Question: What, exactly, constitutes a 'balanced diet,' and will a balanced diet help me lose weight or stay slim?
Answer: A balanced diet contains all the essential nutrients.
The best way to get all these essential nutrients is to eat a variety of foods, and usually this means eating foods from all the different food groups; by doing that, you can get the most complex array of nutrients and the best way to improve your health.
A balanced diet will help you to manage weight if it's controlled in total calories. Calories count. For example, if somebody is eating a healthy balanced diet, but they're eating twice as much food as someone else, they won't be able to manage their weight as efficiently. So you can eat a healthy weight, it still needs to be controlled in total calories in order to best manage weight.
The best way to do this is to control portion size, watch when eating out, because you're not as in control of total calories, try and eat breakfast and eat a lot of vegetables, fruits, and whole foods as opposed to processed foods, such as sugar and refined carbohydrates.