I found the follwing fact about lactose intolerance (LI) in "Milk is not
for everybody", by Steve Carper, 1995.

1) Lactose is a disaccharide (sugar) found in most mammals' milks.
Its digestion produces 2 simple sugars:

    lactose + water --> glucose + galactose

2) LI refers to symptoms like bloating, gas, diarrhea after ingestion of
lactose. It is caused by insufficient secretion of lactase. Diarrhea
is caused by the fact that indigested lactose draws the body's water
in the intestine by diffusion; gas is produced by colon bacteria which
are fed by undigested lactose.

3) Among adults, about 75% of Black Americans, 25% of White Americans
and 100% of Japanese are LI. Few Scandinavians are LI. The production
of lactose usually decreases with age.

4) It is very difficult to know if you are LI. The only reliable way
is to test for it, by drinking 50 grams of lactose in solution (in water),
on empty stomach, and watch symptoms. That quantity corresponds to about
1 litre of milk, which is unrealistic. Moreover, 1 litre of milk would
be better tolerated because the other compounds slow down the transit.

5) Most (>75%) of LI can still digest 1 glass of milk, even on empty
stomach. And even lactose malabsorbers don't necessarily have symptoms
because the intestinal flora adapts. When strains of bacteria which
digest lactose and don't produce gas develop, you don't experience
symptoms.

6) If you drink milk after having been on a lactose-free diet for some
time, you are likely to experience bad symptoms, NOT because you
produce less enzymes, but because the intestinal flora has changed.
However, 1 or 2 weeks are usually enough for the flora to readapt.

7) Some factors can improve lactose digestion:
  *drink small quantities; most people can tolerate 1 glass.
  *slowing down the transit, to allow enough time to the enzymes: drink
whole milk, after meals.
  *temperature: human lactase works best at the body's temperature, i.e.
37 degree C (98.6 F). Thus, warm milk is better digested than
refrigerator-cold milk.

8) Even with all these precautions, a small percentage of the population
still have LI symptoms.

9) Lactose is not the only problem in milk. A small percentage of the
population is intolerant to milk protein (usually casein). What
happens is that a tiny quantity of undigested proteins enter the
blood stream. Normally, the immune system eliminate them silently,
but allergics have an exaggerated immunological response.

10) Milk (in moderation, and with precautions) can however benefit
to most SADers because they would otherwise be calcium deficient.
Except milk, the most important source of calcium is green vegetables
(but gnawing fish bones can work too). Of course, getting the correct
amount of calcium is not sufficient to assimilate it, you also
need vitamin D, correct ratio Ca/P... And for women, hormonal factors
and exercising are important too.

Best wishes,

Jean-Louis
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