A glass of milk is trouble, but is cheese? Lactose varies enormously from one cheese to the next, and long-aged cheeses have effectively none left.
Most of the lactose leaves while the cheese is made
When milk is set and split into curds and whey, the lactose — dissolved in water — goes with the whey. What becomes cheese is the curd, the knitted protein and fat, so a large share is gone at that step already. Bacteria then eat what remains: they consume lactose and give back lactic acid, and that souring is what sets and firms the cheese. Losing lactose is not a side effect here. It is how cheese is made.
The longer the ageing, the less is left
Ageing keeps breaking down whatever lactose survived. In testing, Parmigiano Reggiano and Grana Padano came in below the limit of quantification even at the minimum mandatory age — under one milligram per 100 grams. Italy's standard for labelling a cheese naturally lactose-free is 0.1 grams per 100 grams, so those measurements sit around a hundred times below the cut-off. Long-aged hard cheeses generally belong on this side.
Aged long enough that almost no lactose remains:
Some cheeses keep a good deal of it
High-moisture cheeses that are never aged hold on to more. Ricotta, cottage cheese and cream cheese are the obvious ones, and processed cheese — the sliced kind — can carry more still, because whey powder and milk solids are sometimes added in the making. When someone says they tried cheese and it was no better than milk, this is usually what they tried. The word cheese covers both ends, and on lactose they sit at opposite ones.
These retain comparatively more lactose:
Ageing matters more than the animal
People often report that sheep or goat cheese sits better with them. Smaller fat globules and a slightly different protein makeup are the usual explanations, but the lactose has not gone anywhere. If lactose is the problem, how long it aged tells you far more than which animal it came from: a long-aged cheese of any milk retains almost none. If cow's milk itself is what bothers you, that is when sheep and goat are worth trying.
A milk allergy is an entirely different thing
The two get confused constantly. Lactose intolerance is a shortage of the enzyme that digests lactose, and it produces mainly digestive symptoms — pain, bloating, wind. A milk allergy is an immune response to milk protein, and the NHS warns that food allergies can cause difficulty breathing and swelling of the lips and face, and can be life threatening. Protein survives ageing, so a long-aged cheese is not made safe by its age. If you have a milk allergy, none of this article applies to you. Follow your clinician's diagnosis and instructions.
Where to start
The NIDDK, part of the US National Institutes of Health, suggests that people with lactose intolerance try yogurt and hard cheeses such as cheddar or Swiss, which are lower in lactose than other milk products. The approach is to start with a small amount rather than a large one and build up gradually to whatever you can manage. Tolerance varies a great deal between people, so begin with a little well-aged hard cheese and find your own line. Eating it alongside other food is usually easier than on an empty stomach.