Is there really a difference between the two and which should be used when it's called for in your cake? I was reading through a recipe I wanted to try and it specifically called for full fat buttermilk but I only had lowfat...would the difference be noticeable? 
you can make full fat buttermilk by adding a little lemon juice or vinegar to whole milk, stores out by me only sell lowfat buttermilk and I use it all the time, can't tell a difference
I have seen where recipes call for the fullest fat buttermilk you can find. The thing is that buttermilk is naturally low in fat. Buttermilk is the liquid left over from making butter. Usually most of the fat is left in the butter.
Buttermilk that you buy in the grocer is no longer true buttermilk anyway. So if you want a full fat buttermilk, I would do what the previous poster suggested. But if you have problems with the outcome, then the first thing you should do is switch back to the regular buttermilk. Sometimes I wonder if these people who publish recipes really know what they are talking about when they write these things. It can send you on a wild goose chase for nothing. I did a lot of research to find out the truth about buttermilk. All because I read the same thing you did in a recipe once.
I think the low-fat buttermilk (the only thing you can find in most stores now) is the same buttermilk we always used when I was growing up -- it tastes the same and seems to be the same consistency. As LindaF said, buttermilk is naturally low in fat anyway, but you would never have guessed that from the name. I think labeling it low-fat was just a marketing thing.
I've been using the "low-fat" labeled version in all my recipes that call for buttermilk, and I can't tell a difference.
You can propogate your own buttermilk too - indefinitely. And by doing this you end up with a higher fat level.
Normal buttermilk is 2% fat or less. When you buy a carton next time, leave a few ounces and place in a clean big jar with severla cups of fresh whole milk. Leave out of the fridge in a coolish spot (not too hot) Withon an hour or two is will have turned into buttermilk, as the stuff we buy now is simply a lowfat milk with a CULTURE added. You are merely feeding the culture fresh milk! As soon as its thick and smells like buttermilk (give it a gentle shake), place the jar in the fridge. When you get down to the last few ounces, do the same thing again. In a FRESH jar.
Its important to keep smelling the buttermilk as you keep regenerating it each time. If the culture starts to smell a bit different, its time to throw it out and start again from scratch. If done properly though, you shouldn't need to do this.
Quote by @%username% on %date%
%body%
