1) When I do a 3-mile or 5-mile run, I run for time and try to finish with nothing in the tank. The closer I get to the end, the more I push myself, because I know I don't need to sustain the effort much longer. When I stop running, I suddenly get very light-headed and dizzy. It never happens while I'm running. I walk briskly while I'm warming down, but that doesn't prevent it. Why does this happen? Is it dangerous?

2)  Sometimes after jogging a while, or cycling a great distance, my head begins to spin and I feel as if I'm going to pass out. Is it normal to have feelings like these, and if so, how intense should these feelings be?

Why is it I can always run a greater distance on a track than I can on a treadmill? On a track, I can run five or six miles, but on a treadmill, I can only go a maximum of three or four, then I tucker out. Is it because I get more bored on a treadmill, or is there a more physical explanation?

I'm curious to know if going to the gym increases sexual drive? I am a female in my 20s and recently started an exercise routine (running and weights), and I have noticed that my sexual drive has increased significantly. There have been no other major changes in my life, and I have been dating the same man for close to two years now, so I can think of no other real reason why my libido would have increased. Is there any real connection between working out in the gym and beneath the sheets?

What is the best way to prepare, health wise, before a big test such as the SAT, LSAT, MCAT? What foods do you recommend to keep my "brain moving?"

Does altitude affect calorie-burning? That is, if I do the exact same workout twice — once in New York City and once in Colorado — will I burn more calories at the high altitude? What things DO affect how many calories you burn in a workout?

I have heard that if you exercise/work out in the morning before you eat breakfast that it will cause your metabolism to increase throughout the day, helping you to burn calories. Is this true?

A friend of mine and I were disagreeing about this point and I thought maybe you could help. I have heard that it is not possible to both lose fat and build muscle mass simultaneously: ideally, a person should do cardiovascular exercise with dieting to lose extra body fat, and then start to focus on building up muscle mass. He says it's possible to both burn fat and build up muscle over the same period of time. Which is the case?