Most diets tell you what to eat. Intermittent fasting is mostly about when. People try it for weight loss, and then they want to know if the hype holds up and what the catch is. Here’s what randomized trials found when they put fasting next to plain calorie cutting, and where it can go wrong.
What Intermittent Fasting Is
It isn’t one diet. These are the versions you’ll hear about most:
- 16:8. You eat inside an 8 hour window and fast for the other 16. This is also called time restricted eating.
- 5:2. You eat normally on five days and eat very little on two.
- Alternate day fasting. A fast day, then a normal day, back and forth. In one trial, fast days were 25% of energy needs.
- One meal a day. One meal, usually with a long fast around it.
A review of 11 trials that lasted 8 to 24 weeks compared fasting regimens with cutting calories every day. Weight loss was comparable. The pooled difference was 0.61 kg, and the range was wide enough to include no difference at all (Cioffi et al., 2018).
Does It Beat Cutting Calories?
Three trials give a clear picture. In a 1 year trial of 100 people with obesity, alternate day fasting and daily calorie cutting produced about the same weight loss at 6 and 12 months (Trepanowski et al., 2017). A 1 year trial put 139 patients with obesity on a calorie limit with or without an eating window from 8 AM to 4 PM. The eating window group lost 8.0 kg and the other group lost 6.3 kg, and that gap wasn’t statistically significant (Liu et al., 2022). A 12 week trial with an eating window from noon to 8 PM and no calorie target found it was no better than eating across the day (Lowe et al., 2020).
Pick a trial below to see the bars side by side.
Fasting versus plain calorie cutting
Average weight loss in each trial, fasting group next to the comparison group. Pick a trial.
Sources: Trepanowski et al. (2017), Liu et al. (2022) and Lowe et al. (2020), as reported in their abstracts. Bars are scaled within each trial, so compare the two bars, not the three trials. Alternate day trial, 1 year eating window trial, 12 week eating window trial.
What Else It May Do
Weight isn’t the only thing people hope fasting changes. The evidence here is thinner, so we’ll keep this short.
Blood sugar and insulin. In the pooled trials, fasting insulin was slightly lower with fasting regimens than with cutting calories every day, and the authors said the clinical relevance is uncertain (Cioffi et al., 2018). A review of fasting regimens says they appear to promote weight loss and may improve metabolic health (Patterson & Sears, 2017). In the 1 year alternate day trial, fasting insulin and fasting glucose didn’t differ between the fasting and calorie cutting groups.
Inflammation. A review of 18 trials found that fasting diets and calorie restricted diets both lowered C reactive protein, a blood marker of inflammation, and fasting lowered it a bit more. The drop was small, and two other inflammation markers didn’t change (Wang et al., 2020).
Brain and long life. Reviews describe how switching between fasting and eating changes how brain cells respond, and some of that may support brain function (Mattson et al., 2018). A separate review in the New England Journal of Medicine covers fasting, aging and disease (de Cabo & Mattson, 2019). Those are reviews of how the body responds. They aren’t trials showing that fasting protects your memory or adds years to your life.
The Drawbacks
Hunger, especially at night. A review of 16 studies found that people with a short eating window had higher hunger at bedtime. People also described fear of hunger and eating when they weren’t hungry. It found no major change in disordered eating scores (Vizthum et al., 2023).
Sticking with it. In the 1 year alternate day trial, 38% of the fasting group dropped out, against 29% for daily calorie cutting. People in the fasting group ate more than planned on fast days and less than planned on feast days. By month 12, their LDL cholesterol was 11.5 mg/dL higher than in the calorie cutting group (Trepanowski et al., 2017).
Fatigue and headaches. This is the common worry, and the trial data is milder than you’d expect. A review of 15 randomized trials with 1,365 people found no significant difference in fatigue or headache between fasting and comparison groups. Dizziness was a bit higher in one subgroup, but the range included no difference (Zhong et al., 2024).
Muscle. In the 12 week eating window trial, a smaller group tested in person showed a lower lean mass index in the eating window group than in the comparison group (Lowe et al., 2020). That trial didn’t test strength training or protein. If you fast and want to keep your muscle, how much protein you need is a good place to start.
Who should check with a doctor first. If you take medication for diabetes or blood pressure, have a medical condition, are pregnant, or have a history of disordered eating, get medical advice before you try it.
Fitting a Fast Around Your Training
If you do try an eating window, it helps to see where your sessions would fall. Slide the first meal to a time that suits you, or start from a window one of the trials used.
Where would your eating window fall?
A 16:8 plan means 8 hours to eat and 16 hours to fast. Pick when your window starts, or start from a window used in a trial.
Eating windows: Lowe et al. (2020) used noon to 8 PM and Liu et al. (2022) used 8 AM to 4 PM. Both trials found weight loss was no better than eating across the day or cutting calories alone. This planner shows timing only. Lowe trial, Liu trial.
If your weight loss stalls whatever you try, here’s why plateaus happen.
Putting It Into Practice
The studies suggest that the pattern you can keep up beats the one with the cleverest name. A coach can help you pick, and make sure your training and protein fit around it. That’s what we do in the studio:
- One on one personal training in Vaughan starts with a Crush It Session: about an hour with a coach. We'll credit your full $100 toward your first training package when you sign up within 48 hours after your session.
- Prefer training with others? Our small group classes are coach led and capped at 16 people, with strength work and conditioning. Try a week of unlimited classes with the $30 seven day trial (+ HST). Join by day 5 and your $30 is credited toward your membership.
- Want your eating to match your training? See nutrition coaching in Vaughan.
Our Vaughan studio is open Monday to Friday 5:45 AM to 8 PM and Saturday and Sunday 8:30 AM to 4 PM. Questions? Call or text us at (289) 536-8066 or email g-squad@innerg.ca.
Frequently Asked Questions
What is intermittent fasting?
It’s an eating pattern that cycles between eating and fasting. Common versions include eating inside an 8 hour window (16:8), eating very little on two days a week (5:2), fasting every other day, and eating one meal a day.
Does intermittent fasting work for weight loss?
It can, mostly because people end up eating less. In a 1 year trial, people on alternate day fasting lost about 6% of their body weight relative to a control group, compared with about 5% for people cutting calories every day. The two groups were not significantly different.
Is intermittent fasting better than cutting calories?
Not in the trials we reviewed. Three randomized trials and a review of 11 trials found no meaningful weight loss advantage for fasting over plain calorie cutting. Pick the pattern you can keep up.
What are the side effects of intermittent fasting?
A review of 15 randomized trials found no more fatigue or headache than in comparison groups. One review found higher hunger at bedtime with a short eating window, and a 1 year trial found more dropouts and higher LDL cholesterol with alternate day fasting.
Does intermittent fasting cost you muscle?
One 12 week trial found the group with an eating window lost slightly more lean mass than the comparison group. It didn’t test strength training. Training with a coach and eating enough protein are what we plan around.
Who should check with a doctor first?
Anyone who takes medication for diabetes or blood pressure, has a medical condition, is pregnant, or has a history of disordered eating. Fasting can change how some medications work, so get medical advice before you start.
References
- Cioffi I, Evangelista A, Ponzo V, et al. (2018). Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta analysis of randomized controlled trials. Journal of Translational Medicine, 16(1):371. PubMed
- Patterson RE, Sears DD (2017). Metabolic effects of intermittent fasting. Annual Review of Nutrition, 37:371-393. PubMed
- Mattson MP, Moehl K, Ghena N, et al. (2018). Intermittent metabolic switching, neuroplasticity and brain health. Nature Reviews Neuroscience, 19(2):63-80. PubMed
- de Cabo R, Mattson MP (2019). Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine, 381(26):2541-2551. PubMed
- Wang X, Yang Q, Liao Q, et al. (2020). Effects of intermittent fasting diets on plasma concentrations of inflammatory biomarkers: a systematic review and meta analysis of randomized controlled trials. Nutrition, 79-80:110974. PubMed
- Trepanowski JF, Kroeger CM, Barnosky A, et al. (2017). Effect of alternate day fasting on weight loss, weight maintenance, and cardioprotection: a randomized clinical trial. JAMA Internal Medicine, 177(7):930-938. PubMed
- Liu D, Huang Y, Huang C, et al. (2022). Calorie restriction with or without time restricted eating in weight loss. New England Journal of Medicine, 386(16):1495-1504. PubMed
- Lowe DA, Wu N, Rohdin-Bibby L, et al. (2020). Effects of time restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Internal Medicine, 180(11):1491-1499. PubMed
- Zhong F, Zhu T, Jin X, et al. (2024). Adverse events profile associated with intermittent fasting: a systematic review and meta analysis of randomized controlled trials. Nutrition Journal, 23(1):72. PubMed
- Vizthum D, Katz SE, Pacanowski CR (2023). The impact of time restricted eating on appetite and disordered eating: a mixed methods systematic review. Appetite, 183:106452. PubMed
This article is for general information and isn’t a substitute for individual medical advice. Check with your doctor before changing how you eat or starting a new exercise program, especially if you take medication or have a health condition.