The 13-Day Diet: Honest Look at a Crash-Diet Fad

Short answer: the “13-day diet” (sometimes called the Danish, Copenhagen, Swedish, or Rigshospitalet diet) is a very-low-calorie fad, in the same family as the 3-day Military Diet. It runs at roughly 600 to 1,000 kcal per day with a rigid menu of black coffee, boiled eggs, small meat portions, and vegetables. It is not a hospital protocol despite the name, and it is not endorsed by any of the hospitals it is attributed to. Most of the weight you drop is water and glycogen, not fat⁽¹⁾⁽²⁾.

This article explains what the 13-day diet actually asks, why the scale drops fast in the first week, what happens when you finish, who should not try it, and what an honest fat-loss alternative looks like if you were drawn in by the promise of quick results.

What the 13-day diet actually is

Despite the “brand new” or “hospital” framing, this diet has been circulating in Europe since at least the 1980s under several rotating names. It has no institutional origin. Rigshospitalet in Denmark has explicitly denied any connection⁽³⁾.

The plan runs 13 days on a fixed menu, then requires a 3-month break before repeating. Daily intake is typically 600 to 1,000 kcal, well below the ~1,200 kcal floor most clinicians consider the absolute minimum for adults doing a supervised very-low-calorie diet⁽⁴⁾.

A representative day looks like this:

• Breakfast: Black coffee, sometimes a sugar cube or a slice of toast.
• Lunch: A boiled egg or two, a small salad, a grated carrot, or a small piece of salami.
• Dinner: ~200 g of steak, chicken, or fish with a plain salad.

There are no snacks, no dairy beyond a small yogurt, no bread beyond a single slice, and effectively no carbohydrate beyond one vegetable and one fruit some days.

Why the scale drops fast (and why it is not fat)

The average adult maintenance intake is about 2,000 to 2,500 kcal per day. Cutting to 600 to 1,000 kcal creates a very large deficit, but the early weight loss is not what people think it is. Three physiological things happen in the first week:

1. Glycogen depletion. Your liver and muscles store roughly 400 to 500 g of glycogen, and each gram binds 3 to 4 g of water. Burning through that releases about 1.5 to 2 kg of water in 48 to 72 hours.
2. Sodium and water shift. Removing bread, processed food, and added salt causes an additional 1 to 2 kg water drop over a week.
3. Reduced gut contents. Eating far less food means less transit content and stool weight.

Klein and Kiat’s 2015 review of short-term restrictive and detox diets is direct: rapid weight loss on very-low-calorie or fixed-menu plans is dominated by water and lean tissue changes, not by adipose tissue loss, and is generally regained within days to weeks of returning to normal eating⁽²⁾. Mayo Clinic’s guidance on fad diets is similar⁽¹⁾.

You will lose real fat too, at roughly 0.2 to 0.4 kg per day of pure fat loss maximum given the deficit size, but you will also lose lean tissue because the diet’s protein content (roughly 40 to 60 g/day) is well below the ~1.6 g/kg required to protect muscle during aggressive dieting.

What happens on day 14

Two things:

1. Refeeding regain. As soon as you eat normal carbohydrates and salt again, glycogen and water return within a week. Expect 2 to 4 kg of scale rebound even if you do not overeat.
2. Adaptive thermogenesis. Very-low-calorie diets down-regulate resting metabolic rate and hormones like leptin and T3. Your daily calorie need falls modestly for weeks to months after the diet, making sustained maintenance harder.

Net result across the literature on short crash diets: most participants regain the majority of lost weight within 6 to 12 months, and a meaningful fraction end up heavier than baseline.

Real risks to know about

• Hypoglycemia in people on insulin or sulfonylureas.
• Dizziness, headaches, fatigue in the first 3 to 5 days as glycogen depletes.
• Constipation from very low fiber intake.
• Gallstones. Rapid weight loss on very-low-calorie diets increases gallstone risk, especially in women and people with rapid loss over 1.5 kg per week.
• Loss of lean mass and strength given inadequate protein.
• Micronutrient inadequacy across 13 days (low B vitamins, calcium, magnesium, zinc, iron).
• Disordered eating triggering in anyone with a history of restriction, binge, or purging.
• Rebound weight gain once normal eating resumes.

Who should skip it entirely

• Anyone under 18, pregnant, or breastfeeding.
• People with type 1 or type 2 diabetes on insulin or sulfonylureas.
• Anyone with a history of an eating disorder.
• People with a history of gallstones or gallbladder disease.
• People with kidney disease or heart disease.
• People on blood-pressure medication (risk of orthostatic hypotension as sodium and water fall).
• People with high cholesterol who are told to “only eat egg whites.” Cholesterol from eggs is a small part of blood cholesterol; the bigger issue on this diet is the plan itself.
• Older adults (higher risk of sarcopenia on low-protein VLCDs).

13-day diet vs. Military Diet vs. sustainable deficit

If you have already started, do this instead of finishing 13 days

If you are 3 or 4 days into a 13-day plan and feeling wrecked, you do not need to finish it. Stopping early is not a failure. Consider:

1. Move to ~1,400 to 1,600 kcal/day with adequate protein (about 1.6 g/kg body weight).
2. Keep the vegetable and lean protein pattern you have built.
3. Reintroduce whole grains, legumes, and fruit for sustainable satiety and fiber.
4. Add 7,000 to 10,000 daily steps and 2 to 3 short resistance sessions per week.
5. Track scale weekly, not daily. Weekly averages remove water noise.

You will lose roughly 0.5 to 1 kg per week of mostly fat this way, and unlike the 13-day plan, that loss tends to stay off.

Is there any place for very-low-calorie diets?

Yes, but under supervision. Structured medically supervised very-low-calorie diets (~800 kcal) are used in obesity clinics for patients with high BMI and cardiometabolic disease, typically with meal-replacement formulas that guarantee protein and micronutrient adequacy, and with lab monitoring⁽⁴⁾. That is a very different animal from a 13-day pop-culture menu based on black coffee and boiled eggs.

Frequently asked questions

How much weight can you lose on the 13-day diet?

Most people lose 4 to 8 kg on the scale, but a large portion is water, glycogen, and gut contents. Real fat loss is closer to 1.5 to 3 kg. Most of the scale loss returns within a few weeks of resuming normal eating⁽²⁾.

Is the 13-day diet safe?

It is not appropriate for many people (diabetics, pregnant women, teenagers, people with a history of disordered eating, people with gallbladder or kidney disease). Even for healthy adults it carries risk of dizziness, muscle loss, gallstones, and rebound. It should not be attempted without medical guidance⁽¹⁾.

Is the 13-day diet from a hospital?

No. Rigshospitalet in Copenhagen (the hospital often credited) has explicitly denied any association with the diet. The “hospital” and “Swedish” and “Danish” labels are folklore, not documentation⁽³⁾.

Why do I lose so much weight in the first 3 days?

Glycogen depletion, sodium and water shift, and reduced gut contents. Each gram of glycogen holds 3 to 4 g of water. You can lose 1.5 to 3 kg of water in the first 72 hours without losing much fat.

Why does the diet ban repeating within 3 months?

The 3-month gap is folklore, not clinical protocol. Practically, however, repeating the diet cycles you through metabolic adaptation and rebound gain and is likely to worsen long-term outcomes.

Can I follow the 13-day diet with diabetes?

Not without medical supervision. Very-low-calorie diets in people on insulin or sulfonylureas carry a real risk of hypoglycemia. Speak with your care team before any restrictive plan.

What is a safer alternative for fast weight loss?

A modest daily deficit (400 to 600 kcal below maintenance), a protein target around 1.6 g/kg body weight, plenty of vegetables, whole grains and legumes, and daily walking plus resistance training produces about 0.5 to 1 kg per week of durable fat loss.

Medical disclaimer: This article is for informational purposes only and is not medical advice. The “13-day diet” is a very-low-calorie fad without institutional origin or clinical endorsement. Do not attempt any diet below ~1,200 kcal without medical supervision, especially if you have diabetes, cardiovascular disease, gallbladder disease, kidney disease, a history of disordered eating, are pregnant or nursing, or take prescription medication. Individual results vary. Not medical advice; consult a registered dietitian before making major dietary changes.