Although many adults in the United States who have overweight or obesity may benefit from weight loss, losing weight too quickly or losing too much body weight can cause unpleasant -- even dangerous -- side effects.
Unless you're being medically supervised during weight loss, a safe rate of weight loss is 1 to 2 pounds weekly, suggests the Centers for Disease Control and Prevention.
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Symptoms of Rapid Weight Loss
Losing weight at a rapid pace of 3 pounds or more weekly can cause fatigue, nausea, diarrhea and constipation, according to the Weight-Control Information Network. The network also notes that losing weight rapidly -- which often occurs when eating 800 calories or less daily -- can cause gallstone formation, which can become extremely painful or require surgical removal. Hair loss may also occur when losing weight at a rapid pace.
Whether you're losing weight too quickly or you have underweight, malnutrition is a concern when you're not eating enough calories. General symptoms of malnutrition include dizziness, weight loss and fatigue. Getting too few vitamins and minerals in your diet makes you more susceptible to a weakened immune system, osteoporosis and fatigue. Kwashiorkor, a form of malnutrition that occurs when you eat too little dietary protein, causes changes in your skin pigment, decreased muscle mass, fatigue, hair changes, irritability and rashes, notes MedlinePlus
Signs You Have Underweight
Having underweight can cause you to experience the same unpleasant side effects of malnutrition -- including hair loss, fatigue and dizziness. Weighing less than what is considered a healthy body weight can cause yellowish dry skin covered in fine hair, confusion, slow thinking, poor memory, dry mouth, extreme sensitivity to cold temperatures, muscle wasting and weak bones, according to MedlinePlus. Having underweight can also cause women to stop menstruating. The National Heart, Lung and Blood Institute reports that you're classified as having underweight if your body mass index is less than 18.5.
Maintaining a Healthy Weight
People with healthy body weights have BMIs ranging from 18.5 to 24.9, according to the National Heart, Lung and Blood Institute. To determine your BMI, multiply the number 703 by your body weight in pounds, divide that number by your height in inches and divide by your height in inches once again. If your BMI is less than 18.5, boost your calorie intake by choosing nutritious, calorie-dense foods -- such as nuts, seeds, nut butters and dried fruits -- and eating regular meals and snacks.