Low fat, it's not, but we've have had some success using lower fat ingredients such as milk and sour cream, though we wouldn't bother with nonfat sour cream. You can also used less cheddar than this recipe calls for...or double everything except the sour cream and cheddar.
2 cups uncooked elbow macaroni
1/2 c. butter
1/2 c. flour
1-1/2 c. milk
1-1/2 c. sour cream
1 t. salt
1/2 T. pepper
10 oz. bar mild Cheddar cheese, grated
Preheat oven to 350°F.
Cook macaroni in salted boiling water according to package directions. Drain and rinse with cold water. Pour into a 3-qt. casserole.
In a saucepan, melt butter and stir in flour. Gradually stir in milk and sour cream. Add salt and pepper.
Cook over low heat, stirring constantly, until sauce bubbles and thickens.
Reserve 1 c. grated cheese for the top of the casserole. Toss macaroni with remaining cheese.
Pour sauce over macaroni and mix thoroughly. Sprinkle with reserved cheese.
Bake for 1 hour, or until bubbly and brown.