Unlike laboratory rats, humans can’t be easily coerced into a new diet. Nutrition studies have had to grapple with that fact from the very start, which is why some of the earliest research was conducted in prisons, where Victorian researchers could control meals and see which ones were most likely to cause malnutrition or scurvy. Those methods hardly square with the ethics of modern research, which leaves investigators with the problem of where to find large populations that willingly undergo a dietary switch-up.

One avenue: Looking at religious fasts. Billions of people across the globe participate in observances of Christian Lent, which began this year on February 26 (March 2 for Orthodox Christians), or Islamic Ramadan, which starts April 23. For many practitioners, the season means reducing calories and skipping meat, among other practices. Others skip meals, or only eat during certain hours, a topic increasingly of interest at a time when “intermittent fasting” has become popular as a way to control weight.

One of the earliest religious diets under investigation has been that of the Seventh-Day Adventists, which recommends practitioners eat two meals per day and follow a plant-based diet year-round. Researchers have studied more than 100,000 Seventh-Day Adventists over the last six decades, and found that, on average, they live longer than non-Adventists of comparable demographics in the United States and have a lower risk of cancer. A number of follow-up studies are trying to tease out what factors may result in the increased longevity.

The vegetarian aspects of the Adventist Health Studies inform dozens of studies each year, and have recently been used to investigate how that diet affects the risk of prostate cancer, breast cancer and cardiovascular disease. Other research is digging into their practice of skipping one meal a day, and a study of 50,000 Adventists has been used frequently as evidence that adopting this pattern of eating can lead to sustainable, healthy weight loss.

Looking at other faiths can help with understanding shorter-term changes in diet. In Ethiopian Orthodox Christianity, Lent is called the “Hudade,” and practitioners eat only plant-based foods and skip breakfast for 55 days. Researchers recently studied a population of fasters and nonfasters at Jimma University, and found that the fasters had, perhaps unsurprisingly, lower body fat percentages and smaller waistlines. But fasters also had higher levels of high-density lipoprotein—the “good” kind of cholesterol, associated with lower risk of heart disease and other problems. The results were published in November 2019 in Scientific Reports.

Still other studies are looking for negative impacts of fasting, especially for people who might be adversely affected by a sudden change in diet, such as diabetics. The Middle East has one of the fastest growing incidence of diabetes, and Pakistan, a Muslim-majority country, ranks fourth in prevalence. Researchers have expressed concern that the effects of skipping food entirely while the sun is up—the traditional observance of the Ramadan holiday—could cause these people to have dangerous spikes and troughs in their blood sugar levels.

“There are so many studies on patients with diabetes during Ramadan, but they’re mostly done without continuous glucose monitoring, and they’re not consistent,” says Targ Elgzyri, an endocrinologist and researcher at Lund University in Sweden who sees many Muslim patients.

While Islam exempts those in poor health from fasting, many diabetics choose to participate anyway. Elgzyri recently embarked on a prospective study that charted the daily ups and downs of individual glucose levels as people began and ended the fast. “Our goal is to know if patients should or should not fast during Ramadan based on evidence, rather than using clinical experience or religious background,” he says.

His study, published in October 2018 in the Libyan Journal of Medicine, recruited volunteers in the United Arab Emirates who had type II diabetes but didn’t take insulin. While he found some variation in glucose levels at the beginning of the fast, there was almost none at the end, suggesting that a new equilibrium might have developed during the fast that reduced their danger.

Other fast-related studies are underway this year, linking the pursuit of health data to millennia-old practices of faith. Elgzyri plans to continue his studies during this year’s Ramadan, and hopes that his findings not only learn from but also help his Muslim patients. “I just want to give them good and safe advice,” he says.