Intermittent fasting has reached a new level of interest in the health industry, with as many people in praise of it as suspicious of it.

Often used as a dieting aid, it's the process of restricting the amount of calories you consume in a day, and has been in the headlines on and off since Jack Dorsey - the CEO of Twitter - revealed that he eats just one meal a day and nothing over the weekend, an extreme version that has spurred on arguments that it is a dangerous fad. 

Now, a major US study has found that restricting the amount of calories you consume in a day can have unexpected health benefits for young and middle-aged adults. 

The study, which was funded by the US National Institutes of Health and published this month in the Lancet Diabetes & Endocrinology, focused a group of 143 healthy men and women who ranged in age from 21 to 50. 

These men and women were asked to practice "caloric restriction" for two years, with researchers allowing them to eat whatever they wanted, as long as they reduced the total amount of food they ate. The goal was to reduce their calories by 25%. 

According to the study, the majority of the dieters didn't reach this target, with most slashing 12% of their total calories - roughly 300 calories a day, or a large bagel or some chocolate chip cookies. 

Still, the group saw many of their cardiovascular and metabolic health markers improve, even though they were already in the normal range of calories consumed.

Participants lost weight and body fat, while their cholesterol levels improved, their blood pressure fell slightly, and they had better blood sugar control and less inflammation. A control group of 75 healthy people who did not cut their calories saw no improvements in these areas. 

The study states that some of the benefits in the calorie-restricted group was due to the fact that they lost a large amount of weight - on average about 16lb (7kg) over the two years of the study.

But for researchers, the improvements in the group's metabolic health exceeded expectations. Dr. William Kraus, the lead author of the study and a professor of medicine and cardiology at Duke University, said that this might suggest that caloric restriction might have some unique biological effects on disease pathways in the body.

"We weren't surprised that there were changes," he said. "But the magnitude was rather astounding. In a disease population there aren’t five drugs in combination that would cause this aggregate of an improvement."

The study also showed the challenges involved in restricting calories, with the group not also to meet even half the goal of cutting calories by 25%, despite undergoing intensive training, learning to cook low-calorie meals and attending check-ins with nutrition experts. 

Speaking to RTÉ Lifestyle, Maeve Hanan, Dietitian from the Dublin Nutrition Centre, says that there are multiple kinds of fasting, each suited to different health goals, but stresses that different people will respond to different diets. 

She stresses that central to caloric restriction is "maintaining a nutritionally balanced diet", and this makes it useful for people who are overweight, especially if they are at risk of type 2 diabetes or heart disease. She says that the most common form of fasting is reducing the amount of calories you consume in a day, much like the study above, while intermittent fasting can work for others. 

"There are two main types of intermittent fasting", she explains. "Alternate day fasting where very few calories are consumed on 1-2 days per week (like the 5:2 diet), and time-restricted feeding where all calories are eaten within a specific feeding window each day (like the 16:8 diet)."

Intermittent fasting has boomed in popularity recently, especially in Silicon Valley circles where tech professionals like Twitter CEO Jack Dorsey use fasting to work in ways they believe are more efficient.

Benefits like more stable blood pressure and blood glucose levels, a reduction in inflammation and damage caused by free radicals, as well as weight loss, have been observed in studies, but - as Hanan says - the majority of this research has been carried out on rats, so we don't know the long term effects on humans. 

"Although intermittent fasting can work really well for some people, having long fasts might not be safe for those who are prone to low blood sugars, or for athletes who need to pace their protein intake across the day and refuel properly after exercising", she adds.

Speaking to RTÉ Lifestyle, Michelle Loughlin, a dietitian with Spectrum Nutrition, added that the study doesn't prove that extreme calorie restriction is needed to improve health, saying: "What this study actually shows is that in contrast to fasting, just a small calorie deficit of 300 calories per day is effective in producing long-term weight loss and health benefits." 

She adds that the true take away from the study is that small, sustainable changes are best: "What I see in practice, is when clients try to drastically reduce their calorie consumption or cut out foods/food groups, they end up feeling deprived and eventually go back to their old eating habits.

"Those who are successful long-term tend to make small changes but sustain them consistently."

"Intermittent fasting definitely isn't recommended for pregnant or breastfeeding women, those who are malnourished, or anyone who has a history of an eating disorder. If somebody is thinking of making a change to their diet like this, it is always best to get individual support from a Registered Dietitian", says Hanan. 

"For some people counting calories can become obsessive and counter-productive, so the approach I usually use with clients focuses on learning how to put together healthy meals and snacks, as calorie reduction happens as a side effect of balancing out the overall diet."

For Loughlin, fasting is not a sustainable, healthy way to lose weight. "When working with individuals, the biggest issues they tend to come up against are environmental such as working crazy hours and being fatigued and reaching for sugary snacks, having a busy lifestyle and not having time to cook, eating out with friends and alcohol consumption", she says. 

"My most successful clients continue to eat out, go on holidays and basically eat how they used to eat incorporating some healthy changes, but their mentality and relationship with food shifts and they no longer try to stick to all or nothing rules. This leads to real food freedom." 

She clarifies, however, that different methods suit different people, so doing what's best for you - safely - is always the best option. 

It is important to note that this study focused on a specific demographic of people and anyone considering a new diet plan should contact their GP beforehand.