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Drawbacks to benefits of short fasting, study finds

Researchers have suggested that several possible health benefits of fasting – done properly and under medical supervision – might, in fact, not occur in shorter or more intermittent fasts.

The collaborative study by teams in Europe and the UK found it took more than three days for all major organs to change protein production in ways that could predict better health in participants undertaking a seven-day water-only fast.

These changes were consistent across all 12 healthy participants (five women and seven men), who had their blood taken before, during and after the week of fasting, reports ScienceAlert.

“For the first time, we're able to see what’s happening on a molecular level across the body when we fast,” said Claudia Langenberg, an epidemiologist from Queen Mary University of London.

“Our results provide evidence for the health benefits of fasting – beyond weight loss – but these were only visible after three days of total caloric restriction, which is later than we previously thought.”

That’s a long time to deprive the human body of calories, which introduces some serious risks that may not be worth the desired outcomes.

The body can survive without food for some time before starvation risks causing significant harm.

And while fasting, if done safely and under health professionals’ supervision, may hold possible health benefits, the downsides also need to be carefully considered. Physicians generally advise that children, teens, pregnant people, or anyone with diabetes or eating disorders avoid intermittent fasting.

There’s also a risk of dehydration when fasting, because about 20% of our usual fluid intake comes from food, so people should also make sure they are consuming plenty of water.

Fasting for days on end can be dangerous, and its potential benefits are still not clearly demonstrated. That said, if more research can be done, scientists might be able to mimic benefits of fasting without anyone actually having to deprive themselves of calories.

In recent years, numerous studies have suggested that intermittent fasting, as opposed to the prolonged fasting in the study, could improve some aspects of human health. These include weight loss, lowered blood pressure, improved bone density and appetite control.

Some experiments have even found evidence that experiencing hunger for short bouts of time could slow down the natural ageing process and possibly extend a person’s lifespan.

For all that, scientists have a very limited understanding of what actually happens to the human body when it adapts to starvation. Clinical trials on the topic are limited, which means health professionals don't have evidence-based advice they can give to patients.

The newest research found that of all 3 000-plus proteins measured in the participants’ blood, about a third showed “profound systemic changes” after consuming nothing but water for seven days. The protein changes that were predicted to have the greatest health benefits, however, were only observed after three consecutive days of fasting.

These include protein changes linked to improvements for rheumatoid arthritis and cardiovascular health.

The results support past trials, which also suggest fasting requires several days before it becomes more useful than simply reducing calorie intake.

Most proteins return to baseline the moment a person starts to eat again, which suggests that biological changes from fasting need to be sustained for a certain amount of time to reap long-term health benefits.

“Lack of food has been the default situation throughout human evolution, and our bodies are the result of a selection process for high metabolic flexibility to survive long periods without it,” said the study authors.

“Our results provide the opportunity to systematically identify the potential health benefits from fasting and translate this knowledge into putative interventions, including for patients who cannot adhere to prolonged fasting schemes or fasting-mimicking diets.”

Given the sample size of this latest current study was extremely small, the results are unlikely to be representative of the effects of fasting on a diverse human population, but the international team of scientists hopes their results will provide an important reference point for future research on fasting.

The study was published in Nature Metabolism.

Study details

Systemic proteome adaptions to 7-day complete caloric restriction in humans

Maik Pietzner, Burulça Uluvar, Claudia Langenberg et al.

Published in Nature Metabolism on 1 March 2024


Surviving long periods without food has shaped human evolution. In ancient and modern societies, prolonged fasting was/is practiced by billions of people globally for religious purposes, used to treat diseases such as epilepsy, and recently gained popularity as weight loss intervention, but we still have a very limited understanding of the systemic adaptions in humans to extreme caloric restriction of different durations. Here we show that a 7-day water-only fast leads to an average weight loss of 5.7 kg (±0.8 kg) among 12 volunteers (5 women, 7 men). We demonstrate nine distinct proteomic response profiles, with systemic changes evident only after 3 days of complete calorie restriction based on in-depth characterisation of the temporal trajectories of ~3,000 plasma proteins measured before, daily during, and after fasting. The multi-organ response to complete caloric restriction shows distinct effects of fasting duration and weight loss and is remarkably conserved across volunteers with >1,000 significantly responding proteins. The fasting signature is strongly enriched for extracellular matrix proteins from various body sites, demonstrating profound non-metabolic adaptions, including extreme changes in the brain-specific extracellular matrix protein tenascin-R. Using proteogenomic approaches, we estimate the health consequences for 212 proteins that change during fasting across ~500 outcomes and identified putative beneficial (SWAP70 and rheumatoid arthritis or HYOU1 and heart disease), as well as adverse effects. Our results advance our understanding of prolonged fasting in humans beyond a merely energy-centric adaptions towards a systemic response that can inform targeted therapeutic modulation.


Nature article – Systemic proteome adaptions to 7-day complete caloric restriction in humans (Open access)


ScienceAlert article – Study Reveals a Major Drawback to The Health Benefits of Fasting (Open access)


See more from MedicalBrief archives:


NHS scepticism over study claiming heart benefits of fasting diet


Evidence review: Intermittent fasting for weight loss and lower cardiometabolic risk


Fasting as a weight-loss intervention in gestational diabetes – Australian trial


How good is the now trendy Intermittent Fasting diet?




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