Wellness Wednesday: A Dietitian’s Take on Monofood Fasts, Short-Term Wins, and Long-Term Risks
As a registered dietitian, my role is to help people separate nutrition fact from fiction and find practical, science-backed ways to support their health goals. Diet culture is full of quick fixes that resurface year after year, each promising effortless weight loss or “detox.” One of the most popular, especially in the summer, is the watermelon diet, a short-term regimen where watermelon becomes the sole or primary food for several days. On the surface, it may seem harmless. After all, watermelon is refreshing, hydrating, and low in calories. Some even report feeling lighter or losing a few pounds during the fast.
But from the perspective of nutrition science, this type of monofood diet raises bigger concerns. While watermelon itself is nutrient-rich, relying on it, or any single food, as the foundation of a diet can lead to predictable consequences: nutrient gaps, energy imbalances, and misconceptions about how the body truly responds to food. In this article, we’ll break down what actually happens during a watermelon fast, why people sometimes see short-term results, and, most importantly, explore safer, sustainable strategies for long-term nutrition, energy balance, and weight management.
What Happens During a Watermelon Diet
A typical version of the watermelon diet involves eating only watermelon for 3–7 days, sometimes with water or tea included. A large wedge of watermelon is about 85–90% water and naturally low in calories, roughly 46 calories per cup (U.S. Department of Agriculture, 2019). Because of this, most people consuming watermelon exclusively will experience a dramatic reduction in daily calorie intake, often dropping from their usual 1,800–2,500 calories per day down to fewer than 800–1,000.
What Happens in the First Few Days
From a metabolic standpoint, this sudden drop in calories creates a sharp calorie deficit. As carbohydrate intake plummets, the body begins tapping into stored glycogen for energy. Glycogen, the storage form of carbohydrate found in muscle and liver tissue, is tightly bound with water, about 3–4 grams of water per gram of glycogen. When glycogen is depleted, the water bound to it is released and excreted. This explains why many people see a 2–5 pound drop within the first few days of a watermelon fast. However, this early weight loss is primarily water, not fat.
Beyond Water Weight: Muscle and Fat Loss
If the fast continues beyond a few days, the body begins adjusting in other ways:
Lean muscle breakdown: With inadequate protein intake (watermelon provides minimal amounts), the body turns to muscle tissue to generate amino acids for critical functions, such as maintaining blood glucose through gluconeogenesis. This means a portion of the weight lost comes from lean body mass rather than fat.
Adipose (fat) tissue loss: Sustained calorie deficits will also tap into fat stores, but this process is slower than water loss and often accompanied by muscle breakdown if protein intake is insufficient.
Electrolyte imbalance and energy decline: Because watermelon is low in sodium and other electrolytes, prolonged fasting increases the risk of fatigue, dizziness, and even more serious complications like hyponatremia.
Longer Durations: 14–30 Days
Some variations of the watermelon diet extend far beyond the typical 3–7 days, lasting 14 to even 30 days. While this is far less common, it significantly increases health risks:
Severe muscle wasting: Without dietary protein, the body continuously breaks down muscle tissue to meet its amino acid needs. This compromises strength, metabolism, and immune function.
Micronutrient deficiencies: Watermelon contains vitamin C, potassium, and some antioxidants, but it lacks essential nutrients like vitamin B12, iron, calcium, zinc, and essential fatty acids. Over weeks, deficiencies become increasingly likely, impacting red blood cell production, bone health, and hormone regulation.
Metabolic adaptation: Extended very-low-calorie diets slow the body’s resting metabolic rate as a protective mechanism. This makes weight regain more rapid once normal eating resumes.
Potential organ stress: The kidneys and liver work harder under prolonged nutrient imbalance, especially with excess water intake from both watermelon and fluids, further heightening risks of electrolyte disruption.
The Rebound Effect
When the fast ends and a regular diet is reintroduced, glycogen stores refill, water weight is regained, and the scale often climbs back up within days. In some cases, the rebound weight may even exceed the starting point due to metabolic slowdown and overeating triggered by prolonged restriction.
Nutrient Gaps and Health Risk
Watermelon provides vitamin C, vitamin A (as beta carotene), potassium, magnesium, and lycopene (a powerful antioxidant). But relying on it exclusively strips away access to essential macronutrients and micronutrients:
Protein: Watermelon contains less than 1 gram per cup, making it nearly impossible to meet the body’s needs for tissue repair, muscle maintenance, and immune function.
Fat: Virtually absent, despite being necessary for hormone production, fat-soluble vitamin absorption, and satiety.
Other vitamins/minerals: Deficiencies in B vitamins, calcium, iron, zinc, and essential fatty acids quickly become a risk with a monofood diet.
Extended restriction can lead to fatigue, muscle breakdown, impaired concentration, and even electrolyte imbalances, particularly if the diet is combined with intense exercise or hot weather conditions.
Why People Feel “Better” at First
One reason monofood diets like the watermelon fast gain popularity is because participants often report feeling lighter, less bloated, or more energized initially. There are a few explanations for this:
Digestive Rest – Processed foods, excess sodium, and alcohol often contribute to bloating. Removing these for a few days naturally reduces water retention.
Hydration Boost – Watermelon is about 90% water and contains potassium, which helps balance fluids. Improved hydration can ease digestion and improve energy temporarily.
Simplicity – With only one food to eat, decision fatigue around meals disappears, reducing stress for a short time.
However, these benefits are not unique to watermelon. Similar outcomes can be achieved by improving hydration, reducing sodium, and adding more fruits and vegetables to a balanced diet, without nutrient deprivation.
A Registered Dietitian’s Perspective: Balance Always Wins
From a nutritional standpoint, sustainability matters more than quick fixes. A few days of eating only watermelon may not cause permanent harm in an otherwise healthy adult, but it teaches the wrong lessons about food: that extremes are necessary, that “detoxing” requires deprivation, and that weight management is best achieved through restriction.
In reality, the liver and kidneys already manage detoxification effectively when supported with adequate hydration, balanced nutrition, and overall lifestyle health. Restrictive diets undermine long-term wellness by creating cycles of deprivation and rebound.
A better alternative is learning how to build meals with balance:
Fruits and Vegetables: For hydration, antioxidants, and micronutrients.
Protein Sources: Lean meats, legumes, dairy, or plant-based proteins to support muscle and metabolism.
Healthy Fats: Nuts, seeds, avocado, and oils for satiety and nutrient absorption.
Whole Grains: To maintain glycogen and energy balance.
This balance promotes steady energy, maintains muscle, and supports a healthy metabolism, far more effective than days of watermelon alone.
Better Alternatives for Health and Weight Management
If someone is drawn to the watermelon diet because they want to feel lighter, refreshed, or reset, there are safer, evidence-based approaches:
Hydration Reset – Start the day with water, reduce sugary drinks and excess alcohol, and include hydrating foods like watermelon alongside cucumbers, oranges, and leafy greens.
Plant-Forward Eating – Aim for half your plate to be vegetables and fruit at meals. This increases fiber, lowers calorie density, and supports gut health.
Structured Calorie Deficit – For fat loss, modest calorie reductions (300–500/day) paired with protein intake of 1.2–1.6 g/kg body weight helps preserve lean mass while encouraging fat reduction (Leidy et al., 2015).
Mindful Eating – Instead of following rigid rules, focus on hunger and fullness cues, reducing overeating while maintaining satisfaction.
These strategies mirror some of the “feel good” effects of a watermelon fast while providing nutrients the body needs to thrive.
The Bigger Picture of Fad Diets
The watermelon diet is just one example of how monofood fasts create the illusion of progress while falling short of supporting true health. Similar patterns exist in other popular approaches, from banana diets to cabbage soup cleanses. Each may deliver short-term results, but they fail to offer balance, sustainability, or respect for the body’s complex nutritional needs.
This article marks the beginning of a series breaking down fad diets, extended fasts, and so-called detox supplements, with the goal of empowering readers with evidence-based insight. By understanding both the science and the psychology behind these trends, individuals can make informed decisions that support not just short-term goals, but lifelong health.
