For a diabetic cat, the Missing Link is a big no-no because it contains molasses which will cause bgs to shoot up:
FLAXSEED, RICE BRAN, PRIMARY DRIED YEAST, BLACKSTRAP MOLASSES, FREEZE DRIED BEEF LIVER, SUNFLOWER SEED, DEHYDRATED ALFALFA, DRIED CARROT, SHARK CARTILAGE POWDER*, FREEZE DRIED FISH PROTEIN POWDER, FREEZE DRIED OYSTER POWDER, BARLEY GRASS LEAF POWDER, DRIED KELP, ZINC MONOMETHIONINE, LECITHIN, CHROMIUM YEAST, SELENIUM YEAST, NIACIN (Vitamin B3), PYRIDOXINE HYDROCHLORIDE (Vitamin B6), GARLIC POWDER, YUCCA SCHIDIGERA EXTRACT, RIBOFLAVIN (Vitamin B2), THIAMINE HYDROCHLORIDE (Vitamin B1), FOLIC ACID, AND COBALAMIN (Vitamin B12)
For constipation, you can give a little canned pumpkin or Miralax. I'm not sure what dose of Miralax a cat would get but someone on the board may know.