Intermittent fasting has been growing in popularity over the past few years, with many swearing by this way of eating for weight loss and health. But what does the evidence tell us?
What is Intermittent Fasting?
Intermittent fasting is a diet that requires dieters to fast on an intermittent basis. This means eating no or little calories during these periods. During a fast our body will switch from using carbohydrates (our primary source of fuel) as a source of energy to using fat. This typically occurs after 8-16 hours without eating. Hence people try to use fasting as a method of weight loss or altering body composition. However, others are also using this way of eating in the hopes of managing conditions such as diabetes, cardiovascular disease, cancer, and improving longevity.
The most studied forms of fasting include:
- Time-restricted feeding. Typically 12 hours fast or more per day.
- Alternate day fasting. One example being the 5:2 diet, whereby 2 days of the week dieters eat minimal calories.
It is important to note that there is no clear definition of what an intermittent fasting regime should look like. Generally, diets provide up to 800 calories on a fasting day, with numbers of fasting days also varying from 2 days per week to over 7 days per week. This means that there tends to be a large variance of different fasting regimes used in research studies. This can make it more difficult to assess findings and pinpoint the most effective regime.
Let’s take a deeper dive into the evidence for intermittent fasting in different settings.
Overweight and obesity
All types of intermittent fasting regimes have been shown to help people consume less calories which in turn creates a calorie deficit. A calorie deficit is essential for weight loss. But where this weight loss comes from is important too. For example, weight loss from muscle may lead to a slower metabolism (simply put, this may make weight loss harder, cue plateau) and may interfere with strength and stamina reducing our energy output (i.e. unsatisfying gym sessions).
We ideally want to be losing weight from our fat stores instead. However, research tends to be fairly mixed, generally showing that body composition (i.e. fat vs muscle mass) changes are comparable from intermittent fasting regimes to other more traditional methods of creating a calorie deficit (i.e. a 100kcal reduction in intake per day). What tends to be important here is pairing intermittent fasting (and traditional calorie deficits too) with adequate training programmes that use our muscles and a diet that is adequate in protein.
Other potential benefits of intermittent fasting for weight loss include changes to appetite hormones that help to reduce leptin. Leptin is a hormone that tells us that we are full, but can become abnormally high in those living with overweight and obesity. Therefore reducing levels may be of benefit for these people. At least in the short term.
From the evidence, it appears that various different intermittent fasting regimes can lead to a weight loss of about 3-8% in the short term. This is comparable to other studies using traditional calorie restriction methods. From the few longer term studies available it also appears that weight loss maintenance is similar to that of traditional weight loss methods too.
Therefore it appears that intermittent fasting is another tool that we can use for weight loss, but it should not be deemed as superior. Of the different types of intermittent fasting regimes, it appears that forms of alternate day fasting are more effective, in the short-term at least.
Type 2 Diabetes
Large studies tend to find that in the short term, intermittent fasting may lead to greater weight loss for those living with type 2 diabetes and overweight or obesity, when compared to other traditional methods of calorie restriction. We know that weight loss is key to managing risk factors associated with overweight, obesity and type 2 diabetes. But despite this, improvements to long-term blood glucose readings (this is what we call HbA1c) and glycemic control may be similar to other traditional methods of calorie restriction. Only one study found that intermittent fasting led to improvements in fasting insulin levels, but these were not clinically relevant. Of all the different intermittent fasting regimes, it appears that alternate day fasting may have a slight advantage for managing type 2 diabetes for those living with overweight or obesity.
Of the few studies with longer follow-ups, there appears to be little promising results relating to weight loss maintenance and glycemic control either. We really need more long term studies before we start recommending intermittent fasting to manage risk and outcomes for those living with type 2 diabetes.
Intermittent fasting doesn’t appear to be any better than traditional methods of calorie restriction for improving risk factors for heart disease. These include blood pressure, cholesterol (total, LDL and HDL) and triglycerides. These findings are the same in both short-term and in two long-term studies.
Animal studies have demonstrated positive outcomes after 48 to 72 hours of fasting whilst receiving chemotherapy, and some larger studies (using mostly animal models) also show tumour suppression and chemoprotective outcomes. Other smaller studies (on humans) fasting prior to treatment report reduced side-effects from treatment, such as weakness and fatigue.
It’s worth noting that findings from animal studies do not always translate to human studies, and the human studies also have their limitations too as they are typically small and rely on people reporting their symptoms which can be open to bias (reducing the reliability of findings). Additionally, studies may be looking at specific cancer types – results can’t be automatically assumed to be the same when considering other types of cancers.
It’s important to note that cancer can lead to malnutrition in patients, so adding a restrictive diet such as intermittent fasting may lead to unwanted weight loss and possibly nutritional deficiencies.
Diets such as intermittent fasting and other weight loss interventions should always be discussed with the oncology team and have input from an oncology dietitian. Whilst results are looking interesting in certain areas, more human research is needed before it’s something we can recommend.
Intermittent fasting is also gaining interest for its potential to reduce the effects of ageing and age-related damage. To date, most of this research is conducted on animals or yeast as measuring outcomes relating to longevity is difficult. Measuring this on organisms with a shorter lifespan is easier to do and can give us more data.
Of the research in humans, it appears that benefits tend to be fairly isolated to those that are metabolically less healthy. For example, for those living with overweight or obesity. Here our bodies tend to be in a state where the body finds it more difficult to repair and recycle damaged cells. This can lead to more damage and less highly productive cells being produced. This may contribute towards disease and ageing. Other benefits from smaller studies suggest that there may be less oxidative stress and inflammation in the short term. However, this may be related to weight loss. These benefits tend to occur with longer time-restricted feeding windows of 16 hours or more, which may be harder to stick to.
The opposite is true for adults that are metabolically healthy, whereby benefits appear to be lacking. This is likely because processes such as autophagy (the process by which cells degrade and recycle their components) are more likely to occur naturally. Ultimately, more long-term human studies are needed.
The gut microbiota
The gut microbiota (the trillions of organisms living within our gastrointestinal tract) has been proposed as one of the mechanisms by which benefits from intermittent fasting occur. However, much of the evidence has been carried out on animals or small human studies (largely adults throughout Ramadan).
It does appear that there may be some benefit for those who have an imbalance in their gut microbiota, otherwise called dysbiosis. Other potential benefits are that possible changes can lead to genetic shifts relating to carbohydrate metabolism, which may help to improve blood glucose and body mass index. These benefits tend to occur with longer time-restricted feeding windows of 16 hours or more.
With any change, there might be some form of side-effects. Dieters in research studies reported a lack of energy, headaches, feeling cold, constipation, bad breath, lack of concentration and bad temper. Often these side-effects are reduced with time. In the longer-term, restriction as seen in intermittent fasting (and other more extreme diets) may trigger disordered eating for some. This is why consulting a medical professional or dietitian before committing to such diets, is the safest approach.
Intermittent fasting could offer a promising approach to help people living with overweight and obesity to reduce their weight and improve health. But it is not superior to other traditional diets. It is also considered generally safe for most people in the short-term, but unfortunately we know little about the long-term effects of intermittent fasting.
With all diets, long-term adherence is generally poor. Ultimately, it will be the adherence to diets that predict long-term outcomes. To overcome this, we should consider approaches that minimise burden by considering individual-specific factors. This is where a dietitian can add value. Dietitians can vet patients for contraindications (such as eating disorders, some anti-diabetic medication etc) to maintain safety and create a tailored intermittent fasting plan to suit individuals that focuses on nutritional quality to preserve muscle mass.