While it’s tempting to jump on a “lose weight fast” diet, crash diets often cause more harm than good and rarely lead to sustainable weight management. A balanced eating plan and regular physical activity remain the healthiest and most effective ways to manage your weight long-term. Here’s what happens to your body when you choose extreme calorie restriction or fad diets.

1. Your Body Burns Glycogen, Not Fat

During the first couple of days of a crash diet, your body uses glycogen—a stored form of carbohydrate found in the muscles and liver—for energy instead of body fat.

As glycogen is broken down, it releases water, causing a loss in water weight. This gives the false impression of rapid fat loss. Once you return to normal eating, this water weight often returns.

2. Your Cortisol Levels May Increase

Crash dieting places stress on the body, raising levels of cortisol, a stress hormone. This can lead to feelings of irritability, poor concentration, and sleep disturbances. The effects may be even more pronounced if the diet is very low in carbohydrates, causing fluctuations in blood sugar levels.

3. Your Metabolic Rate Slows

When calorie intake drops significantly, your body adapts to conserve energy by slowing your metabolic rate. While this survival mechanism is helpful in times of food scarcity, it can make it harder to burn calories and may lead to weight gain over time. This can leave you feeling constantly fatigued and unmotivated.

4. Your Muscle Mass May Decrease

In addition to slowing your metabolism, crash diets can lead to the loss of lean muscle tissue. Instead of using stored fat for energy, your body may break down muscle, particularly when protein intake is insufficient. Losing muscle further reduces your metabolic rate, making long-term weight management even more challenging.

5. Fatigue Sets In

Extreme dieting reduces essential nutrients, leaving you feeling tired and lethargic. It can also affect thyroid function, which regulates energy levels. The loss of glycogen and water further compounds feelings of exhaustion.

6. Irregular Bowel Habits May Occur

A poorly balanced diet lacking in fibre and nutrients can lead to changes in bowel habits, including constipation. Over time, this can disrupt gut health, including the microbiome balance, and hinder the body’s ability to flush out toxins.

7. Hair and Skin Quality Can Suffer

Rapid weight loss often deprives your body of essential vitamins, minerals, and healthy fats. This can result in dull, dry skin and brittle hair. Acne breakouts and other skin issues are also common, leaving you looking and feeling less healthy.

8. Dehydration May Develop

Many crash diets, such as juice cleanses, cause significant water loss. This can lead to dehydration, triggering side effects like dizziness, headaches, and fatigue. While increasing water intake can help, rapid weight loss often reverses once normal eating resumes.

A Smarter Approach to Weight Management

Crash diets may provide quick results, but they are rarely sustainable and can negatively impact your health. Instead, consider working with a qualified dietitian or nutritionist to create a balanced eating plan tailored to your lifestyle and body type.

Sustainable approaches such as:

  • Low-carb eating plans
  • Keto diets
  • Intermittent fasting

…can be effective when guided by a professional and paired with regular exercise. These methods focus on long-term health rather than short-term fixes, supporting your overall well-being while helping you manage your weight effectively.

Before starting any weight management plan, it’s always best to consult a healthcare professional to determine the right approach for you.