Mandi Knowles, RDN · Senior Director of Research & Development, Wellina
Published: September 17, 2026
Intermittent fasting has been one of the most talked-about approaches to weight loss for the better part of a decade. It's simple to explain, it doesn't require counting anything, and it comes with a compelling story about metabolism and fat burning.
The research is less dramatic than the story. Fasting does work, but mostly for a reason that has little to do with fasting itself. Here's what the evidence shows, and how to decide whether it's worth trying.
What is intermittent fasting?
Intermittent fasting isn't one diet. It's a family of eating patterns that share one feature: they set windows when you eat and windows when you don't.
The three most studied versions are:
Time-restricted eating. You eat within a set window each day, commonly eight hours, and consume nothing but water, coffee or tea outside it. The 16:8 pattern is the best known.
Alternate-day fasting. You alternate between normal eating days and fasting days. Most research uses a modified version where "fasting" days allow roughly a quarter of your usual calories rather than nothing at all.
Whole-day fasting. You eat normally most days and fast, or eat very little, on one or two non-consecutive days a week. The 5:2 pattern is the common example.
What none of these specify is what you eat. That turns out to matter more than the schedule.
Does intermittent fasting work for weight loss?
Yes, modestly, and about as well as simply eating less.
The largest analysis to date, published in The BMJ in 2025, pooled 99 randomized clinical trials covering 6,582 adults. Participants had a median age of 45 and a median BMI of 31.3, and nearly 90% had an existing health condition. Every fasting approach produced small reductions in body weight compared with eating without restriction — and so did ordinary continuous calorie restriction.
When the researchers compared fasting directly against continuous calorie restriction, only alternate-day fasting came out ahead, by an average of 1.29 kg (2.84 lbs.). That's worth knowing, but it's worth reading carefully too: the researchers had set 2.0 kg (4.4 lbs.) as the smallest difference that would be meaningful to a person. A 1.29 kg (2.84 lbs.) edge falls below their own threshold for a difference that matters.
The honest summary is that fasting and calorie counting get you to roughly the same place. Both work by reducing how much you eat overall. Fasting just does it by limiting when rather than how much.
Is fasting better than just eating less?
Not by enough to matter on the scale. But "equivalent on average" doesn't mean "equivalent for you."
Some people find a closed eating window far easier to follow than tracking portions all day. If you routinely graze from dinner until bedtime, an eating window removes that pattern without requiring you to think about any individual food. That's a real advantage — for the person it suits.
Others find the opposite. If you're hungry, irritable and preoccupied with food by mid-afternoon, an approach that makes you white-knuckle your way to dinner isn't sustainable, however well it performs in a trial.
The evidence supports treating this as a question of fit rather than a question of superiority. The best approach is the one you'll still be doing in six months.
Does fasting make you less hungry?
This is one of the more common claims, and it doesn't hold up well.
Restricting your intake tends to make you hungry, and changing the schedule doesn't change that much. If you've tried fasting and found yourself genuinely hungry rather than serenely energized, you're having the normal response, not doing it wrong.
What about the other health benefits?
Beyond weight, the picture is mixed and mostly modest.
Fasting approaches show some improvements in blood pressure, waist circumference and fasting glucose compared with unrestricted eating. But in the BMJ analysis, those improvements were broadly similar to what continuous calorie restriction achieved. Most of the benefit appears to come from losing weight, not from fasting specifically.
One finding deserves particular attention: time-restricted eating has been associated with reductions in fat-free mass, that is, muscle. Losing muscle alongside fat is a concern with any weight loss approach, and it's a bigger concern when a compressed eating window makes it harder to get enough protein across the day. If you fast, protein and resistance training become more important, not less.
Who should skip intermittent fasting?
Fasting isn't appropriate for everyone, and a few situations call for talking to your doctor or a dietitian first:
- If you take insulin or certain diabetes medications. Extended periods without food can cause blood sugar to drop too low. This needs medical supervision, not self-experimentation.
- If you're taking a GLP-1 medication. These already suppress appetite substantially. Adding long fasting windows makes it harder to meet your nutrition needs and increases the risk of blood sugar dropping. Our guide to eating well on a GLP-1 covers what experts recommend instead.
- If you're pregnant or breastfeeding.
- If you have a history of disordered eating. Rules about when you may and may not eat can reinforce unhelpful patterns. A registered dietitian can help you find an approach that doesn't.
- If you're older or working to preserve muscle. The muscle-loss signal above matters more as we age.
What actually predicts whether you lose weight?
Across the research, the pattern is consistent: what separates approaches that work from approaches that don't is whether people stick with them, and whether the food is good enough to support them while they do.
That's less satisfying than a mechanism. But it points somewhere useful. Rather than asking which schedule is optimal, ask what makes eating well easier on an ordinary Wednesday with enough protein to stay full, enough fiber to stay satisfied, portions you don't have to calculate, and meals that don't require an hour of cooking at the end of a long day.
Frequently asked questions
Does intermittent fasting work for weight loss?
Yes, modestly and about as well as simply eating less. In a 2025 analysis of 99 randomized trials covering 6,582 adults, every fasting approach produced small reductions in body weight, and so did continuous calorie restriction. Only alternate-day fasting beat calorie restriction directly, by 1.29 kg, which fell below the 2.0 kg threshold the researchers had set as meaningful to a person.
Is 16:8 fasting better than counting calories?
Not on the scale. Time-restricted eating and continuous calorie restriction produced similar weight loss in the trial evidence. The better approach is whichever one you'll still be following in six months, for some people a closed eating window is easier than tracking portions, and for others it's considerably harder.
Which type of intermittent fasting is most effective?
Alternate-day fasting showed the largest advantage over ordinary calorie restriction in the 2025 analysis, but the difference was small enough to fall below the study's own threshold for clinical importance. In practice, adherence separates approaches that work from approaches that don't.
Does intermittent fasting reduce appetite?
Not reliably. A systematic review of 17 randomized trials involving 1,111 participants found that intermittent fasting doesn't reduce appetite more than continuous calorie restriction does. If you've tried fasting and found yourself genuinely hungry, that's the normal response.
Does intermittent fasting cause muscle loss?
It may. Time-restricted eating has been associated with reductions in fat-free mass, and a compressed eating window makes it harder to get enough protein across the day. If you fast, protein intake and resistance training become more important, not less.
Can you do intermittent fasting on a GLP-1 medication?
It isn't recommended. GLP-1s already suppress appetite substantially, and adding long fasting windows makes it harder to meet your nutrition needs while raising the risk of blood sugar dropping too low. Our guide to eating well on a GLP-1 covers what experts recommend instead.
Is intermittent fasting safe?
For most healthy adults, yes. It isn't appropriate if you take insulin or certain diabetes medications, if you're pregnant or breastfeeding, if you take a GLP-1, or if you have a history of disordered eating. Talk to your doctor or a dietitian before starting.
The bottom line
Intermittent fasting is neither a fad nor a breakthrough. It's one reasonable way to eat less, roughly as effective as other reasonable ways to eat less, and genuinely easier for some people than counting.
If a defined eating window fits your life and your doctor has no concerns, it's a legitimate option. If it leaves you hungry, distracted and eventually raiding the kitchen at 11pm, that's useful information rather than a personal failure and there are other approaches that work just as well.
Either way, what you eat inside the window still decides the outcome.
Wellina meals and snacks take the guesswork out of that part. They are portioned, dietitian-designed, and ready in minutes, whatever schedule you're eating on.
About the author
Mandi Knowles is a Registered Dietitian Nutritionist and Senior Director of Research & Development at Nutrisystem, where she leads nutrition standards for Wellina.
This article is for general information and is not medical advice. Talk to your doctor or a registered dietitian before making significant changes to how you eat, particularly if you take medication or have a health condition.
References
- Semnani-Azad Z, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomized clinical trials. BMJ. 2025;389:e082007. doi:10.1136/bmj-2024-082007
https://pmc.ncbi.nlm.nih.gov/articles/PMC12359177/ - Elsworth RL, et al. The Effect of Intermittent Fasting on Appetite: A Systematic Review and Meta-Analysis. Nutrients. 2023.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10255792/ - Intermittent Fasting for the Prevention of Cardiovascular Disease Risks: Systematic Review and Network Meta-Analysis.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12289860/ - Wu X, et al. Comparison of Different Intermittent Fasting Patterns or Different Extents of Calorie Restriction for Weight Loss and Metabolic Improvement in Adults: A Systematic Review and Network Meta-Analysis. Nutrition Reviews. 2026;84(3):487–499. doi:10.1093/nutrit/nuaf056
