Intermittent fasting: between trend and tradition

It sounds simple: eat whenever you like – as long as it fits within a specific time window; in between, you fast. No calorie counting, no complicated diet plans. Intermittent fasting is considered one of the major dietary trends of recent years. But there is more to it than just a lifestyle fad, and the method has been the focus of research for years. The background is a serious one: worldwide, obesity and metabolic diseases are reaching epidemic proportions. In Austria, too, according to a health survey, more than 46 per cent of the population aged 15 and over are overweight or obese. This makes it clear that metabolic diseases are by no means confined to individual specialist fields, but represent a key challenge for the medical profession as a whole.

At the Medical University of Graz, Harald Sourij is investigating precisely this trend. As head of the Cardiometabolic Trials Unit, he and his team are examining how closely metabolic disorders are linked to other conditions, such as cardiovascular, renal, hepatic or haematological-oncological diseases. The approach is deliberately interdisciplinary: metabolic diseases are not considered in isolation, but within the context of their diverse interactions. The focus is on clinical trials aimed at scientifically evaluating both new drug therapies and lifestyle interventions.

Research at the interface between metabolism and the heart

A particular focus of the research team is on dietary strategies such as intermittent fasting. Various fasting regimens, such as alternate-day fasting or time-restricted eating, are specifically investigated to determine how they influence the cardiovascular and metabolic risk profile. “It has been well established that reducing energy intake and body weight in cases of overweight or obesity leads to a reduction in the incidence of diabetes and an improvement in cardiovascular risk profiles. However, achieving a sustained reduction in energy intake and, above all, long-term weight loss proves difficult. Intermittent fasting may offer a helpful alternative, as people often find it easier to consume no meals or just one meal (e.g. breakfast only) on selected days of the week than to continuously reduce their energy intake,” explains Harald Sourij. 

As with traditional diets, reducing energy intake has a positive effect on body weight and established cardiometabolic risk factors. However, the scientist explains the fundamental difference as follows: “What happens in addition is that regular, prolonged periods of fasting lead to a ‘metabolic switch’ towards increased lipolysis, fatty acid metabolism and ketone body production. In various mechanistic studies using animal models, this promotes not only autophagy but also cell repair processes and the slowing down of ageing processes. This field of research is therefore currently of great interest – particularly at the Medical University of Graz,” says the expert.

Metabolism in transition: current research 

Whilst we are eating, our body is in ‘building mode’: energy is stored, insulin is released, and nutrients are processed. During fasting phases, however, the body switches gears. Insulin levels fall, fat reserves are mobilised, and repair processes within the cells are activated. These processes are particularly relevant in the context of modern widespread diseases. Conditions such as type 2 diabetes or fatty liver disease are closely linked to disrupted metabolic processes, and this is precisely where many research approaches to intermittent fasting come into play.
At the Medical University of Graz, research into this topic is currently being conducted on several fronts. In a clinical trial funded by the FWF, the Cardiometabolic Trials Unit is currently investigating, in people with type 2 diabetes not receiving insulin therapy, how intermittent fasting, regular physical activity (a combination of endurance and strength training) or a combination of both affect body weight, glucose metabolism, sleep patterns and sleep quality, as well as patient-reported outcomes. The first phase of the study has been completed and is currently being analysed; however, the long-term effects two years after the intervention are also to be investigated. 
As part of the ERA4Health project OPTIMAMIND, in which Harald Sourij’s Trials Unit and the Medical University of Graz’s biobank are participating, the influence of intermittent fasting on markers of neurocognitive function is being investigated. The MetAGE Cluster of Excellence project, led by Thomas Pieber at the Medical University of Graz and involving numerous research groups from the Medical Universities of Graz and Vienna as well as the University of Graz, is also addressing this topic. The focus here is on the various aspects of ageing processes, the impact of obesity on metabolic control in age-related diseases, and how these might be influenced by dietary interventions or, for example, the administration of spermidine.

Intermittent fasting: a concept for everyone?

Fasting is not a new concept; it has been part of daily life in many cultures and religions for centuries. What is new, however, is the perspective taken by modern medicine, which examines its specific effects on health. Intermittent fasting is not primarily about what we eat, but about when we eat. The so-called 16:8 method is particularly widespread: fasting for 16 hours, eating for eight. Other variants, such as 5:2 – eating normally for five days and severely restricting intake on two days – are also popular. With all these options, many people wonder which intermittent fasting method makes the most sense for them. “I see several factors at play here – if the main aim is weight loss, the preference of the person who is to follow this method over a longer period is a crucial factor. When it comes to issues of cell repair or ageing processes, the duration of the fasting period is arguably a key parameter, as the fasting period is longer with the 5:2 method, for example. However, what this ultimately means for humans is still the subject of scientific research, as a great deal of the data to date comes only from animal models,” says Harald Sourij. Lifestyle factors such as sleep, stress and exercise also play a role. “Lifestyle appears to have a bidirectional influence – on the one hand, stress and reduced sleep, or even poor sleep quality, can have negative effects on the ‘metabolic switch’ mentioned above; on the other hand, intermittent fasting can influence sleep. In our ongoing studies, we are also collecting data on sleep and sleep quality in order to gain further insights into this area,” explains the researcher. 

As promising as intermittent fasting sounds, it is not a cure-all. For some people, it can even be problematic: for example, for pregnant women, people with certain pre-existing conditions or eating disorders. Everyday life also plays a role. For those with irregular working hours or heavy family commitments, a strict eating schedule can be difficult to maintain. “In any case, I would advise against it for underweight people, those with serious illnesses, children and pregnant women. For people with diabetes mellitus who are on insulin therapy, medical supervision is important, as the insulin dose needs to be adjusted, particularly on fasting days,” emphasises Harald Sourij.

A beacon of hope for health?

The list of potential benefits is long: weight loss, improved blood sugar levels, and positive effects on cardiovascular diseases. Intensive research is also being carried out in the areas of cellular ageing and inflammatory processes. However, not all expectations are clearly substantiated. Many studies are based on animal models or are only short-term in nature. Long-term data from human trials is often still lacking. This is precisely why clinical trials, such as those being conducted in Graz, are of central importance: they provide reliable data on whether, and for whom, intermittent fasting actually brings lasting health benefits.

The INTERFAST-2 study, led by the Medical University of Graz, investigated whether intermittent fasting is safe and effective for people with insulin-treated type 2 diabetes. In people with type 2 diabetes receiving insulin therapy, a difference in body weight of five kilograms was observed after twelve weeks compared with the control group. This is a considerable weight loss that can rival the effect of average ‘weight-loss injections’. The chosen adjustment to the insulin dose in the study (a 20 per cent reduction in the basal insulin dose on fasting days) helped to prevent severe hypoglycaemia.

A follow-up examination took place two years after the start of the study. In the intermittent fasting group, almost all of the weight loss was maintained, even though just under 60 per cent of participants continued to practise intermittent fasting (even if only irregularly) beyond the twelve-week intervention phase. “In none of our studies to date have we observed a change in basal metabolic rate due to intermittent fasting, which could help explain these lasting effects,” reports Harald Sourij. A lasting effect on weight loss was therefore observed, whilst no lasting effect was seen after two years regarding the significant improvement in blood glucose control observed in the first twelve weeks. The researchers attribute this to the progressive nature of diabetes, as all study participants were already receiving insulin therapy involving several injections a day.

“In our current study involving people with type 2 diabetes, there will also be a two-year follow-up. However, dietary intervention studies in people that run over longer periods and in which participants are randomised to a dietary intervention are challenging, burdensome for many people and difficult to carry out, as one can imagine,” explains the scientist.

Between discipline and flexibility

A major advantage of intermittent fasting is its simplicity. Many people report that it means they have to think less about food and develop a better sense of their bodies. At the same time, the method requires discipline and a certain degree of flexibility: after all, not every day is the same and not every body reacts in the same way.

Intermittent fasting is more than just a passing fad. It exemplifies a broader approach in medicine to incorporate lifestyle factors more strongly into prevention and treatment. Given the rising number of people with metabolic disorders, this could be a key lever for change. Research by Harald Sourij and his team shows that diet is not merely a personal choice, but a key factor in overall health. The most important advice for people wishing to try intermittent fasting is: ‘Everyone should find their own optimal fasting interval; this can vary from person to person. If you have any existing medical conditions, you should seek medical guidance. “Often, when people are motivated to lose weight, they try to take extreme measures to shed as much weight as possible in the shortest possible time – but this approach is usually doomed to failure,” says Harald Sourij. Professionally supervised intermittent fasting can be the way to achieve the desired goal.”