More information about your presentation would be helpful, but based on what youve given Id say start of with something like,
Im about to present about a very sensetive, very real and present health issue affecting many people around the world. Obesity. Almost 30% of people in the world is obese. In my presentation I will be talking about the causes and effects of obesity and what it truly means to be overweight.
(This is a vague beginning since I dont have much to work on, add detail based of what you are explaining in your presentation about obesity)
