Changing when you eat may not rewire how the heart fuels itself – but it may rapidly restore how well blood can reach it. Volunteers followed alternate-day fasting, and researchers found striking improvement in arterial function – a key predictor of heart disease outcomes, essentially how well the heart can increase blood flow when it needs more oxygen. Such changes are usually achieved only with prolonged drug treatment.
What if the timing of your meals could improve how easily blood reaches your heart? To test that idea, researchers from Aarhus University in Denmark asked volunteers to abstain from food every other day.
Their findings, presented in the Journal of Clinical Endocrinology and Metabolism, overturn a popular idea about fasting: the expected shift in fuel never materialises. Instead, something else changes.
“The ability to open up the arteries increased dramatically,” says co-author Lars Christian Gormsen, a professor, physician and nuclear imaging specialist at Aarhus University. “That is one of the strongest predictors of survival in heart disease.”
Extreme fasting: a tough test of the idea
Alternate-day fasting is a more extreme diet than most people are willing to sign up for. “You have an eating window of around 12 hours one day, then you fast for 36 hours, and then you eat for 12 hours again,” explains Esben Søndergaard, a professor at Aarhus University who researches metabolism. During the fast, the only intake is plain water. Alternate-day fasting is not for people younger than 18 years or anyone with a history of disordered eating. “I would not be able to do it myself,” Gormsen adds.
The researchers saw alternate-day fasting as a clear test of the concept. “What we are really interested in now is whether some less severe dieting may have some of the same effects,” Gormsen says. He points to time-restricted eating – limiting meals to a 6- to 11-hour window per day – as a more approachable alternative.
At the heart of the study was a simple expectation: fasting would rewire how the heart fuels itself. Our muscles typically rely on glucose as fuel. But some research suggests that the heart can switch to ketones – energy molecules made from fat – and run more efficiently, producing the same energy while using less oxygen. Studies in heart failure and experimental ketone infusion support this idea.
A fuel shift that never really happens
Whether ketogenic diets – which also raise ketone levels – help or hurt cardiovascular health long term is not clear.
“We have known for a century that it has an effect for epilepsy – for children with severe epilepsy, it helps to reduce their seizures,” Søndergaard says. The keto diet became a household phrase in the 2010s as it exploded as a weight-loss strategy. But studies suggest that the high-fat, very-low-carb approach can raise cholesterol levels enough to undermine potential benefits.
“But raising ketones by diet is quite different from raising them by fasting. We wanted to measure how many actually enter the heart,” Gormsen says.
The scientists tracked several measures of cardiovascular health as 16 volunteers with obesity followed alternate-day fasting for three weeks in a controlled crossover study. Each participant also served as their own control under a standard diet.
In the past, researchers would have needed to insert a catheter into the heart to evaluate such effects. Here, PET scans enabled the team to measure how blood and fuel molecules move through the heart in real time using tiny radioactive tracers – without an invasive procedure.
No fuel shift – but a striking effect on blood flow
The participants experienced several familiar benefits from fasting, including lower blood pressure and resting heart rate. These effects are well documented in both human and animal studies of intermittent fasting. But the expected shift never came: the heart did not take up or use more ketones.
“We thought the heart would adapt to alternate-day fasting and be able to transport more ketones into each cell,” Gormsen says. “But the heart did not really adapt that much. It did not get more efficient.”
Instead, the effect showed up somewhere else entirely – in the arteries.
“You want to be able to open your arteries to supply as much oxygen as the heart needs,” Gormsen says. After just three weeks, myocardial flow reserve – how much blood flow can increase from rest to stress – was up 34%.
That is a “really dramatic” jump, he says – an effect that often requires months of treatment with multiple prescription drugs. In some studies, it appears only after long-term statin or beta-blocker therapy.
Why the arteries respond is still unclear
Why this effect appears – despite no change in fuel use – remains unclear. Several lines of evidence suggest similar mechanisms, including improved endothelial function, changes in nitric oxide signalling and reduced vascular resistance found in both human and animal fasting studies.
“Maybe there is a direct effect on the endothelium – the thin inner lining of blood vessels that controls how wide they open,” Gormsen says. Or it may simply reflect lower blood pressure reducing resistance in the vessels – a well-documented effect of intermittent fasting, Søndergaard adds.
The authors emphasise that weight loss alone is unlikely to explain the vascular changes. Analyses show that improvements in blood flow reserve were not strongly linked to weight loss. Participants lost a modest 3% of their body weight over three weeks.
And future studies will be needed to assess whether the same effects prevail among people who do not have overweight or in other patient groups, since most previous fasting studies have focused on metabolic outcomes rather than direct measurements of heart function.
From extreme fasting to everyday habits
Despite the striking results, the researchers do not envision prescribing alternate-day fasting long term.
It is not yet clear how long the arteries stay widened after returning to normal eating. And few people can sustain alternate-day fasting long term. The next step is to see whether these effects can be replicated at a smaller, more practical scale. “Let’s see whether we can replicate this with time-restricted eating,” Gormsen says.
He and his colleagues say they are already working on a follow-up study to test the same metrics of heart health for intermittent fasting with an eating window of six hours a day.
Who could benefit the most from this kind of diet change? “I would say people with coronary microvascular disease, in which small vessels cannot widen properly and limit blood flow – a key treatment target. Maybe also people who have had diabetes for some time but do not have heart failure outright,” he says.
“Skipping breakfast or avoiding evening meals may have a real, measurable effect on how blood reaches the heart,” Gormsen says.
