Traditionally butter shouldn't contain lactose. The definitive ingredient is butterfat, and in practice any remaining lactose would have been fermented rendering the content negligible. They had little use for uncultured butter that spoiled quickly. Modern industrial practice allows for a small amount of lactose to be preserved in the water typically remain in butter products. This is small enough to be tolerable by most people, however the most sensitive may prefer clarified butter (almost pure butterfat) or artificially cultured butter. Or they could avoid it altogether.
It may have some trace amounts; however, both lactose and casein make me quite sick and I still cook with lots of butter and have heavy-cream lattes almost every day. There's not enough in there to make me feel it (and I did the typical remove it for 30 days and reintroduce to see if it's a problem thing). So I'd say if you're fine with it, then it's a great source of fat (and flavor), but if it does make you sick, there's lots of other tasty sources of fat out there, so you're not missing much.
Approaching from the other direction: lactose is water soluble (ie. more lactose in skim milk than heavy cream), therefore there won't be any lactose in the butter fat, BUT there will be some (small quantities) in the milk solids (generally not enough for most lactose intolerant). Clarifying butter (or making ghee) is the easiest way to remove what's left and has the advantages of cooking at higher temperatures and storing longer.
@balor123, you probably already have what you need but I'll add this:
Those with messed-up gut flora/metabolism who have trouble with dairy will probably react to butter--I know I did. For quite a while I had to make ghee.
Now, with healthier metabolism and a much happier gut I handle butter (and even home-made full-fat yogurt) just fine although butter is an occasional ingredient. I'm a major user of whatever fat's on/in the meat or my tub of beef tallow.
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