One of the best ways to write a topic sentence is to use the question or prompt as your guide. Take the key words from the prompt and incorporate them into the topic sentence. Since I don't know the exact prompt, the words "healthy food" are in your original question and should be included. Another great thing to include in your topic sentence is to make sure it answers the questions how or why. For example,
Eating healthy foods is important because it helps maintain a good body weight and fight off diseases. The highlighted "because" phrase tells the reader "why". By including a how or why statement, it tells the reader exactly what you will be writing about in the paragraph. This also works well for thesis statements in essays.
Healthy food is necessary for an athlete because it gives him the energy he needs to perform on the field.