Chemistry Reference
In-Depth Information
6.4.1.
The Blood Glucose Test
A baseline measurement of blood glucose is taken before ingestion of a
lactose load, and then at various time intervals (usually every 30 min) for the
following 2 h. An increase in blood glucose indicates lactose digestion (lactase
cleaves the lactose molecules into glucose and galactose, allowing absorption
into the bloodstream and subsequent detection), and no increase, or a 'flat line',
is indicative of a lactose non-digester/maldigester or intolerant phenotype.
6.4.2.
The Breath Hydrogen Test
This test measures hydrogen production by colonic bacteria. If a lactose
dose meets the enzyme lactase in the small intestine, no changes in breath
hydrogen will be observed, and a diagnosis of lactose tolerance is made.
Conversely, in lactose maldigesters, the lactose load passes through the
small intestine and into the colon where it is digested by bacterial fermenta-
tion, one by-product of which is hydrogen. Some of this hydrogen is absorbed
into the bloodstream and released into the breath (where it can be detected) as
the blood passes through the lungs. A baseline measurement of breath hydro-
gen is taken, prior to ingestion of the lactose load, and further readings are
taken at 30 min intervals from the time of ingestion for the following 3 h.
In both cases, somewhat arbitrary cut-off points have to be set for
distinguishing the two phenotypes and both methods inform on the person's
ability to digest lactose rather than the given individual's lactase expression.
We infer the lactase persistence status of an individual from these tests, and
must therefore keep in mind that there will be an error in both directions.
Some of these observed errors could be attributed to test design, parti-
cularly the quantity of lactose administered. Non-persistent individuals express
a residual amount of lactase, approximately 10% of adult levels (Semenza et al.,
1999) and so when only a low-lactose dose is used the quantity passing through
to the colon may not be large enough to increase breath hydrogen by the
standard
>
20 ppm increment. Using the blood glucose method would, on the
other hand, have the opposite effect. Lactase-persistent individuals may show
an insufficient rise in blood glucose to cross the nominated threshold (usually
1.1 mmol/L). Also, some studies suggest that even lactase-persistent people fail
to digest a proportion of consumed lactose, and therefore an increase in breath
hydrogen could be observed even in persistent subjects when a high-dose
challenge (such as 50 g) is used (Bond and Levitt, 1976).
Apart from dose, many other factors can impact upon the test result; gastric
emptying and intestinal transit times could exert an effect both on blood glucose
and breath hydrogen measurements. Diarrhoeal disease is known to reduce
lactase expression temporarily as a result of villus flattening and loss of the cells
