Why fasting for weight loss is not a good idea

Why fasting for weight loss is not a good idea

You’ve probably heard from countless fitness gurus on the Internet that the only way to lose weight is through a caloric deficit—when you take in fewer calories than you burn.

While this adage has some validity, it does not fully explain the healthiest and most effective approaches that will lead to sustainable and long-term weight loss.

Because of this, many people resort to fasting, which can be extremely harmful to health.

In this article, you will learn why fasting for weight loss is not a good idea and how to implement healthier weight loss strategies.

What is the difference between fasting and intermittent fasting?

If you are not familiar with the term, you may think that intermittent fasting is the same as fasting. However, if done correctly, intermittent fasting can be a healthy and sustainable practice. (1) (2)

Intermittent fasting is a way of eating that involves cycling between “eating” and “fasting” periods. For example, the most typical form is the 16:8, which includes an 8-hour feeding period and a 16-hour fasting period. ( 3 )

Although intermittent fasting can help you lose weight, excessive calorie restriction is not the goal. Instead, you simply eat your normal daily calories or go into a caloric deficit for a short period of time each day.

Fasting, on the other hand, is usually defined as an extended period of time without food or with very limited food intake significantly below the body’s daily caloric needs. This puts your body in too much of a calorie deficit and leads to unsustainable weight loss.

Nutritionists typically define a very low calorie diet as eating 450-800 or fewer calories per day, which is not healthy and sustainable in the long term. Exposing the body to such fasting can pose many health risks and is not recommended. (4) (5) (6)

How fasting affects your body

To lose weight, your body must be in a caloric deficit, which involves burning more calories through exercise and/or eating fewer calories from food. However, a larger calorie deficit doesn’t always mean you’ll lose weight and keep it off.

Although significant weight loss may be seen initially, it may be difficult to maintain this weight loss in the long term.

Even more problematic, if you’re starving, your body’s survival mechanisms can adapt to a large calorie deficit. This can hinder your weight loss plan.

Your metabolism slows down

During a prolonged calorie deficit, your body begins to use fat stores as a primary source of energy, and muscle and skeletal tissue as a secondary source of energy.

Over time, your body responds to the calorie deficit by lowering your resting metabolic rate (RMR) through adaptive thermogenesis (metabolic adjustment). This makes your body less efficient at burning calories to conserve as much energy as possible. (7) (8) (9)

This was proven by a groundbreaking study of 14 contestants from The Biggest Loser. Over the course of 30 weeks, the show’s contestants lost an average of 58.3 kg, and their RMR dropped from an average of 2,607 calories per day to 1,996 calories per day. ( 10 )

Although they later regained an average of 41 kg, their average RMR remained low (1,903 calories per day).

However, recent studies show that the metabolic adaptation moderates when you are no longer in a caloric deficit. Most cases of weight regain are thought to be the result of excessive caloric intake, which can be attributed to increased hunger and the feeling of “freedom” from the calorie deficit. (11) (12) (13) (14) (15)

A slowed metabolism can also cause fatigue more easily. This is a strategic mechanism your body uses to prevent you from using up too much energy. Your body also increases the release of hunger hormones that motivate you to eat. (16) (17) (18)

Over time, your body will work hard to prevent further weight loss by slowing your metabolism, especially during prolonged fasting.

Your body works less efficiently

Depending on the severity of your fast for weight loss, the number of calories you are restricting, and the length of the period, your body may begin to prioritize basic bodily functions such as breathing and heart rate and slow down other bodily processes, such as:

  • Hair and nail growth. Your hair and nails can become brittle.
  • Immunity. Your immune system may have a harder time fighting infections and illnesses.
  • Regulation of digestion and appetite. You may experience irregular or increased hunger, recurring bloating or abdominal discomfort.
  • Reproductive health. Your menstrual cycle may change or stop.
  • Skin health. Improper or delayed wound healing or premature aging may occur.
  • Bone health. It can weaken your bones.

Starvation puts your body in an unhealthy state that it is desperate to get out of. Although you may lose weight quickly at first, your body needs enough calories to function properly and will work hard to regain weight and health as soon as possible.

It can harm your mental health

Fasting and other unhealthy eating habits can harm your mental state.

Fasting dieting can lead to the development of eating disorders such as food restriction, fear associated with food choices, negative association with food, excessive exercise and obsession with reducing body weight and size. (19) (20) (21) (22)

In more severe cases, prolonged fasting can develop into an eating disorder such as anorexia nervosa, bulimia nervosa, or binge eating disorder.

If you think you have an eating disorder or disordered eating patterns, it’s important to talk to your doctor, who can refer you to a specialist.

Tips for healthy weight loss

Instead of extreme fasting for weight loss and compromising your health in the name of weight loss, it is better to adopt healthy, sustainable habits.

Here are some science-based ways to lose weight in a healthy way and keep it off:

  • Aim for a small calorie deficit. Most research shows that a deficit of 10-20% is sustainable and manageable. For example, if your maintenance calories are 2,500 calories per day, aim for a deficit of 250-500 calories per day through healthy eating and exercise.
  • Increase physical activity. Aim for a combination of strength training and cardio (running, walking, etc.) for at least 200 minutes per week, or about 30 minutes each day.
  • Add strength training to your routine. Strength training helps maintain and build muscle tissue during weight loss. Building more muscle mass can increase your metabolism.
    Limit processed foods. Try to make the majority of your meals consist of whole, minimally processed foods that are generally lower in calories and higher in protein, fiber and healthy fats to promote satiety.
  • Eat more protein. A high-protein diet can help preserve muscle tissue during a caloric deficit.
  • Drink mostly water. Limit sugary drinks, energy drinks, and drinks that are high in sugar and calories. Instead, choose water, flavored water, coffee and tea most often.
  • Be patient and take your time. Most studies show that a sustainable and healthy rate of weight loss is about 0.45-0.9 kg per week. So slowly add new healthy habits to help you reach your weight loss goals.

The best diets are affordable, enjoyable and sustainable. Remember that not all weight loss is healthy. Focus on a healthy lifestyle that gives you energy and that you enjoy.


Fasting for weight loss is not healthy or sustainable.

While it may be tempting to deprive yourself of food to lose weight faster, your body will suffer. After a prolonged fast, your body’s metabolism may slow down, your body may not function properly, and your mental health may deteriorate.

Although you may lose a little weight at first, you will likely quickly regain the previous pounds.