My vegetarian diet is well belanced so I'm pretty sure I have no deficiency BUT I read that it makes sense supplementing magnesium as an athlete and that it especially makes sense when you go easy on carbs while having a lot of protein throughout the day.
My diet consists of whole grain cereals (wholemeal bread, pasta and brown rice) and I try to eat green leafy vegetables once a day as well as nuts and seeds, peas, beans and lentils. I eat tofu, drink soy milk and supplement whey powder for protein as well as iron and Omega-3 fatty acids in pill form. I do have dairy products though as well as eggs from time to time so I'm not a vegan. I'm trying not to have too many carbs because I'm a boxer and I have to weigh comparatively little compared to my height (I compete at 69kg with a height of 178cm). So my diet needs to supply me with enough energy for rigorous workout while not making me gain weight.