Lowering the Calorie Count of Sauces by bookseller

In terms of sauces, adding a little cornstarch in a slurry (i.e., dissolved in liquid, typically in a 1:3 ratio) to ygurt, even lower-fat yogurt, will keep the yogurt from breaking and allow for a creamy sauce. If you, for example, braise some chicken with onions and greens, remove the chicken, boil down the liquid and veg to concentrate the flavors and evaporate most of the liquid, puree the veg and the remaining bit of braising juices, return this puree to the pot, stir in the yogurt slurry, simmer, return the chicken to the pot for a few minutes, and dish it up, maybe with some chopped dill or capers or lemon zest, something that will go well with the yogurt's mild acidity, you'd probably have a very nice dish that would taste much more calorically expensive than it really was.

I also find that Greek yogurt, or strained yogurt, makes a very good substitute for either sour cream or cream cheese, depending on the thickness. However, it can be a bit tricky, because the full-fat stuff doesn't save you a lot of calories, but the reduced-fat stuff can be REALLY sour once it's strained, so you'd need to experiment with brands and levels of concentration and also your own taste for sour.

I used to make a lower-fat maynnaise substitute, using roasted onions and tofu and fresh thyme, that I liked quite a lot; I think the recipe is in the Otters' recipe log. In fact, tofu can "creamify" things (like soup, especially, and desserts -- makes a very convincing mousse) very nicely. There will be lots of recipes online, and you might be interested in Deborah Madison's I Can't Believe It's Tofu. She's an extremely well respected vegetarian chef, was the chef at Chez Panisse before branching out on her own.

Worth knowing, btw, that almond milk (widely available here, not sure about where you are, though it's easy enough to make yourself) makes for a very pleasant bechamel, substituting for milk or cream, and certainly lower in carbs than the one and in calories than the other. You do need the flour to thicken it, though; on its own it has about the consistency of skim milk, so not much use as a sauce.

Finally -- and I think this is the last trick I know -- in southern Italy, pasta is often eaten with a topping of toasted coarse breadcrumbs, rather than cheese. Partly this is because that part of Italy has historically been desperately poor, and bread is cheaper than cheese but it performs some of the same functions; it helps drink up a bit of the sauce and thus thickens it and helps it cling to the pasta. Bread is also a caloric bargain over cheese; if you make pasta this might be an easy and tasty switch.