Grant Tinsley has been studying the diet phenomenon since 2013.
To most people, it's probably no surprise that dieting has changed over the years – after all, we've witnessed it. From Atkins to Paleo to Zone, various fad diets have pushed a wide spectrum of advice: cut the carbohydrates, eat less fat, eat more fat, cut out gluten, eat superfoods, take pills, try a cleanse. The list goes on and on.
One of the newest is the idea of intermittent fasting, an umbrella term that refers to a number of eating patterns that contain regularly occurring periods of fasting that are longer than the standard overnight break between dinner and breakfast.
Texas Tech University's Grant Tinsley, who researches intermittent fasting, says he can understand why the phenomenon is getting so much attention.
"I think there are a few factors that have contributed to the rapid rise and sustained popularity of intermittent fasting," said Tinsley, an assistant professor of exercise physiology in the Department of Kinesiology & Sport Management. "Of course, cyclical trends in fad diets are unfortunately a mainstay of the health and fitness world. However, I think intermittent fasting has stuck for a lot of people because it ends up being a very sustainable strategy.
"For some people, limiting all food intake to relatively short periods of time is a simple way to eat less without going through the burden of avoiding certain foods or keeping track of everything they eat. Many people have found success following intermittent fasting for this reason, and those people sometimes become vocal proponents of the method. I think a large part of the appeal of intermittent fasting is the simplicity of 'eat during these times, but not these times.'"
He cautions that just because intermittent fasting is getting attention, it doesn't mean it's a miracle diet – and no one should interpret it that way.
"While there could be unique health benefits of intermittent fasting independent of eating less, there is more research to be done before these potential benefits are clarified," he said. "Although I think intermittent fasting is a great weight-loss or weight-maintenance method for some people, it's my opinion that many online sources may overstate the benefits of these programs relative to the actual research that has been conducted."
Tinsley should know. He's one of the people conducting the research.
Tinsley first became interested in intermittent fasting as a doctoral student at Baylor University in 2013. Common dietary recommendations at the time suggested eating frequent, small meals throughout the day to maintain metabolism and promote a healthy body composition – and Tinsley's adviser didn't buy it.
"As noted by my adviser in his work, the research did not indicate there were actual benefits to this approach," Tinsley said. "Our discussions of high meal frequencies led to a consideration of the other end of the spectrum: What influence does less frequent eating have on body composition, exercise performance and health? Are there unique health benefits to regular periods of fasting? For active individuals, will muscle be lost and exercise performance compromised by this approach?"
Intermittent fasting was just beginning to emerge within academic research and the general health-conscious population, and there were more questions than answers. Tinsley decided to do something about that.
"Due to my interest in active populations, I noticed a distinct lack of intermittent fasting research in exercising individuals," he said. "This led me to conduct my first trial of intermittent fasting plus exercise and started me off on this line of research."
Still a doctoral student, Tinsley led his first study using a form of intermittent fasting called time-restricted feeding (TRF), which limits all calorie intake to a specific number of hours each day and leaves all other hours for fasting.
The study randomly assigned recreationally active men to either continue their regular diet or follow a TRF program, in which they consumed all their calories within a four-hour period on four days each week. All participants completed an eight-week, weight-training program.
At the end of the study, both groups showed similar improvements in exercise performance, but members of the group eating normally had gained about 5 pounds of muscle – and TRF group members hadn't.
"While this showed apparent detriments of time-restricted feeding for lean mass gain, there were some other factors to consider," Tinsley explained. "For example, based on dietary analysis, participants in the TRF group selected a lower-protein diet, which could have contributed to the results. While not definitively different between groups, the control group also appeared to gain body fat, about 1.8 pounds, whereas the TRF group lost a small amount, about 1.3 pounds."
Because the study was one of the first of its kind, it received substantial attention. The paper publishing its results was awarded the 2018 Best Paper Award by the European Journal of Sport Science.
Tinsley then collaborated with a colleague, Antonio Paoli at the University of Padova in Italy, for a second study – this one using one of the most common implementations of TRF, in which all calories are consumed within an eight-hour period each day.
This time, well-trained men were randomly assigned to either the control group, which would follow a 13-hour feeding schedule each day, or the TRF group, which would consume all calories between 1 p.m. and 9 p.m. Because of the possible protein differences between groups in study 1, both groups in this study received dietary counseling in order to maintain similar intake of calories, protein and other nutrients. Again, both groups completed an eight-week, weight-training program.
"This study revealed more favorable effects in the TRF group as compared to our other trial," Tinsley said. "Significant loss of body fat was observed, with no change in the control group. Both groups maintained their lean mass and muscular performance throughout the study. However, a variety of health markers were improved only in the TRF group. These included markers of inflammation, blood sugar, insulin and triglycerides."
This study received even more attention than its predecessor, showing there was obviously a need for, and an interest in, continued research on intermittent fasting in active individuals.
That led to Tinsley's most recent research, which differed from the prior studies in several ways. For one, it examined TRF in active women instead of active men.
"Based on a small amount of research on the topic, it has been questioned whether females and males respond differently to intermittent fasting programs," he said. "We felt it was important to conduct a trial that exhibited some similarities to our previous work, but in active females."
The new study also incorporated a third group in addition to the TRF group and the control group. This group performed TRF but also took a dietary supplement called beta-hydroxy beta-methylbutyrate (HMB) when they were fasting.
"This supplement has been shown to decrease muscle breakdown, and we were interested in investigating whether consuming this supplement during fasting periods would enhance the results of the exercise program, particularly in regards to body composition," Tinsley said.
Lastly, the new study had several methodological differences from its predecessors, including a more advanced method of body composition assessment, an objective evaluation of physical activity levels using accelerometers, and provision of whey protein supplements to promote optimal protein intake in all participants.
In comparisons of lean muscle mass, muscle strength and endurance, metabolism, mood and sleep quality, the results between the control group and the TRF group were essentially the same. But there was a difference in fat mass.
Both TRF groups lost about 2-3 pounds of fat, while the control group gained about 1 pound of fat.
The observation of fat loss without compromised muscle gain surprised Tinsley.
"It appears that TRF in combination with weight training is a suitable strategy for active individuals wanting to increase muscle and lose fat at the same time," he said. "While I think some people overstate the uniqueness of the benefits obtained from intermittent fasting, I do think our data demonstrate this is a dietary strategy worthy of attention.
"For active individuals, simultaneous fat loss and lean-mass gain is often viewed as very difficult or impossible. However, we demonstrated that a simple TRF program, with very minimal dietary advice, was able to produce these changes when combined with an evidence-based exercise program."
While no major side effects or adverse events occurred during the study, some participants reported greater hunger, irritability or fatigue when following the TRF program.
"Intermittent fasting may be difficult for certain individuals to follow," Tinsley said. "Since adherence is known to be one of the most critical factors for long-term success with a dietary program, intermittent fasting is probably not a good strategy for those who find it especially difficult. With that said, a number of our participants reported that the intermittent fasting program got substantially easier after they had followed it for several days or weeks."
He adds that all study participants were already healthy and active, so individuals with medical conditions should discuss intermittent fasting with a medical professional before trying it for themselves.
And, because of the similarity of results between the TRF and control groups, he doesn't want anyone to mistakenly believe that following an intermittent fasting program is a necessity.
"I think intermittent fasting deserves attention as a strategy to be considered by active individuals," he said, "although it certainly isn't the strategy for everyone."
Even as he publishes his latest findings, Tinsley definitely isn't finished studying the intersection of intermittent fasting and exercise. His next angle is to examine differences in when exercise is performed.
"I have an interest in evaluating whether exercise adaptations differ when exercise is performed fasted versus non-fasted within the context of intermittent fasting," he said. "In our studies to date, all exercise has been performed in the non-fasted state. Although my previous research has used weight training as our exercise modality, I am also interested in the effects of intermittent fasting on endurance training adaptations.
"There also are a lot of forms of intermittent fasting that have still not been evaluated in active individuals. While my research to date has focused on TRF, I may branch out into other forms in the future."