Humans are the only animal with lactose intolerance. We consume milk after being weaned from our mother's.
Milk was not available to us, until after our hunter gatherer forebears stopped roaming around, and domesticated goats and cows.
Our intestines reflect this, in that lactase enzyme responsible for lactose digestion normally starts to decline during pre-school years and gradually diminishes thereafter.
There are also racial differences in lactase "deficiency". In the 1979 report by Barr et al¹, lactose malabsorption was documented in 12 of 16 black, 16 of 59 white and 4 of 5 Hispanic children. These children could not be picked on symptom scores. People of Asian background are also more frequently affected.
Some families are more affected, as well.
Symptoms also include gas and diarrhoea produced by the fermentation.
Savaiano et al² did a meta-analysis of 21 lactose versus dummy (placebo) trials, where the people didn't know what they were getting.
In these trials using 7 to 25gm of lactose, the subjects, who believed they were lactose intolerant, were unable to tell what they had received.
Cows milk has about 9gm of lactose per 200ml standard cup, so you will need to have more than that to test yourself.
Symptoms of lactose malabsorption are non-specific, so any test will need to be repeated 2 or 3 times, to be sure it wasn't just something else. Temporary lactase deficiency sometimes follows gastroenteritis in children.
Milk allergy (or intolerance) is a different condition, due to a reaction to the protein content and capable of causing any symptom in the book.
If you suspect you are allergic to milk, consider doing the 5 day elimination test described on the
irritable bowel syndrome page.
Hydrogen breath tests can detect increased levels of hydrogen gas produced by fermentation of lactose in the intestines and excreted through the lungs.
lactose tolerance test, which measures blood levels of glucose after a person has had a drink containing lactose. The test with 50gm of lactose in 400ml of water, should show a rise of greater than 20mg/100ml in the blood glucose and cause no symptoms. An abnormal test causes symptoms and has a flat blood glucose response.
Stool acidity test, which measures acids found in stool when the lactose is fermented.
A small intestine biopsy can be taken at endoscopy and tested for disaccaridase enzymes, looking at lactase, maltase and sucrase. In primary lactose intolerance, only lactase will be deficient. In lactose intolerance secondary to other causes of damage to the lining of the small bowel, all 3 may be low.
Usually you can get away with having small amounts of dairy products more often.
Hard cheese is virtually lactose free, and yogurt is usually OK.
While we are on the subject (adequacy of diet) it may be a good time for you to look at the whole subject, as on the USDA's ChooseMyPlate.gov site
1. NEJM Volume 300:1449-1452 June 28, 1979 Number 26
2. J Nutr. 2006 Apr;136(4):1107-13.
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