Nutrition is very tricky. The best place to start is to look at the Westin Price foundation westinprice.org and read two books Nutrition and Physical degeneration by Wesin A. Price and Nourishing traditions by Sally Fallon.  One reason it is so tricky is when you tell an athlete to eat differently you potentially change or challenge the culture of the family/sport as food is so parallel with culture. And this is one reason why it is so hard to get people to eat differently because you are asking them to go counter culture. I have many talks that I have given about food over the past years. As a starting point for food and athletic performance is to talk first about getting the junk out of diets, yes poutine has got to go as a pre or post food to athletic performance. Eat  Whole Foods, i.e. meat, potatoes, rice, vegetables, fruit, etc. if it looks like something that was once alive then eat it. Avoid highly processed food, fast food (i.e. Macdonalds, KFC, A&W, Burger King), pre made, pre packaged meals i.e. kraft dinner, pop, juice (as much sugar as pop), energy drinks (too much sugar and caffeine) the whole chip isle, white bread, some people need to avoid all bread and grains (research gluten and phytic acid). Milk and many milk products are going to be difficult for many to digest as well.

When you look at nutrition from an active for life stand point many young athletes can maintain weight/health when eating lots of junk food because they are growing and very active they can metabolize just about anything. As kids get into their later teens and become adults this is very challenging especially if they stop being as active and have not changed their junk filled diet. It is at this time that their weight increases dramatically and their health will decline. If they are eating a healthy diet of complex whole foods it will be very unlikely that they gain a lot of weight as there is not the calorie density in whole foods. 1 litre of coke and a large bag of chips is 2000 calories and very little nutrition. 

The next steps is going to be understanding how to use a glycemic index (http://www.glycemicindex.com/foodSearch.php) knowing that high glycemic (sugar) foods should only be eaten directly after physical activity. Understand the difference between good fats(westin price has good info on this) omega 3, 6 , 9, medium chain fatty acids, saturated fat for cooking and where to get this (i.e. coconut oil, fermented cod liver oil, butter (raw is best), ghee, olive oil (not heated), avocados, animals fats) and bad fat (high processed high heated vegetable oils i.e. soy, canola), trans fats (and how they are made). 

If there are athletes with low energy, have extra weight, getting sick a lot, overly emotional diet is likely playing a role. It would be very interesting to have a food sensitivity test done as they are likely eating foods that they are sensitive to but not allergic. This is irritating their digestion and making it difficult for them to stay healthy and balanced.