The reason that unsaturated fats are better is down to the molecular structure of fat. Saturated fat molecules form regular shapes that clump together easily; unsaturated fat molecules however, form irregular shapes that cannot clump together so easily. Saturated fat is therefore more likely to stick to the sides of arteries and allow other saturated fat molecules to build up; this can gradually clog the arteries leading to higher blood pressure and making it more difficult for the heart to pump oxygen rich blood around the body.