Just to say that I've been ploughing through cookbooks and home economics books from the early part of the 20th century over the last year for my housekeeping blog, and working out the costs and calories involved in these diets, heavy as they were in saturated fat and meat.
Most of us simple could not afford to eat as much meat as the average working family put away (you'd end up spending £70-£80 a week on meat and fish alone for a family of four), and we could not afford to home grow as much produce as many families did - we rely on mass produced fruit and veg which works out a lot cheaper, but which are probably lower in nutritional values. The calorific values of their diets were much greater than ours, as the meals had a heavy emphasis on animal fats like suet, lard, whole milk, and carbohydrates.
However people's average weights were lower, and the only reason for this as far as I can see is the amount of walking they did, and the absence of TV, which meant they engaged in a lot more low level exercise throughout the day instead of slumping on the sofa for hours on end like many of us do. There was less snacking as well, which may have meant that blood sugar and leptin levels were controlled differently.
They were also shorter and children matured later, probably because of illness in early childhood and in some cases a poor quality diet deficient in calcium and other vital minerals in the case of poor households.
So taxing fat seems pointless - it would make more sense to focus on increasing low level exercise for the whole population. However taxing things actively raises money for the government, which makes me suspicious about the motives here, given there is no evidence that simply avoiding fat makes you slim (which it doesn't - if only it were that simple!)