More research backs a popular but controversial diet hack
Key Takeaways
A recent animal study published in Nature looked at how intermittent fasting (along with caloric restriction, or CR) affects health and lifespan.
The researchers found that metabolic boosts due to dietary restriction aren’t necessarily tied to longevity.
Experts say that while intermittent fasting is a reasonable way to promote health for most patients, more research is needed.
You’ve probably heard of intermittent fasting (IF): an approach to eating that reduces your food window to a specific number of hours. Maybe you or your patients have tried IF for health or weight management purposes. People either swear by it or they’re wary of it. So, what does the latest science say?
A recent animal study published in Nature looks at how IF (along with caloric restriction, or CR) affects health and lifespan.[] Plenty of studies have affirmed the helpfulness of both IF and CR, but the researchers say that identifying the most helpful forms of dietary restriction (DR) is the goal.
It’s tough because it is hard for people to stick to CR, even though, as the authors say, caloric restriction “delays the onset of age-related diseases and extends lifespan in multiple species.” IF has become popular since it shifts the focus from how much a person eats to when a person eats.
To examine these approaches more closely, the researchers looked at the effects of graded levels of CR (20% and 40%) along with IF (1 and 2 days fasting per week) on 960 genetically diverse female mice, each with differing physiological traits.
The study found that both CR and IF—working in unison—contribute to health and longevity, but not without some caveats mentioned below. “In mice, regular periods of fasting can convey considerable benefits without reduction in overall energy intake,” the authors state. “Mice on CR also experience prolonged periods of daily fasting, and the health benefits of CR can be optimized by feeding at a specific time of the day, suggesting that both caloric intake and timing of feeding contribute to physiological response and lifespan extension.”
While DR may contribute to health (in this case, reduced adiposity and lower fasting glucose). it was the mice’s individual genetic backgrounds that were most important in determining lifespan: “Collectively, our study highlighted physiological resilience, in particular the maintenance of body weight, body composition and key immune cell populations, as major biomarkers for longevity and suggested that the pro-longevity effects of DR may be uncoupled from its effects on metabolic traits.
Despite the powerful effects of DR in this study, genetic background proved to be the more important factor in determining lifespan.”
In short, dietary restriction boosts metabolic health, but it may translate into an extension of lifespan. “This insight has important implications for the choice of biomarkers in human dietary intervention studies, which frequently focus on metabolic health,” the authors write.
The authors also note that while DR offers obvious metabolic benefits, it could also hinder health in other ways. The mice who ate 40% fewer calories experienced lower body temperature and displayed food-seeking behavior (indicating that they were hungry). They also experienced immune issues that could result in infection. This, to the authors, signals a warning sign for humans. It is still unclear whether IF and CR could extend the lifespan of human adults. The ultimate message is that it depends on an individual’s genetic background. “Specific blood biomarkers can predict an individual’s ability to benefit from certain physiological effects of DR, while withstanding others, to maximize its health benefits and longevity effects,” the authors report.
What does all of this mean for humans? First, a quick explanation of IF: “When you fast, it sends a signal to your body to start mobilizing the energy stores, which is primarily fat. After a period of time, after using immediate sources (glucose), the body still needs more energy, so it needs to break down fat to get that energy,” says Mir B Ali, MD, Medical Director at MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center. “Basically, IF gives your body that chance to use other sources.” Dr. Ali says that the study’s findings are a good start. “This is an animal study, so take that with a grain of salt. Everything we see in animals doesn't immediately translate to humans.” He says that he’d like to see more research on how IF benefits people depending on their age and body type. He would also like to see what types of fasting and what periods of fasting time might help. In short, there are a lot of questions still left unanswered.
The role of genetics
That said, Dr. Ali notes that the individual genetic component adds up: “You see a lot of health issues in the 90s, but patients do OK. And then there [are] young people who die of unknown causes,” he explains. “It comes down to genetics in many cases.” Genetics aside, Dr. Ali still supports patient use of IF to boost health and manage weight, unless they are diabetic. “IF is a great tool for a lot of people, but it’s all about what they’re eating during their eating window,” he says. “Someone who is doing IF, but during the eating period eating the wrong things … they don’t see the results.”
Rather than obsessing over a perfect eating window, Dr. Ali suggests putting good foods into your body: “Eat healthy. Reduce carbs and sugars. Unfortunately, there is no one thing that works for everyone.”
What this means for you If your patients want to dive into IF, a good starting point is to have them fast for just 8 hours—usually during their sleeping time, Dr. Ali suggests. That means no food and noncaloric drinks.
“As the body adjusts to that interval, aim for 12 to 16 hours,” Dr. Ali says. “Start with a couple days per week. See how their body tolerates it.”