Welcome to Elgg melly. Regarding your requirements, they need some heavy coding so that you can implment it in Elgg. But many of the required classes like BMI calculator, calorie calculator etc.. are already available free in the Web. You can integrate these classes with Elgg. To start your development with Elgg, digg in to the code itself, its the best starting point. You can also have a look at http://docs.elgg.org. If you still have problem ask your questions here and the community will help you.