Intermittent fasting and calorie counting are two of the most popular ways to achieve weight loss. But in a head-to-head comparison, which one comes out on top?
According to a 2025 study published in Annals of Internal Medicine, a so-called 4:3 intermittent fasting program outperformed calorie reduction for helping people lose weight over the course of a year.
Unlike some intermittent fasting (IF) programs that restrict eating to certain windows of the day, the study’s 4:3 program restricted total calories only on certain days of the week.
“This paradigm involves three non-consecutive days a week with significant calorie restriction and four days a week with no calorie restriction,” study authors Victoria Catenacci, MD, and Danielle Ostendorf, PhD, affiliated with University of Colorado Anschutz Medical Campus and the University of Tennessee, Knoxville, respectively, told Health in a joint email.
Krista Varady, PhD, a professor of nutrition at the University of Illinois, Chicago, who was not involved with the research, told Health in an email that the results were surprising. Previous studies, such as a 2024 systematic review, concluded that fasting strategies are not more effective than calorie restriction. “This is the only study to date in humans that has shown this,” she said.
To conduct their 12-month trial, the researchers divided 165 participants into two groups. One group was instructed to cut calories by 80% on fasting days. For women, this meant decreasing intake to about 400 to 600 calories three days per week, while men reduced calories to around 500 to 700. These participants were only required to count calories on their fasting days.
“Then, on the other four non-fasting days, participants did not have to count or restrict calories but were encouraged to make healthy food choices and portion sizes,” the authors explained. “Overall, this amounted to a target weekly calorie deficit of about 34% in the 4:3 intermittent fasting group.”
A second group received instructions to reduce their daily calories by 34% to match this overall calorie target.
For as much accuracy as possible, both groups received instructions on tracking their calories and kept a written log of their food consumption. A registered dietitian reviewed these logs weekly to ensure that participants stayed on track calorie-wise. As an additional measure of accuracy, the researchers used the doubly labeled water intake balance method, which measures how quickly isotopes in water leave the body and determines the number of calories burned.
At the end of 12 months, the researchers determined that those in the 4:3 fasting group had lost significantly more weight than those in the calorie-counting group—an average difference of 6.4 pounds.
Study Limitations
Despite its impressive findings, the study did have some limitations.
Catenacci and Ostendorf noted that their participant pool did not include children, adolescents, older adults, pregnant or lactating women, or people with diabetes, cardiovascular disease, or cancer. “That limits the generalizability of our results to these groups,” they said.
Additionally, although the study lasted an entire year, it didn’t address longer-term outcomes of intermittent fasting and calorie counting.
The authors also said that meeting with a registered dietitian every week could have influenced outcomes (and is a perk many people don’t have access to when pursuing weight loss).
The study’s results—and how they differ from previous research—raise the question of why 4:3 intermittent fasting might yield more weight loss than calorie counting alone. According to the study’s authors, there’s nothing magical about IF. Rather, they found that people who followed its principles three days a week simply ate less than those who counted calories daily.
Varady agreed. “In this study, the fasting group produced greater calorie restriction than the [calorie-counting] group, which led to greater weight loss. It’s as simple as that!” she said.
Catenacci and Ostendorf theorize that the 4:3 form of fasting could also be a “sweet spot” for many IF users. “IF diets featuring more fasting days per week (like in alternate-day fasting) may be too rigid and difficult to stick with—but too few fasting days per week (like in 5:2 IF) may not produce enough of a calorie deficit during the week to enhance weight loss over calorie counting.”
That said, there may be some metabolic benefits to intermittent fasting as well.
In general, intermittent fasting, like the 4:3 program utilized in the study, may promote greater weight loss than simple calorie restriction due to its purported effect on key hormones, such as leptin (which stimulates fullness) and ghrelin (which stimulates hunger), Michael Snyder, MD, medical director of the Bariatric Surgery Center at Rose Medical Center in Denver and in-house obesity specialist at FuturHealth, told Health.
“In addition,” Snyder said, “when fasting, insulin sensitivity may be improved, helping the body to burn fat for energy rather than store it.”
Despite the study results, intermittent fasting isn’t the best weight loss method for everyone. Varady pointed out that a 4:3 program can be tough to stick with: “It involves very heavy calorie restriction on several days per week, which can lead to extreme hunger.” On the other hand, more modest, consistent daily calorie counting might not leave you feeling as deprived.
Meanwhile, fasting could pose health risks for some groups of people. “It can lead to hormonal disruptions, particularly in women, that might affect reproductive hormones and menstrual cycles and, in some cases, thyroid function,” Snyder said.
He also noted that fasting can increase cortisol, the body’s stress hormone, if done too frequently or for too long. Blood sugar imbalances are another possibility, he added, especially for people with diabetes.
Looking to start a weight loss journey but aren’t sure where to begin? It’s worth a conversation with your doctor or a registered dietitian. They can help you determine whether intermittent fasting, calorie counting, or another approach is right for you.
