Is a BMI of 15.8 Bad? Understanding the Health Risks of Underweight
                                
                                
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                                    4 min read                                
                            
                                According to the World Health Organization (WHO), a BMI below 18.5 is classified as underweight, and a BMI of 15.8 falls into the "moderate and severe thinness" category. This significantly low number is a serious indicator that a person may be malnourished and is at an increased risk for various health problems.