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

In fact, when done properly and under medical supervision, fasting is thought to offer a range of health benefits to people – but those benefits may not occur in shorter or more intermittent fasts, new research shows.

A new study by researchers 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.

"For the first time, we're able to see what's happening on a molecular level across the body when we fast," explains 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 – 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.

When done safely and under the supervision of a health professional, fasting may hold health benefits, but the downsides also need to be carefully considered for each individual. Physicians generally advise that children, teens, pregnant people, or people with diabetes or eating disorders do not partake in intermittent fasting.

There's a risk of dehydration when fasting, because around 20 percent of our usual fluid intake comes from food, so individuals 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 – may 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 aging process and possibly extend a person's lifespan.

For all that, scientists have a very limited understanding of what is actually happening 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-some 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," explain the authors of the study.

"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 the current study is extremely small, the results are unlikely to be representative of the effects of fasting on a diverse human population. The international team of scientists hopes that their results will provide an important reference point for future research on fasting.

The study was published in Nature Metabolism.