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People at the opposite ends of the weight spectrum — those who are significantly underweight and those who are morbidly obese — are at increased risk for serious complications and hospitalization from the flu, according to an observational study conducted in Mexico. Results of the study, which suggest health care providers should consider a patient’s weight when deciding on flu treatment, were published in the journal Influenza and Other Respiratory Viruses.

The study involved 4,778 people treated for influenza or flu-like illnesses in six hospitals, either as inpatients or in outpatient departments. Study participants presented with at least one respiratory symptom — cough or difficulty breathing, for example — and either fever or another non-respiratory symptom, such as headache or malaise (feeling unwell). Compared with people of normal weight, those a with low (<18.5) or high (35 or above) body mass index (BMI) were at higher risk for flu complications, including pneumonia, and hospitalization.