Fast fact

Fasting found to have a transformative effect on diet results

A new study suggests fasting can benefit difficult-to-treat metabolic problems.

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Claims about the health benefits of fasting tend to edge on hype, with scientists racing to produce study results that can match the clip of fandom.

Proponents of intermittent fasting tout its ability to prompt rapid weight loss, improve heart health, and even reduce the risk of cancer. Liquid fasters talk about better digestion and hydration. Meanwhile, juice cleanses and fasts promise to fix basically everything.

But new research reveals some of the strongest evidence yet that fasting can have cascading benefits for multiple aspects of health.

This study explored the effects of another kind of fast: one that happens before beginning a new diet. Ultimately, the research team found this action can significantly influence some dieters’ health as far as three months down the line. This result was published in March in the journal Nature Communications.

Participants with metabolic problems who fasted briefly before starting a new food regimen saw long-term differences in their:

The same results were not seen in the group who did not fast before adopting the new diet. The study illuminates the ways a fast can influence many parts of the body points toward an effective way to treat stubborn health problems like high blood pressure and obesity.

Is fasting a good way to start a diet?

Senior author Sofia K. Forslund is the group leader of the Forslund Lab at the Max Delbruck Center for Molecular Medicine in the Helmholtz Association in Berlin, Germany. She describes fasting as a form of “controlled damage.”

These results suggest when controlled damage is induced, “then the restoration of the gut ecosystem either results in a better ecosystem or results in a period of activity that is associated with health,” Forslund tells Inverse.

The study participants were put on a meal plan similar to the Mediterranean diet.


Previous studies have examined links between intermittent fasting and improved cardiovascular health, limited weight loss, and increases in anti-inflammatory gut bacteria in mice. Meanwhile, mouse experiments suggest intermittent fasting can spur changes in gut bacteria, which in turn can affect cognition.

But studies on gut health after a single period of fasting before a diet are less common — especially in humans. They’re also often conflicting.

For example, a study published in May 2020 focused on the gut microbiome of a small group of people fasting for Ramadan and noted marked microbiome changes. Meanwhile, research published in 2018 on the effects of a 24-hour fast found gut bacteria produced a chemical compound thought to increase the risk for cardiovascular disease.

Another study, this time published in 2019 in the Journal of Nutritional Science, looked at the effects of a 10 day fast on 15 healthy men in Germany. This fast induced positive changes in the gut microbiota and other health benefits, including lower blood pressure.

Forslund’s work is one of the first studies to closely examine the microbiome in relation to other health indicators after fasting before and during a diet.

What’s new — The team recruited 145 participants with metabolic syndrome, which describes a number of conditions observed together, including high cholesterol, high blood pressure, high blood sugar, and high body fat.

The participants were split into two groups. Both groups would adopt a diet called DASH for about three months, which is a Mediterranean-style diet high in vegetables, fruits, and whole grains and low in dairy, sodium, and some fats.

However, only one group began their three months with a five-day fast, only consuming very limited amounts of vegetable juices and broths.

Researchers compared the two conditions, examining changes in their microbiome composition, immune cells, blood pressure, BMI, and body weight from their baseline, after one week and after the three months.

People who fasted before dieting, compared to those who did not, had marked differences in how their gut bacteria changed.

Nature Communications / Maifeld et al.

They discovered:

  • There was a distinct change in the microbiota of the fasting and DASH group that did not occur in the DASH alone group.
  • There was a distinct change in the immune cells of the fasting and DASH group that didn’t occur in the DASH alone group.
  • All participants saw reductions in their blood pressure, weight, and BMI after three months. However, only the fasting and DASH group experienced significant changes, sometimes lasting up to three months.
  • A subset of the fasting and DASH group called “responders” had different gut microbiome and immune cell responses from the others in their group.
  • This “responder” subset had a longer-term reduction in blood pressure that could be predicted by their baseline microbiota and immune cell composition.

How fasting affects the microbiome

This is one of the first studies to examine the effects of fasting prior to a diet and the corresponding changes to the human microbiome.

The study team hypothesizes that the lasting effects on health markers like BMI, weight, and blood pressure in the fasting group came from a jolt to the gut microbiome system. This jolt, in turn, affected immune cells and augmented the diet.

Forslund describes it as a kind of “controlled insult to the system that allows it to adapt and regain homeostasis, that involves both the microbiome and the host.”

For example, during the fast, some disease-related bacteria in the gut increased. When the fast was over, their numbers fell. Meanwhile, fasting also caused a reduction in some beneficial or “commensal” bacteria. But post-fast, this good bacteria grew back and induced anti-inflammatory effects.

Some of the characteristics of high blood pressure, obesity, and heart problems are related to an overactive response similar to inflammation of subsystems in the body, Forslund says. The effect fasting has on the body could be why these conditions were positively influenced.

What’s next — In this study, it was hard to tell whether or not the relative success of the fasting and DASH group had to with how well they actually stuck to the diet, Forslund says. Though the study provided food counseling and some degree of monitoring, it could be that those who fasted before their diet had an easier time adhering to it. Redoing the study with more detailed dietary tracking could help tease out this question.

The key to further research will be examining the changes in microbiota more closely, and seeing what the effects of a pre-diet fast are in other populations, Forslund explains.

Currently, she’s working on a similar study with participants who have multiple sclerosis to see if the same results can be achieved.

“I think this is probably a set of mechanisms that are shared between many different diseases,” Forslund says. “I think it could be potentially applied more widely.”

This wider application is perhaps this work’s most intriguing potential. Forslund said she looked at data on the microbiome of people with chronic fatigue and found almost the opposite microbiome composition as in the fasting group. “There might be a number of these sort of poorly recognized complex diseases that affect quality of life, in particular for women, where perhaps fasting could be used [as an intervention],” she says.

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