Fasting Weight Loss Results: 1 Week

If you just started 16:8 fasting or another eating window, the first week can feel like an exam graded by the scale. The number will probably move, in both directions. This guide follows the week in the order you live it.

Why the Scale Drops Fast in Week 1

Your body keeps a reserve of carbohydrate called glycogen in your liver and muscles, and it is stored with about three to four parts water (Kreitzman, 1992).

When you eat less, or eat fewer carbs, your body uses that glycogen and releases its water. That is why many people see a quick drop in the first days. Its size varies a lot between people and says little about body fat.

The water returns when carbs come back. A pasta dinner or a weekend of pizza and pancakes refills glycogen, and the scale can climb with little or no fat gained. The authors of that paper call the early drop an illusion of easy weight loss and warn of an exaggerated regain after carbohydrate loading. Neither swing is a verdict on your week.

What Decides How Much Fat You Lose

Body fat goes down when you use more energy than you take in, day after day. An eating window can make that easier for some people, but only if the meals inside it add up to less than you burn.

That gap depends mostly on what you eat in your window, with body size and activity also playing a part. A modeling study in The Lancet showed that body weight responds slowly to a change in eating, and that people with more body fat can expect to lose more from the same change (Hall, 2011).

What Trials Found About 16:8 and Weight Loss

The TREAT trial followed 116 adults with overweight or obesity for 12 weeks (Lowe, 2020). One group ate freely between noon and 8 p.m. and nothing outside that window. The other ate three structured meals a day.

The 16:8 group lost an average of 0.94 kg (about 2 lb) and the three-meal group 0.68 kg (about 1.5 lb). The difference was not statistically significant, and the authors concluded that time-restricted eating alone was no more effective for weight loss than eating throughout the day. In the subgroup measured in person, the 16:8 group also lost more lean mass in the arms and legs.

A year-long trial in China gave 139 adults with obesity the same reduced-calorie diet, and one group also ate only between 8 a.m. and 4 p.m. (Liu, 2022). After 12 months, the group with the window did not lose significantly more weight, waist or body fat than the group on the diet alone.

So the window is a tool that can help you eat less, and what you eat inside it matters more than the clock. If 16:8 makes eating less easier for you, it suits you.

How to Judge Your First Week

A single weigh-in is noisy. Weigh yourself each morning after the bathroom and before breakfast, then compare this week's average with next week's.

Expect the weekend to show up. In 80 adults who weighed themselves daily, weight was higher on Sunday and Monday and fell during the week (Orsama, 2014). A European study of more than 1,400 adults in a weight maintenance program found the same pattern, about 0.35% of body weight on average (Turicchi, 2020). Starting on a Monday after a big Saturday can make your first number look high.

Once a week, also measure your waist at belly-button level and notice how your jeans fit. Both change slowly, so they tell you more over a month.

What you may notice in week 1	What it usually means	What to do
A quick drop in the first days	Mostly glycogen and its water	Judge progress by weekly averages
Higher weight on Monday	The usual weekend rise	Compare Monday with Monday
A jump after a carb-heavy meal	Glycogen refilling with water	Keep your usual plan the next day
Hunger at your old meal times	Hunger tends to rise around the meal times you are used to	Water or black coffee, same window daily (see hunger while fasting)
Headache or feeling lightheaded	Can come from too little fluid or salt	Drink water and salt meals normally. Stop if you feel faint
Scale flat after the first days	Water shifts can hide slow fat loss	Wait for two to three weekly averages (see intermittent fasting plateau)

Habits That Help in Week 1

Build Each Meal Around Protein

Start each meal in your window with eggs, Greek yogurt, cottage cheese, chicken, canned tuna or black beans. The lean mass result in TREAT is one reason to give protein a fixed place while you eat less.

Drink Enough and Keep Some Salt

During the fast, stick to water, black coffee, plain tea or unsweetened seltzer, and keep a bottle at your desk. Salt your food normally in your window. An electrolyte mix without sugar or calories is fine during the fast (see whether electrolytes break a fast).

Protect Your Sleep

In a two-week study, 10 overweight adults followed the same reduced-calorie diet twice, with 8.5 or 5.5 hours in bed at night. With short sleep, they lost less than half as much fat and more lean mass. They also felt hungrier (Nedeltcheva, 2010). If a late window pushes your bedtime back, move it earlier.

Keep the Same Hours, Weekend Included

Keep the same window every day, Saturday and Sunday included, and walk most days. If you use FastTrack, the timer shows each fasting phase and your water and weight logs sit in the same app, so the whole week is in one place.

When to Stop or See a Doctor

Stop fasting and get medical advice if:

• you feel dizzy or faint, or you pass out
• your heart races or pounds
• you are vomiting or cannot keep fluids down
• eating in your window turns into binges you cannot control

Talk to your doctor before you start if:

• you have diabetes and take insulin or another medicine that lowers blood sugar
• you take a medicine that has to be taken with food
• you are pregnant or breastfeeding
• you have had an eating disorder
• you are under 18

The Short Version

Week 1 is mostly about learning a routine. The quick drop is largely glycogen and water, and it can come back. Fat loss comes from a steady energy deficit, at a pace that varies and slows over time. Judge it by weekly averages and your waist over several weeks.

Ready to start and track your first week? Download FastTrack and let it handle the timer, the reminders and the progress, so you only have to show up. The fasting timer is free.

Weight Loss in a Week: Common Questions

How much weight can you lose in a week of fasting?

There is no reliable number to expect, because it varies widely between people. An early drop is mostly glycogen and the water stored with it, and it can return when you eat carbs again. Fat loss is much slower: in a 12-week trial, the 16:8 group lost 0.94 kg (about 2 lb) on average over the whole three months.

Is losing weight fast in a week healthy?

A large first-week drop is mostly glycogen and water, so it says little about fat. Eating very little to push the number down makes the week harder to keep up, and the water returns once you eat normally.

What are the side effects of the first week of intermittent fasting?

Some people feel hungry around their usual meal times in the first days, and some get headaches or feel low on energy. Drinking enough and salting your meals normally can help. Dizziness or fainting is a reason to stop and see a doctor.

How much weight can you lose in one week on 16:8?

No one can predict it for you. In the TREAT trial, adults who ate only between noon and 8 p.m. for 12 weeks lost about the same as those eating three meals a day. What you eat inside the window matters more than the first week's number.

Why is my weight higher after the weekend?

Studies of daily weigh-ins found that weight tends to be highest on Sunday and Monday and to fall during the week. Compare the same weekday from week to week, or use your weekly average.