Dad and I are on a sort-of low fat diet. It's not a no-fat diet, nor deliberately selected to be real low fat; neither do we buy "Low Fat" foods. I just use care in cooking to avoid having or putting excess fat, oil and grease in the food.
I use olive oil for most all baking and cooking needs. I don't use shortening, since I can replace it with a lesser amount of oil.
In the 8 months I've been doing the cooking, I've lost maybe 5 lbs. , while Dad's dropped nearly 15. Zocor has gotten his cholesterol and triglycerides down to 'acceptable' levels for the first time ever.
I believe the trick to maintaining an acceptable weight is to avoid eating too much and to eat a diet that is balanced over time. If you eat more than you need, you will gain weight. And if your diet is not balanced over time, you will gain weight, too.
By 'balanced over time', I mean that, over one, two, three, four weeks, your diet should be balanced among all the food groups (grain, dairy, meat, vegetables, chocolate and ice cream). Well, maybe not so much chocolate and ice cream,

but you get the idea.
There is no need to have *every* meal or *every* day be perfectly balanced. But over a period of a few weeks, your food intake *should* be fairly balanced.
Fest3er