I once increased my weight very fast (around 2-3kg/week), that time without doing workout, and using so called "supplement" by doctor. However, it left some stretch marks when my weight is decreased quite faster after did not consume that "supplement" anymore.
How much is the most ideal weight increment if I aim to do it again + workout this time + help of Personal Trainer and consumption of gainer + whey (I prefer whey better to avoid bloating) without leaving stretch marks in future.
You look like you are trying to gain weight for whatever reason. Most sources state that when losing or gaining weight you should aim for a 0.5kg change per week.
However, it is highly individual and you should diet plan to eat 10-15% above your Total Daily Energy Expenditure and then measure your weight change.
Make sure your breakdown of protein, fat and carbs is sensible and you are exercising regularly.
Stretch marks are a “tearing” of the layers of the skin (dermis). As you’ve already figured out, they’re usually the result of a rapid expansion of the skin surface and can have many causes. Unfortunately, since each of us is an individual, there’s no standard “rule” as to how much weight the body can add without resulting in stretch marks. Logic would, however, dictate that to avoid them you should try to gain weight gradually over an extended period so that your body can adapt and adjust to a larger skin surface.
