Mononucleosis is an infection caused by the Epstein-Barr virus, or EBV, one of the most common human viruses. While symptoms of EBV are indistinguishable from other mild illnesses in children, up to 50 percent of cases lead to mono, the drawn-out, glandular infection. Symptoms include high fever, swollen lymph nodes, sore throat and sometimes a swollen spleen. In the early stages of mono it's not uncommon to lose weight from nausea, loss of muscle tone and decreased appetite.
Appetite
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Loss of appetite is one of the main symptoms of mono. A sore throat, swollen neck glands, fever and weakness in the early stages of the infection can make eating difficult. An enlarged spleen, a symptom in some mono cases, can cause discomfort or a feeling of fullness and pain in the abdomen and can further diminish your appetite. The amount of weight you are at risk of losing as a result of a decreased appetite combined with a lengthy virus depends on your calorie intake. Going from a 2,000-calorie a day diet to a few hundred calories can result in a total weight loss of several pounds.
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Muscle Atrophy
The Mayo Clinic suggests waiting at least one month before returning to vigorous activities, heavy lifting or contact sports. The symptoms of mono can ease after a few weeks, but it could take several months before you feel completely normal. In that time, lack of physical activity, especially if you're bedridden, could lead to significant atrophy of lean muscle mass. Muscle wasting is defined by unintentional loss of five to 10 percent of body weight.
Nausea
Nausea and diarrhea, both of which can cause weight loss, are some distinguishing features of mono. The best way to treat these symptoms is by reducing your food intake, consuming a liquid diet and eliminating fats, dairy and sugar. Another danger of vomiting and diarrhea is that your body won't absorb and retain fluids. Patients with gastrointestinal symptoms are at risk of losing weight from dietary restrictions as well as losing water weight. Total weight loss, however, should be minimal since these symptoms rarely last longer than a few days.
Weight Loss Dangers
While weight loss is one characteristic of mono, it is not a distinguishing feature or a huge concern. The Mayo Clinic explains that if you lose 10 percent of your body weight in six months, you should see your doctor. Rapid weight loss could be indicative of a more serious condition. It could also interfere with your ability to recover from mono in a timely manner.