Why do we gain weight? From before birth to old age

Summary

This is the first session of Dr. Om’s 2025 obesity course, a conversation with a fitness coach about the most basic question people ask: why did I gain weight?

His central message is that weight gain is not about laziness or lack of willpower. It is a complex condition with causes that stack up across a lifetime:

• Before birth. Most family obesity comes from many genes acting together. A few rare single-gene conditions cause severe hunger and obesity from early childhood and need a specialist. He also explains fetal programming, using the Dutch famine of the 1940s and Dr. C. S. Yajnik’s research in India: babies who are undernourished in the womb and then grow up with plenty of food are more prone to obesity and diabetes. Babies of mothers with high sugar in pregnancy also carry higher risk.
• Childhood. Breastfeeding seems to protect against later obesity. Early antibiotic use, rapid weight gain after a small birth weight, screen time while eating, sitting most of the day, using food as a reward, and some chemicals in plastics and cosmetics may all contribute. Obesity can also bring on earlier puberty in girls.
• Adulthood. Long hours of sitting, disturbed sleep and night shifts, stress and emotional eating all play a part. Some hormone conditions cause weight gain, but he points out that a mildly raised TSH rarely explains much weight gain, whereas steroid medicines often do.
• Later life. Losing muscle while gaining fat, called sarcopenic obesity, is a particular concern, and strength exercise helps at any age.

Read more about whether thyroid problems cause weight gain and what obesity is.