Author: Professor Walter J. Veith, PhD
Summary: Intolerance to products containing lactose is widespread. Could this be an indicator that dairy products are not meant for human consumption?
Milk and dairy products are advertised as wonder foods that will supply all the nutrients required for healthy growth. The calcium levels in milk, in particular, are stressed as an essential component of the human diet, and the impression is created that a loss of this dietary source of calcium will lead to abnormal bone development.
It is certainly true that dairy products are packed with nutrients, but this does not mean that the combination of nutrients is suited to human nutrition.
Mother's milk is essential for infants, but then infants are specially designed to cope with this growth-promoting food. Prior to weaning, the necessary enzyme systems needed for the digestion and assimilation of milk components are active, but they are progressively deactivated with age. The milk of other mammals also differs in composition from human milk, and this, together with the potential danger from ingested antigens, makes cow's milk unsuitable for human consumption.
There is considerable resistance from industry, and even from the established scientific world, to the idea that dairy consumption is detrimental to health, but the evidence from recent scientific findings seems fairly conclusive with regard to this issue. Dairy consumption is being coupled with a host of other diseases, and as consumption rises worldwide, so the evidence is becoming more and more conclusive.
In the past, the detrimental effect of the consumption of dairy products may have been masked by the positive effects of other lifestyle choices, such as higher consumption of grains, fresh fruits, and vegetables with their high fibre content. Western diets have, however, increased in the consumption of animal products, including dairy products, with a corresponding decline in the consumption of grains, legumes, fruits and vegetables, and this may explain the increase in the incidence of degenerative diseases in industrialized countries.
Lactose, the sugar in milk, is broken down in the intestines by the enzyme lactase. Most people are able to digest lactose properly during infancy and early childhood, but as they grow older this ability declines. Approximately 75% of adults worldwide are lactose intolerant.
Native Americans and Asians have the highest intolerance, and only slightly lower than these are the Blacks, Jews, Hispanics, and southern Europeans. Lactose intolerance is lowest among northern Europeans and their descendants. In the US some 25% of Caucasians, 51% of Hispanics, and 75% of all African Americans have insufficient levels of lactase to digest dairy products, whereas 90% of Asian Americans are lactose intolerant.i,ii
It has been found that 90% of African people are lactase deficient, and in the case of the rural Zulu of South Africa it was found that they showed no change in blood glucose concentrations after ingesting 50 g of lactose.iii When milk and dairy products are digested, lactose is broken down by the enzyme lactase into glucose and galactose.
The presence of lactose is a feature of mammalian milk, but the concentration of this sugar is normally geared to the needs of the species, as are the concentrations of all the other components of milk. Human milk does not only contain the essential nutrients that are required for growth and development, but also contains the bacterium bacillus bifidus that assists in the digestion of lactose.
In cow's milk, however, the bacterial composition differs from that of human milk. If cow's milk is fed to infants, this can interfere with the digestion of lactose. The lower protein and higher carbohydrate content of human milk is also more suited to the needs of infants, because their growth rates are considerably lower than those of calves.
After the conversion of lactose to glucose and galactose, the available galactose is not used as such, but is converted to glucose in the liver by a series of steps requiring the initial presence of the enzyme galactokinase. The production of both lactase and galactokinase declines with age, and the capacity to digest and utilize the products of lactose in adult life is thus curtailed. A deficiency in the enzyme lactase will result in fermentation of lactose by intestinal bacteria, which can result in abdominal distress such as gas, cramping, bloating, stomach rumbling, altered bowel habits, and diarrhea.iii The severity of the symptoms depends on the quantity of lactose consumed and the level of intolerance.
A further problem with milk is encountered in the digestion of the milk protein casein. In comparison to human milk, cow's milk contains 300% more casein and more than double the amount of total protein. Casein and beta-lactoglobulin are the two main proteins in milk and they are unique in that they contain a perfect blend of amino acids, which is precisely what is needed during early infant growth. Human infants, however, double their mass on average 180 days after birth, whereas cows achieve the same feat in only 47 days. Cow's milk is therefore geared to meet the rapid growth requirements of cows, but is not suitable for humans.
Casein also naturally stimulates thyroid function in infants, and as the thyroid is involved in many developmental processes, including the development of the nervous system, casein from other mammals could have adverse effects on metabolic processes of infants particularly since a portion of the dietary casein can be absorbed undigested and serve as antigen.
As with lactase, the concentration of the enzyme rennin, that breaks down the casein, also declines with age in all mammals. By the time milk teeth develop it is virtually non-existent in the human digestive tract. Without renin, the digestion of casein has to be carried out by the normal proteolytic enzymes which are not as efficient in breaking down casein.
The presence of casein in the diet of mammals has also been linked to elevated cholesterol levels and various degenerative diseases such as arteriosclerosis. Rabbits fed casein developed arteriosclerosis, but the effect could be reduced if a plant protein source, such as soybean flour was introduced into the diet. This shows that the amino acid pool produced by casein probably no longer meets the requirements of weaned or adult mammals.
Casein also produced higher cholesterol levels than soy protein in a number of animal species, including rates, hamsters, guinea pigs, pigs, and monkeys. In humans, a reduction in cholesterol levels was also found if meat and dairy proteins were replaced by soy proteins.iv,v Casein also seems to have an adverse effect on insulin secretion, hormone levels, and calcium metabolism.
Does milk really provide the calcium we need for strong bones? Find out the truth.
Updated January 2009.
i Editorial, "Lactose intolerance" Lancet 338 (1987): 663-664.
ii B. Levine, "Most frequently asked questions about lactose intolerance," Nutrition Today 31 (1996).
iii S. O'Keefe et al., "Milk induced malabsorptions in malnourished African patients," American Journal of Clinical Nutrition 54 (1991):130-135.
iv K. Carroll, "Review of clinical studies on cholesterol-lowering response to soy protein," Journal of the American Dietary Association 91 (1991):820-827.
v R. Van der Meer, A. Beynen, "Species-dependent responsiveness of serum cholesterol to dietary protein" Journal of the American Oil Chemists Society 64 (1987):1172-1177.