Should You Count Calories on Intermittent Fasting?

You don't have to count calories to try 16:8. But if you want to know why the scale isn't moving, you do.

Time-restricted eating is a schedule, not a measurement. Two large randomised trials found that a fasting window, on its own, produced no significant extra weight loss beyond what a matched comparison group lost without one. So a flat scale after two weeks isn't proof that intermittent fasting failed. It just means the window hasn't told you anything yet about what you're eating inside it, and logging is the only way to find that out.

Should you count calories while intermittent fasting?

The window controls when you eat, not how much. If you eat the same total food inside eight hours as you would across sixteen, the schedule has changed but the input hasn't.

In the TREAT trial, people ate only between noon and 8pm. They lost 0.94 kg over 12 weeks. A control group, instructed to eat three structured meals a day with no fixed clock window, lost 0.68 kg (Lowe et al., JAMA Internal Medicine 2020). The gap was 0.26 kg, and it wasn't statistically significant.

That result only makes sense if a share of the fasting group ate close to what they normally would, just compressed into fewer hours. The window did not force a deficit by itself. Whether your version of 16:8 creates one depends on what actually goes in your mouth during those eight hours, and a timer app can't see that. Counting calories is how you check.

What did the trials actually measure?

Two trials looked at this from different angles, and both point the same way.

TREAT compared a fasting window against a group eating three structured meals a day without a fixed window, with neither group given a calorie target. The difference in weight loss was small and not significant, and neither was the difference in measured energy intake between groups.

The NEJM trial went further. It put both groups on the same calorie range, then added a fasting window to only one of them. The window group lost 9.0% of baseline weight, about 8.0 kg, over 12 months. The calorie-only group lost 7.2%, about 6.3 kg (Liu et al., NEJM 2022). The 1.8 kg gap wasn't statistically significant either.

When the calories matched, the window added nothing measurable. That's the mechanism behind your stalled scale. The window is doing its job as a schedule, but it can't do the job of a deficit unless your eating inside it happens to create one.

How do you find out if your window is a deficit?

You log what you eat during the window for a stretch of days, without changing anything yet. The point isn't to hit a number, it's to see one.

Start by logging every meal and snack that falls inside your fasting window, exactly as you'd normally eat it. Don't diet while you check. The question isn't "can I eat less," it's "what am I already eating."

Some meals inside your window are homemade dishes rather than single foods. Working out a homemade dish's total calories takes an extra step. Add up the ingredients, weigh the cooked result, then split it into portions. The recipe calorie calculator takes a recipe's total calories, plus either your number of servings or the cooked dish weight and your portion weight. It returns calories per serving so you can log that dish accurately.

After a week or so, look at the pattern. If your logged intake during the window looks close to a normal, unrestricted day for you, that's consistent with the TREAT and NEJM results: same input, same output, flat scale.

If it looks lighter than a normal day, either because the shorter window naturally crowds out a snack or because hunger genuinely drops, then you have a deficit. The scale should reflect it, given more time than two weeks allows.

This step doesn't require a target or a cutoff. It just turns a guess into a number you can look at.

What does two weeks of a flat scale actually tell you?

Not much, on its own. Daily weight moves for reasons that have nothing to do with fat, and two weeks is a short window even in a real deficit.

The cited trials measured meaningful weight change over 12 weeks and 12 months, not 12 days. TREAT's own 12-week result was a fraction of a kilogram. Expecting a visible move in two weeks sets a bar the underlying research doesn't clear either.

A flat scale over two weeks is normal, whether or not you're in a deficit. It becomes informative only alongside a log of what you actually ate, because that's the variable the trials show explains the outcome, not the timer.

Do you have to log every day, or just enough to know?

You need enough days to see a pattern, not a permanent habit. A handful of typical days inside your window is usually enough to tell you whether the schedule is doing any of the work you're hoping for.

If those days look like a normal eating day for you, that matches what the trials found: the window alone isn't the mechanism. If they look noticeably lighter, you likely do have a deficit building, even if the scale hasn't caught up yet.

Either answer is useful. It replaces "I don't know why this isn't working" with a specific thing to look at next. For the method behind the logging itself, how to log food accurately enough to trust the trend covers weighing and reading labels in more depth.

Worked example: matching the trial arithmetic to a week of logging

The following weekly totals are illustrative, invented to show the arithmetic; they are not from the cited trials. Imagine two versions of the same person's week, one where the eating window changes nothing about food choices, and one where it does.

The middle column mirrors the TREAT result. The window changed nothing about intake, so there's no reason to expect the scale to move differently than the no-window column. The third column mirrors what would need to happen for a window to help: the schedule nudged food choices down, creating an actual gap.

You can't tell which column you're in from the timer alone. Both look identical on a fasting app that only tracks hours. The only way to see the difference is to log what's eaten inside the window and add it up, the same way this table does. That's the entire argument in one sum. Same calories, same trial result. Different calories, different result.

What quietly breaks this check?

• Don't assume a shorter window means less food. Some people naturally compress their eating; others just eat the same amount faster. The trials found no automatic effect either way.
• Don't judge the window by the first two weeks. Both trials ran for 12 weeks or longer, so a shorter check will look flat even in a genuine deficit.
• Don't skip logging on "normal" days. The days that feel unremarkable are exactly the ones that reveal whether your baseline habits changed.
• Don't extend the fast to fix a flat scale. Neither trial found that a longer window added weight loss beyond what the calorie difference explained; the fix here is measurement, not more hours fasted.
• Don't confuse the fasting timer with a food log. A timer records when you ate, not what or how much, and only the second one answers this question.

Where does an app fit into this?

An app that pairs a fasting timer with a food log can show you both halves of this question in one place: how long you fasted, and what you logged inside the window. Avocalo's timer covers 16:8, 14:10, 20:4 and one meal a day, alongside photo and manual food logging and statistics over 7 days, 30 days and 3 months.

It can't tell you whether your window is a deficit before you log anything, and it doesn't run its own clinical trial. What it can do is put the two numbers next to each other so you're not guessing. If you want to check your own window the way this article describes, you can get the app with the fasting timer and food log together.

Frequently asked questions

Does a longer fast, like 20:4 or one meal a day, burn more fat than 16:8 at the same calories?

Not according to the trials cited here: neither found that a fasting window itself added weight loss beyond what a matching calorie deficit explained. A longer window may make it easier or harder for a given person to eat less, but that's an effect on intake, not an extra metabolic bonus from the fast itself.

Can you gain weight during intermittent fasting?

Yes. The window restricts when you eat, not how much, so eating more than usual inside a shorter window can still produce a surplus. Both trials found that the fasting groups' actual intake, not just the schedule, determined the weight outcome.

Is it normal for the scale not to move in the first two weeks of 16:8?

Yes, and it doesn't mean the approach failed. Even the shorter trial's 12-week changes were often under a kilogram. Two weeks is too short a window to expect a clear signal either way.

Do you need to log fasting hours if you already log food?

Not for this question specifically. The fasting hours tell you the schedule was followed; the food log tells you whether that schedule produced a deficit. Logging food is what answers "why hasn't the scale moved," which is the question this article is about.

Sources

• Lowe et al., JAMA Internal Medicine 2020 (the TREAT trial): reports the 12-week weight change in a time-restricted eating group. It's compared against a group eating three structured meals a day, with no calorie target in either arm.
• Liu et al., NEJM 2022: compares 12-month weight loss between a calorie-restricted group with a fasting window and a calorie-restricted group without one, isolating the window's effect from the calorie target.
• Calorie and Time Restriction in Weight Loss, PMC review: secondary source confirming the NEJM trial's reported kilogram and percentage figures.
• For the logging method itself, see how to log food accurately enough to trust the trend.

This article was written by the Avocalo Editorial Team for Avocalo, published by Petros IT Solutions SRL. See the editorial policy for how figures are sourced and checked. Corrections: [email protected].