Can You Gain Weight From Not Eating? Examining the Myths and Facts

Losing weight is a common goal for many to potentially support overall health, but many myths surround weight loss. Reducing caloric intake is a common tactic for reducing weight, and while calorie reduction can be an effective strategy, it also has some drawbacks. Learn more about undereating, weight loss, and weight gain facts below.

Can you gain weight from not eating?

While the typical theory is that you lose weight by eating less than your body needs for energy each day, the truth isn't as simple as this. You might not lose weight if you're not eating enough, and some weight gain is even possible when undereating.

How energy needs change with weight loss

Energy needs are based partly on body weight, so when you lose weight, your energy needs decrease, and future weight loss may become harder. If you return to your regular eating habits after weight loss without shifting to your new, somewhat lower calorie needs, you may regain some or all of the lost weight.

When losing weight, your body composition changes, and you don't only lose extra body fat. You also may lose some of the lean body mass (muscle), which can account for as much as 20–40% of the total weight loss. Losing muscle mass also may decrease your resting metabolic rate, making it easier to gain weight in the future.

Eating adequate amounts of protein, which, according to the Recommended Daily Allowance, is around 0.8 grams per kilogram of body weight per day, can offset the loss of lean muscle mass. Incorporating physical activity into your weight management efforts, especially strength and resistance training, can also help potentially decrease the loss of lean body mass.

Adaptive thermogenesis and hormonal changes

Adaptive thermogenesis after weight loss may further decrease energy needs. Adaptive thermogenesis is a greater-than-expected decrease in energy expenditure beyond what you would expect from the loss of fat mass and fat-free mass alone.

In a systematic review of 33 studies, adaptive thermogenesis after weight loss was observed in 27 studies. However, this effect lessens after weight stabilizes for some time, so it may not play a long-term role in weight maintenance or weight regain after weight loss. Also, some studies mentioned in this review on adaptive thermogenesis have found contradictory results, so more research is needed to determine how adaptive thermogenesis may impact weight management.

Weight loss is also accompanied by hormonal changes, some of which may increase your appetite and decrease satiety levels or feelings of fullness.

What is undereating?

Put simply, undereating is consuming fewer calories and nutrients than your body needs to perform all its basic functions, including breathing, moving, temperature regulation, and maintaining your weight. This causes a calorie deficit. If you're trying to lose weight, you might undereat by aiming for a modest calorie deficit over time.

A modest calorie deficit for weight loss is unlikely to cause short-term health issues, but chronically undereating may lead to nutritional deficiencies, malnutrition, or other health complications. When severely undereating, you're likely eating significantly less than your body requires to maintain your weight and basic bodily functions.

Symptoms of undereating

Symptoms of undereating are wide-ranging and include physical and emotional symptoms.

Physical symptoms

People who undereat may experience certain physical symptoms. It's common to feel low energy, fatigue, weakness, or shakiness when not eating enough to meet body needs. Some people also feel cold all the time when undereating, though this is more likely to happen with chronic undereating. When undereating, it can be challenging to meet all your micronutrient needs from the diet alone, which may lead to potential nutritional deficiencies long-term.

In a study on meal skipping, those who skipped a meal had lower diet quality, which could impact health over time. Long-term undereating may cause hair loss or brittle hair due to not meeting nutrient needs. Loss of lean muscle mass is another symptom of undereating, but this can be combated partly by resistance training and eating adequate protein.

Emotional symptoms

When not eating adequately to maintain optimal health, emotional symptoms may also occur after a while. Undereating may cause irritability or mood changes. Restrictive eating can cause a person to think about food more often or have more frequent food cravings, making it difficult to focus on other areas of life.

Eating too little may also potentially cause increased stress, especially in your personal relationships, if people are concerned about you and your health. Increased anxiety and depression-like symptoms may also be possible due to undereating and nutritional deficiencies, though more research is needed to understand food's impact on overall mental health.

Causes of unintentional weight gain

Unintentional weight gain has many potential causes. Some weight regain is common after losing weight. If you previously strictly monitored your dietary intake by carefully measuring portion sizes or portioning foods into individual servings and then stopped being as diligent, portion sizes may creep up over time.

Increasing exercise but not monitoring food intake may potentially cause some weight gain. Physical inactivity can also lead to unintentional weight gain. Even if you do some formal exercise sessions, having a sedentary lifestyle outside of a structured exercise routine can slow down your overall weight loss efforts. Non-exercise activity thermogenesis is a type of daily energy expenditure that comes from general daily life activities and not from structured exercise. Examples include walking, moving on the job, standing up and moving throughout the day, or performing chores.

If undereating, you might eat more at subsequent meals and snacks due to increased hunger. In a study that examined the effects of skipping meals in the United States, researchers found that adults who skip one of the three standard meals of the day — breakfast, lunch, or dinner — consumed more calories at the next meal. If they skipped both breakfast and lunch, they ate, on average, 783 more kilojoules at dinner, or around 187 calories. However, their total daily energy intake was still reduced, even with eating more at the next meal after skipping.

Researchers in this study also found that people who skipped meals had decreased diet quality overall since certain food groups were more likely to be eaten at certain meals of the day, such as whole grains and dairy at breakfast or vegetables and protein at dinner. Lower diet quality from skipping meals could also negatively impact weight management.

Not all causes of weight gain are within a person's control. Some hormonal conditions, such as hypothyroidism or polycystic ovary syndrome, can make weight loss more difficult, even in a calorie deficit. Certain medication use sometimes may potentially cause some involuntary weight gain.

How to break the cycle of undereating

If you're stuck in a cycle of undereating and want a healthier relationship with food, start by focusing on nourishment.

1. Eat foods from most of the basic food groups daily: Fruits, vegetables, proteins, grains, healthy fats, and dairy or dairy substitutes. This will help ensure you're eating essential nutrients that your body needs.
2. Aim for three meals per day: If you're not sure when to eat, start by eating a meal shortly after waking up in the morning, then try to eat at least every three to four hours while awake to become used to eating routinely again. Add a snack in between meals if you feel hungry.
3. Include a few different foods at every meal: Ideally, this should be a source of protein, carbohydrates, and healthy fats at most meals. If you're unsure how much to eat of a food, you could try measuring food for a time or eating the serving size suggested on packaged foods.
4. Learn your hunger cues: Try to learn your hunger cues and focus on body sensations to understand when you're full after eating.

Keep in mind that no one-size-fits-all approach exists for correcting a cycle of undereating. Diet patterns are different from person to person, so adjust the general suggestions above to work for you and your individual needs as needed.

Seeking professional help

Consider seeking help from a registered dietitian if you think you might be undereating and are unsure how to make changes. Dietitians can help you gain or lose weight healthfully, understand how much to eat, the beneficial foods to include in the routine, and how to find a balance between food intake and your level of exercise. A therapist specializing in areas such as body positivity, emotional eating, or disordered eating would also be helpful when trying to develop a healthier relationship with food and your body.

Note that chronic undereating is associated with some potential eating disorders, so please seek professional help if you're experiencing symptoms of an eating disorder or you feel challenges in terms of your eating pattern.

Bottom line

Eating less than your body needs for daily energy may lead to potential weight loss, but undereating in some instances may also result in gaining weight, among other health consequences. Instead of only focusing on calorie reduction, work on improving the overall quality of your diet, eating nourishing foods for better overall health, and increasing physical activity over time. Seek professional help if you're having trouble finding balance in your diet or are experiencing physical or emotional symptoms of overeating.