Stress can affect your endocrine system in regards to increasing chances of dysfunction with abnormal hormone or insulin production. Hormones are the chemical controls of your body and due to the stresses the rates of hormones overall are changed. This can become chronic changes, that affect your body.
Anyone who falls between the 5th percentile and the 85th percentile is a healthy weight. If someone is at or above the 85th percentile line on the chart (but less than the 95th percentile) is overweight.
firmly say no showing signs of weakness allows the person to keep pressuring you into doing it since you seem weak.