Is it posible to avoid weight gain just through a very healthy diet?

Yes: It is possible with avoiding non- complex carbs, takes at least of 64 ounces of water per day, limit your calorie count to not more than your daily calorie expenditure and increasing your fiber intake. There are multiple ways to control your wt, but be careful of extreme calorie intake or starvation diets.

Yes: Although most recommend both a very healthy diet as well as exercise to avoid weight gain more recent research is showing that diet is more important than exercise in maintaining body weight.