For many cancer patients, malnutrition is the guest they didn’t expect, or want. It may show up shortly after you’ve been diagnosed with cancer, if nausea or vomiting prevents you from eating well, or if the disease disrupts how you digest or metabolize food. Or it may creep in during treatment. It may affect you even if you appear to be eating plenty of calories and protein but, because of your cancer, are unable to maintain enough fat stores and muscle mass.