For Dietetics and Nutrition, it's the Chemistry which is crucial - in the first year of your course you'll study Biochemistry, and you'll also study macronutrients such as protein, carbohydrate and fat. From a biological point of view, you'll need a good grounding in cell biology, and preferably a decent grasp of major physiological systems.
Anything is do-able, but the scale of the task shouldn't be underestimated. Added to that, you've made it more difficult by joining late. I'd see what extra support is available from your tutors (after all, they have a responsibility now having accepted you on to the course) and in the meantime really try to keep on top of the work. Also make sure that your basic maths is up to scratch, and seek help if not.