What Are the Side Effects of Not Eating?

Not eating leads to a host of side effects, which may be minor at first; however, they can quickly progress to result in more serious, life-threatening consequences.
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Food is energy. It provides the raw materials that all your body systems rely on for proper functioning. Not eating leads to a host of side effects, which may be minor at first; however, they can quickly progress to result in more serious, life-threatening consequences.



Not eating can lead to fatigue, gastrointestinal distress, nutrient deficiencies and slowed metabolism.

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Short-Term Fasting Side Effects

The recent rise in popularity of calorie restriction strategies such as intermittent fasting has a lot of people forgoing food for up to a few days at a time. Going three days without food isn't a big deal; your body can survive without sustenance for brief periods of time. But that doesn't mean you won't experience side effects.

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The most common symptom of undereating is fatigue. Calories from food are your body's energy currency. Without enough calories, you'll feel tired when you exert yourself, whether it's exercising or simply walking up a flight of stairs. Low blood sugar may cause you to feel dizzy. According to the Academy of Nutrition and Dietetics, you may find it harder to think clearly due to the short supply of glucose that is your brain's main source of energy.

Other side effects of undereating are gastrointestinal. Your stomach rumbles, and you feel a little a queasy and nauseous. You may also experience either diarrhea or constipation.

Read more:13 DOs and DON'TS of Intermittent Fasting

Potential Nutrient Deficiencies

Food supplies not only calories, but also nutrients, such as protein, fats, vitamins and minerals. Your body requires these substances in sufficient amounts to function properly. Although it can go a short time without nutrients, effects of undereating and nutrient deficiencies will begin to be noticeable. Some important nutrients and their deficiency symptoms are:


  • Carbohydrates are your body's main source of energy. Deficiency results in noticeable deficits in physical and mental effectiveness.
  • Vitamin B12 is crucial for making healthy red blood cells that carry oxygen throughout your body. Low levels result in anemia, or not enough red blood cells, the most common side effects of which are fatigue and reduced exercise tolerance. Prolonged deficiency can cause neurologic deterioration.


  • Iron also plays an important role in the creation of healthy red blood cells. Deficiency symptoms are similar to those of B12 deficiency — chronic fatigue, weakness, dizziness and shortness of breath when doing everyday tasks.


  • Zinc is an essential mineral that supports immune system health. If you become deficient, you may experience impaired wound healing, decreased sense of taste, hair loss, diarrhea and changes in your skin.


  • Sodium is an electrolyte mineral that helps regulate the body's fluids; it's also involved in muscle function. Low sodium levels may cause nausea, cramps, headache, confusion, exhaustion, irritability, seizures and coma, according to Mayo Clinic.


Starvation may result from illness, eating disorders, famine and fasting. In response, the body will begin to mobilize its tissues, leading to the destruction of organs and muscle. The skin typically becomes dry and thin, and hair thins and falls out. According to ‌Scully's Medical Problems in Dentistry (Seventh Edition)‌, starvation can be fatal within eight to 12 weeks.

Metabolic Side Effects

If your weight loss plan consists of giving up food altogether, you're doing yourself a disservice. Eating too few calories can actually hinder your weight loss success. According to the Academy of Nutrition and Dietetics, eating too few calories slows your metabolism so you burn fewer calories and less fat. This is because your body goes into starvation mode when it doesn't have what it needs to function optimally, slowing your metabolism to conserve energy.


If the symptoms of undereating make you tired all the time, you're also not going to be able to do one of the most important things for weight loss: exercise. Regular aerobic and muscle building exercise — in addition to a healthy diet — is a surefire way to weight loss success.

Additionally, when you severely restrict calories, your body needs to find alternate energy sources. As a result, it may resort to breaking down lean muscle mass for energy, which is detrimental not only to your overall health, but also to efficient metabolism. Muscle mass is metabolically active — more so than fat — so the more you have, the faster your metabolism. This means you don't have to starve yourself to lose weight — you just need to build more muscle mass, and maintain what you have.



If you're struggling with an eating disorder, it's important to get help as soon as possible. Talk to a trusted friend, family member or your physician. Eating disorders such as anorexia nervosa, which involve restricting food intake — sometimes severely — have physiological and emotional causes that can be addressed with medical treatment and therapy.

Intermittent Fasting: Good or Bad?

Intermittent fasting (IF) involves restricting food intake for parts of each day or for entire days at a time. Popular IF patterns are 16:8, which means fasting for 16 hours a day and eating all your meals inside an eight-hour window, or 5:2, which means fasting for two full days per week. IF is different from simply not eating; the emphasis is on getting proper nutrition but limiting food intake to a specific period of time.



There are many claims about the potential benefits of IF, including:

  • Weight loss
  • Disease prevention
  • Heart health
  • Metabolic health

Much of the existing research has been done on animals; however, there are a handful of small human trials that show potential for this type of dietary pattern.

A study published in the journal Cell Metabolism in June 2018 involved 12 men with prediabetes. Participants were randomized to two groups following either an early time-restricted eating pattern with all meals in a six-hour period and nothing after 3 p.m., or a 12-hour feeding period. After five weeks, the groups switched eating schedules.

The researchers found that the six-hour, early time-restricted eating pattern improved cardiometabolic factors including insulin sensitivity, blood pressure and oxidative stress. It also helped the men control appetite for weight loss.

Another small study published in January 2013 in the Journal of Diabetes and Metabolic Disorders found that eight weeks of alternate day fasting reduced body weight, waist circumference, blood pressure and cholesterol in a group of 15 women who had overweight or obesity.

Read more:Why Intermittent Fasting May Help You Lose Weight

However, both studies and others like them are too small to determine whether IF is really a viable option for weight loss or disease prevention, or whether it's simply the practice of eating a healthy, reduced-calorie diet or not eating late in the day that is responsible for the favorable outcomes.

If you're healthy, there's no reason not to give IF a try, as long as you do it in a safe manner. When you do eat, choose nutrient-rich foods packed with fiber, protein and healthy fats. It's a good idea to ease into this sort of dietary pattern by slowly increasing the time you restrict eating, so your body and appetite have a chance to get used to the change.




Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.

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