to needing a regular supply of energy (41). Therefore long periods without food may disrupt an important signalling pathway called the hypothalamic-pituitary-gonadal (HPG) axis (42). This in turn can impact hormones such as gonadotropin-releasing hormone, luteinising hormone, follicle stimulating hormone, oestrogen and progesterone. Fasting can also increase levels of the stress hormone cortisol (43). This can put additional pressure on the HPG axis and compound things for women who are trying to restore their hormone levels and period. As with female health studies in general, much more human research is needed in this area.

It’s important to highlight that fasting, including IF, can be a symptom, trigger or socially-acceptable manifestation of disordered eating. So 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 (44, 45, 46). For more information about disordered eating and IF check out this article. 

For some people, fasting can also lead to unpleasant side effects like (47, 48): 

  • Headaches
  • Reduced energy levels
  • Less movement
  • Anxiety
  • Irritability and low mood
  • Poor sleep

So there are a lot of significant risks to consider when it comes to fasting, including IF.

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 more research is emerging related to this all the time. 

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. 

But there are a lot of significant risks related to fasting, including IF, so this isn’t a suitable approach for many people. 

And importantly, 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. 


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Intermittent fasting was last modified: May 30th, 2022 by Maeve Hanan

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