I wouldnt say that thier is a best food to give racers, i wood say as long as u give them a balanced meal with low fat u dont want them getting to heavey.
I give mine a mix of raw meat either mince or tripe or tripe and chicken together, with veg soup and a couple of slice (brown bread) or they get rabbit or chicken or hare.
As long as u keep them at their racing weight , daily exercisse.