Fasting Weight Loss and Fat Loss: What to Expect
The most popular question people ask when they start getting interested in fasting is simple: how much weight can I lose?
Before getting into mechanisms, it helps to look at real-world data showing how fasting weight loss actually behaves across different fast lengths.
Fasting weight loss vs length of fast
So, the honest answer is that fasting weight loss varies a lot, and the number on the scale does not always reflect how much fat you are actually losing. To understand what fasting really does, it’s important to separate fasting weight loss from fasting fat loss.
⏱️ Early Fasting Weight Loss Is Mostly Not Fat
During the first 2-3 days of an extended fast, most fasting weight loss comes from lean mass rather than fat. This includes water, glycogen, and gut content. Main contributors to early weight loss:
Glycogen - The body stores about 0.9-1.3 lbs (400-600 grams) of glycogen in the liver and muscles, providing roughly 1,600-2,400 calories that are used up early in fasting
Water - Each gram of glycogen binds about 3-4 grams of water; losing glycogen typically releases about 3-6 lbs (1.4-2.7 kg) of water, but actual water loss varies widely with hydration and sodium balance
Gut content - The digestive tract can hold roughly 2-6 lbs (0.9-2.7 kg) of food and waste, which gradually clears during fasting
Total effect - Typical early fasting weight loss is around 5-7 lbs (2.3-3.2 kg), but depending on diet, hydration, sodium intake, and gut content, it can be lower or reach 10+ lbs in some people
Even though this early drop on the scale is exciting, it is temporary. Most of this lean mass returns during refeeding as glycogen is refilled, water is retained again, and normal gut content returns. Because of that, early fasting weight loss should not be mistaken for fasting fat loss.
🔥 When Fasting Fat Loss Begins
Once glycogen stores are depleted and the body enters ketosis, fasting fat loss becomes more steady.
Based on available data, fasting fat loss during extended fasts averages around 0.5 lbs (0.2 kg) of body fat per day. This rate is not the same for everyone and depends on several factors, including activity level, starting body fat, hydration, electrolyte balance, and previous fasting experience.
As a result, fasting fat loss can vary significantly between individuals.
Fasting fat loss vs length of fast
📊 What a 7-Day Fast Typically Looks Like
Looking at real-world results, a 7-day fast may lead to total fasting weight loss of roughly 7.5 to 15 lbs (3.4-6.8 kg). This wide range reflects differences in early lean mass loss, individual fat loss rates, and water balance.
This is why fasting weight loss alone can be misleading. Two people may lose a similar amount of fat but see very different changes on the scale.
⚖️ How Much Fat I Lost During My Fasts
In my own extended fasts, fat loss was measured using Dexa scans and body composition tracking. Across fasts lasting 7 to 10 days, my fasting fat loss ranged from about 3.8 to 5.5 lbs (1.7-2.5 kg) of body fat. Shorter fasts showed less fat loss, while longer fasts generally resulted in more total fat loss, though not in a perfectly linear way.
These numbers line up well with the average fat loss rate of roughly 0.5 lbs (0.2 kg) per day once ketosis is established. At the same time, the data also shows clear variability from fast to fast, even in the same person, depending on conditions like activity level, hydration, and refeeding strategy.
⏳ My Last 10-Day Fast: Weight Loss, Regain, and Fat Loss
My most recent 10-day water fast is a good example of how fasting weight loss and fasting fat loss play out in real life.
At the start of the fast, my body weight was about 164 lbs (74.4 kg). Over the 10 days, my weight dropped to roughly 149 lbs (67.6 kg), for a total fasting weight loss of about 15 lbs (6.8 kg). As expected, a large portion of this loss happened early and was driven by glycogen depletion, water loss, and emptying of gut content.
After the fast, during the refeeding period, my weight rebounded quickly. Within about 12 days of refeeding, my weight increased to around 160 lbs (72.6 kg). This rapid regain highlights an important point: most of the weight lost during an extended fast is not fat and comes back once normal eating resumes.
Daily body weight during my 10-day water fast and refeeding period
Body composition data makes this much clearer. Before the fast, my fat mass was about 23.9 lbs (10.8 kg). After the 10-day fast, fat mass dropped to approximately 18.4 lbs (8.3 kg), meaning fasting fat loss of about 5.5 lbs (2.5 kg).
After 12 days of refeeding, fat mass increased slightly to around 19.7 lbs (8.9 kg), but it remained well below the pre-fast level. In contrast, most of the lean tissue lost during the fast returned during refeeding.
Body composition before the fast, after the 10-day fast, and after 12 days of refeeding - fat mass, lean tissue, and bone mass
This is the key distinction between fasting weight loss and fasting fat loss. Scale weight can drop quickly and rebound just as fast, while fat loss happens more slowly and tends to persist if refeeding is done reasonably.
These graphs help show why focusing only on body weight can be misleading and why body composition provides a much clearer picture of what fasting actually does. I hope it helps! Stay strong, and happy fasting!