What causes weight gain and water retention?

Many things: The list is virtually endless and may be different for each person. In general however, foods high in sodium can lead to water retention as can high carbohydrate meals due to the amount of water needed to store carbohydrates as glycogen. Hormonal and even environmental changes can also affect total body water. Lastly many medications can lead to fluid retention.

Quite a lot!: Hypertension, Heart disease such as valve problems, Pregnancy, Hormone changes, Obesity, Excessive salt intake, Alcohol ingestion, Inflammatory diseases, many medications, and sedentary living / excessive sitting days. I encourage all my patients to live a healthy active life with as little medication as needed. Good luck.