I get a lot of these every late winter and early spring for summer lab internships for undergrads. Tell them where you're from, and what you want to be when you grow up. Then, tell them you read a paper or four of theirs, and are interested in the approach they are taking to the study of X. Focus on the approach. Don't ask to join a lab that does obesity research and just focus on obesity as the reason you want to join. There are hundreds and hundreds of obesity research labs, why this one? Are they studying feast/famine networks in nematodes? Talk about why that seems interesting to you. But don't be so specific and tell them that you want to do exactly what they did in their latest paper, because that makes you seem rigid and naive.
Something like this:
Dr. X,
My name is X, and I go to Y university. I'm currently a sophomore majoring in neuroscience, with a goal of attending medical school starting in 2019. One of my goals over the next two years is to deepen my laboratory experience in pursuit of an MD/PhD program. I'm interested in the neural networks governing appetite and satiety, and I was very intrigued by your recent paper in Nature Neuroscience in which you identified SUR1-mediated pathways for famine resistence in the nematode. I would very much like the opportunity to briefly discuss any opportunities in obesity-related invertebrate modeling that might be available for me to work on with you and your lab. My CV is attached. Thank you for taking the time to consider my request.
Sincerely,
AlphaBeta<3