It's a guinea pig!
There are a lot of nutritionists and trainers that have said that if you don't do the whole food diary, counting calories and macros, you will never fully understand dieting (as in a healthy diet) and nutrition.
Every time I have gone through this process it has been a major learning process.
There are a number of really interesting programs on British telly right now about diet and obesity.
One interesting thing that I remember being said is that all it takes is a 70 kcal/day excess of calories you need, and in 10 years, a chubby 18 year old will be an obese 28 year old.
It really is that simple, and you can also get fat eating really healthy foods (I have done that one as well a couple of times).