reproduced from Physicians Committee for Responsible Medicine -
Physicians Committee for Responsible Medicine  

(articolo in italiano)
  What is lactose intolerance?
Understanding Lactose Intolerance

Lactose intolerance is the inability to digest the milk sugar lactose, causing gastrointestinal symptoms of flatulence, bloating, cramps, and diarrhea in some individuals. This results from a shortage of the lactase enzymes which break down lactose into its simpler forms, glucose and galactose.

Virtually all infants and young children have the lactase enzymes that split lactose into glucose and galactose, which can then be absorbed into the bloodstream. Prior to the mid-1960s, most American health professionals believed that these enzymes were present in nearly all adults as well. When researchers tested various ethnic groups for their ability to digest lactose, however, their findings proved otherwise. Approximately 70 percent of African Americans, 90 percent of Asian Americans, 53 percent of Hispanic Americans, and 74 percent of Native Americans were lactose intolerant.1-4 Studies showed that a substantial reduction in lactase activity is also common among those whose ancestry is Arab, Jewish, Italian, or Greek.5

In 1988, the American Journal of Clinical Nutrition reported, "It rapidly became apparent that this pattern was the genetic norm, and that lactase activity was sustained only in a majority of adults whose origins were in Northern European or some Mediterranean populations."6 In other words, Caucasians tolerate milk sugar only because of an inherited genetic mutation.

Overall, about 75 percent of the world's population, including 25 percent of those in the U.S., lose their lactase enzymes after weaning.7 The recognition of this fact has resulted in an important change in terminology: Those who could not digest milk were once called "lactose intolerant" or "lactase deficient." They are now regarded as normal, while those adults who retain the enzymes allowing them to digest milk are called "lactase persistent."

There is no reason for people with lactose intolerance to push themselves to drink milk. Indeed, milk and other dairy products do not offer any nutrients that cannot be found in a healthier form in other foods. Surprisingly, milk-drinking does not even appear to prevent osteoporosis, it's major selling point.

Milk Does Not Reliably Prevent Osteoporosis

Milk is primarily advocated as a convenient fluid source of calcium in order to slow osteoporosis. However, like the ability to digest lactose, susceptibility to osteoporosis differs dramatically among ethnic groups, and neither milk consumption nor calcium intake in general are decisive factors with regard to bone health.

The National Health and Nutrition Examination Survey (NHANES III, 1988-1991) reported that the age-adjusted prevalence of osteoporosis was 21 percent in U.S. Caucasian women aged 50 years and older, compared with 16 percent in Hispanic Americans and 10 percent in African Americans.8

A 1992 review revealed that fracture rates differ widely among various countries and that calcium intake demonstrated no protective role at all.9 In fact, those populations with the highest calcium intakes had higher, not lower, fracture rates than those with more modest calcium intakes.

What appears to be important in bone metabolism is not calcium intake alone, but the balance. The loss of bone integrity among many postmenopausal white women probably results from genetics and from diet and lifestyle factors. Research shows that calcium losses are increased by the use of animal protein, salt, caffeine, and tobacco, and by physical inactivity.

Animal protein leaches calcium from the bones, leading to its excretion in the urine. Sodium also tends to encourage calcium to pass through the kidneys, and is even acknowledged as a contributor to urinary calcium losses in the current Dietary Guidelines for Americans.10 Smoking is yet another contributor to calcium loss. A twin study showed that long-term smokers had a 44 percent higher risk of bone fracture, compared to a non-smoking identical twin.11 Physical activity and vitamin D metabolism are also important factors in bone integrity.

The balance of these environmental factors, along with genetics, is clearly as important as calcium intake with regard to the risk of osteoporosis and fracture. For most adults, regular milk consumption can be expected to cause gastrointestinal symptoms, while providing no benefit for the bones.

Commercial Lactase Enzymes: Not the Best Choice

Lactose-reduced commercial milk products are often depicted as the "solution" to lactose intolerance. These products are enzymatically modified to cleave lactose into glucose and galactose, preventing stomach upset and other symptoms of lactose maldigestion. But even the lactase pills don't solve the problem, as individuals can still experience digestive symptoms.

Iron deficiency is more likely on a dairy-rich diet since cow's milk products are so low in iron.12 A recent study linked cow's milk consumption to chronic constipation in children.13 Epidemiological studies show a strong correlation between the use of dairy products and the incidence of insulin-dependent diabetes (Type 1 or childhood-onset).14,15 Women consuming dairy products may have higher rates of infertility and ovarian cancer than those who avoid such products.16 Susceptibility to cataracts17 and food allergies are also affected by dairy products.

Humans typically get the vitamin D needed from small amounts of daily exposure to the sun. Some foods, such as cow's milk, soymilk, and some cereals, are fortified with this nutrient. Unfortunately, samplings of milk have found significant variation in the vitamin D content, with some samplings having had as much as 500 times the indicated level, while others had little or none at all.18,19 Too much vitamin D can be toxic and may result in excess calcium levels in the blood and urine, increased aluminum absorption in the body, and calcium deposits in soft tissue.

Healthier Sources of Calcium

While the focus on calcium intake appears to have resulted from the prevalence of osteoporosis among Caucasian women (not to mention the influence of the dairy industry), this is not to say that a certain amount of dietary calcium is not needed by those in other demographic groups. However, calcium is readily available in sources other than dairy products. Green leafy vegetables such as broccoli, kale, and collards, are rich in readily absorbable calcium (Table 1).

Many green vegetables have absorption rates of more than 50 percent, compared with about 32 percent for milk. In 1994, the American Journal of Clinical Nutrition reported calcium absorption to be 52.6 percent for broccoli, 63.8 percent for Brussels sprouts, 57.8 percent for mustard greens, and 51.6 percent for turnip greens.20 The calcium absorption rate from kale is approximately 40 to 59 percent.21 Likewise, beans (e.g., pinto beans, black-eyed peas, and navy beans) and bean products, such as tofu, are rich in calcium. Also, about 36 to 38 percent of the calcium in calcium-fortified orange juice is absorbed (as reported by manufacturer's data).

Green leafy vegetables, beans, calcium-fortified soymilk, and calcium-fortified 100-percent juices are good calcium sources with advantages that dairy products lack. They are excellent sources of phytochemicals and antioxidants, while containing little fat, no cholesterol, and no animal proteins.

Table 1.
Calcium in Foods (milligrams)
Food Source Serving Calcium

Dried figs

10 figs 269 mg
Total cereal, General Mills 3/4 cup 250 mg

Calcium-fortified orange juice*

8 ounces 250 mg
Collards, frozen, boiled 1/2 cup 179 mg

Tofu, raw, firm

1/2 cup 130 mg
Vegetarian baked beans 1 cup 128 mg

Great northern beans, boiled

1 cup 120 mg
Kale, boiled 1 cup 90 mg

Raisins, golden, seedless

2/3 cup 53 mg

Broccoli, boiled

1 cup 72 mg
Brussels sprouts, boiled 1 cup 46 mg
Kale, boiled 1 cup 90 mg

Chick peas, canned

1 cup 77 mg
Kidney beans, canned 1 cup 69 mg
Source: J.A.T. Pennington, Bowes and Church's Food Values of Portions Commonly Used. (Philadelphia: J.B. Lippincott, 1998.)
*Package information

1. Cuatrecasas P, Lockwood DH, Caldwell JR. Lactase deficiency in the adult: a common occurrence. Lancet 1965;1:14-8.
2. Huang SS, Bayless TM. Milk and lactose intolerance in healthy Orientals. Science 1968;160:83-4.
3. Woteki CE, Weser E, Young EA. Lactose malabsorption in Mexican-American adults. Am J Clin Nutr 1977;30:470-5.
4. Newcomer AD, Gordon H, Thomas PJ, McGill DG. Family studies of lactase deficiency in the American Indian. Gastroenterology 1977;73:985-8.
5. Mishkin S. Dairy sensitivity, lactose malabsorption, and elimination diets in inflammatory bowel disease. Am J Clin Nutr 1997;65:564-7.
6. Scrimshaw NS, Murray EB. The acceptability of milk and milk products in populations with a high prevalence of lactose intolerance. Am J Clin Nutr 1988;48:1083-5.
7. Hertzler SR, Huynh BCL, Savaiano DA. How much lactose is low lactose? J Am Dietetic Asso 1996;96:243-6.
8. Looker AC, Johnston CC, Wahner HW, et al. Prevalence of low femoreal bone density in older U.S. women from NHANES III. J Bone and Mineral Research 1995;10:796-802.
9. Abelow BJ, Holford TR, Insogna KL. Cross-cultural association between dietary animal protein and hip fracture: a hypothesis. Calif Tissue Int 1992;50:14-8.
10. Nordin BEC, Need AG, Morris HA, Horowitz M. The nature and significance of the relationship between urinary sodium and urinary calcium in women. J Nutr 1993;123:1615-22.
11. Hopper JL, Seeman E. The bone density of female twins discordant for tobacco use. N Engl J Med 1994;330:387-92.
12. Pennington JAT. Bowes and Church's Food Values of Portions Commonly Used, 17th ed. New York: Lippincott, 1998.
13. Iacono G, Cavataio F, Montalto G, et al. Intolerance of cow's milk and chronic constipation in children. N Engl J Med 1998;339:110-4.
14. Scott FW. Cow milk and insulin-dependent diabetes mellitus: is there a relationship? Am J Clin Nutr 1990;51:489-91.
15. Karjalainen J, Martin JM, Knip M, et al. A bovine albumin peptide as a possible trigger of insulin-dependent diabetes mellitus. N Engl J Med 1992;327:302-7.
16. Cramer DW, Harlow BL, Willet WC. Galactose consumption and metabolism in relation to the risk of ovarian cancer. Lancet 1989;2:66-71.
17. Simoons FJ. A geographic approach to senile cataracts: possible links with milk consumption, lactase activity, and galactose metabolism. Digestive Disease and Sciences 1982;27:257-64.
18. Jacobus CH, Holick MF, Shao Q, et al. Hypervitaminosis D associated with drinking milk. N Engl J Med 1992;326(18):1173-7.
19. Holick MF. Vitamin D and bone health. J Nutr 1996;126(suppl);1159S-64S.
20. Weaver CM, Plawecki KL. Dietary calcium: adequacy of a vegetarian diet. Am J Clin Nutr 1994;59(suppl):1238S-41S.
21. Heaney RP, Weaver CM. Calcium absorption from kale. Am J Clin Nutr 1990;51:656-7.

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