Fasting day: what it provides and what it doesn't
A fasting day is a 24-hour period with sharply reduced calorie intake, usually consisting of a single food item. It almost always works on the scale, but almost never on body fat, and the difference between these two statements is the whole story.
Where does the lost kilogram come from
The morning after a fasting day, the scales show 0.7–1.5 kg less. This much fat is not lost in a day: even a complete refusal of food with usual expenditure creates a deficit of about 2000 kcal, which is approximately a quarter of a kilogram of adipose tissue.
Something else is lost:
- Glycogen and associated water. Each gram of glycogen retains about three grams of water; with a sharp reduction in carbohydrates, the reserve is used up, and water leaves with it.
- Intestinal contents. On a kefir or apple day, the volume of food consumed is many times less than usual.
- Salt and water. A typical diet contains significantly more sodium than a kilogram of apples.
All three return within one or two normal days. The mechanism is the same as described in the analysis of why weight changes by a kilogram in a day.
What the weekly arithmetic says
The result is determined by the weekly average, not by a single day. A fasting day of 800 kcal instead of the usual 2000 provides a deficit of 1200 kcal — a significant contribution if the other six days remain unchanged.
The problem is that they usually change. Sharp restriction intensifies hunger the next day and the day after, and compensation eats up most of the gain. This is not a weakness of character, but a measurable physiological response: the more severely the diet is restricted, the stronger the hunger signal.
Hence the practical conclusion: if fasting days are done against the background of a normal diet and do not lead to overeating afterward, they work like a regular deficit. If the diet is disrupted after them, they work against you.
How this differs from intermittent fasting
| Fasting day | Alternate-day fasting | |
|---|---|---|
| Frequency | Usually once a week or less | Regular pattern |
| What has been studied | Almost not studied separately | There are controlled comparisons with a regular deficit |
| Result in studies | — | Comparable to a regular deficit of equal calories |
| Composition | Often a single product | Normal diet, calorie-restricted |
The key takeaway from the second column: where alternating schemes were compared with a regular deficit at equal weekly calorie intake, no advantages were found for them. That is, it's not about the fact of “fasting” itself, but the total. More details on eating patterns with breaks — intermittent fasting and cheat meals and refeeds.
Single-food days
Classic options include kefir, apple, buckwheat, cottage cheese, and cucumber days. From a daily perspective, the difference between them is small, but there are a few things to know.
Cottage cheese and kefir days provide protein and are tolerated more easily than others: satiety lasts longer. Kefir, however, has almost no protein compared to cottage cheese — 3 g per 100 ml versus 17.
Apple and cucumber days are almost entirely without protein. Nothing will happen in a day, but it is often after these that a rebound occurs.
Buckwheat days provide carbohydrates and fiber, but do not provide complete protein.
None of them “cleanse” the body or remove toxins: the liver and kidneys are constantly engaged in this work, and food cannot help them — analysis.
Who should avoid them
Fasting days are not recommended:
- for diabetes, especially when on insulin or sulfonylurea drugs — risk of hypoglycemia;
- during pregnancy and breastfeeding;
- for children and adolescents;
- for a history of eating disorders — sharp restriction followed by a binge is their typical cycle, signs are listed separately;
- for kidney disease, liver disease, gout — sharp fasting can provoke an attack, see gout and purines;
- when taking medications that are taken with food or that affect glucose levels.
What to do instead
If the goal is to compensate for a feast, there's nothing to compensate it with: a one-time excess has little impact on the monthly total, and an attempt to “work it off” often triggers a restriction-binge cycle. It's more sensible to return to your usual diet with the next meal.
If the goal is weight loss, a moderate daily deficit works: it does not cause compensatory hunger of such intensity and does not require willpower once a week. The boundaries of what is reasonable are in the analysis of how to lose weight correctly.
And if you simply enjoy fasting days and they don't disrupt the rest of your week, there's no harm in them. They should simply be counted as one day with lower calorie intake, rather than as a separate method.
Frequently asked questions
- How much weight can be lost in a fasting day?
- The scale usually shows a drop of 0.7–1.5 kg, but this is glycogen with water, intestinal contents, and sodium. About a quarter of a kilogram of adipose tissue is lost per day even with complete fasting, and what is lost on the scale returns within one or two normal days.
- Do fasting days help with weight loss?
- Only as part of a weekly deficit. In studies where intermittent fasting schemes were compared with a standard calorie-equivalent deficit, no advantages were found, and severe restriction increases hunger in the following days.
- Who should avoid fasting days?
- Those with diabetes, during pregnancy and breastfeeding, in childhood and adolescence, with a history of eating disorders, with gout, kidney or liver diseases, and when taking medications that affect glucose levels.
Read next
- 16/8 intermittent fasting: what happens during the fasting window — People usually say the same thing about 16/8: at some point, the body "switches to fat," and then "autophagy kicks in.
- Why weight changes by a kilogram in a day — Fat cannot be gained overnight: a kilogram of adipose tissue is thousands of kilocalories that cannot be acquired in eight hours of sleep.
- How to lose weight correctly — Weight loss is boring: a slight energy deficit, enough protein, strength training, and time.
This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.
Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.