Alkaline phosphatase (ALP) is on enzyme that is produced mostly by the liver and bones.  There are other organs in an adult that produce alkaline phosphatase including the kidneys and the small bowel. Placental of a pregnant female also produces some alkaline phosphatase. The amount of alkaline phosphatase produced by the liver is more than the combined level of alkaline phosphatase by all other sources in an adult.

There are laboratory studies that can distinguish between 2 primary sources of the ALP.  The two “isoenzymes” are bone ALP and liver ALP.

The presence of an elevated level alkaline phosphatase is significant in post weight loss surgical patients because of its relationship to calcium and vitamin D absorption.  When a patient has inadequate calcium and vitamin D absorption (or intake) this will result in elevation of the parathyroid hormone (PTH).   Elevated level of parathyroid hormone will the cause, increased bone breakdown, increased absorption of calcium from the GI tract, increased resorption of the calcium from the kidneys.  All of these measures are to normalize the level of the calcium in the blood.  One of the byproducts of bone breakdown is alkaline phosphatase. 

When the patient has an elevated ALP consideration should be given to liver sources including biliary obstruction.  In post duodenal switch operation this can only be studied by a magnetic resonance cholangiopancreatography (MRCP.)  Patients with a duodenal switch operation cannot have an endoscopic retrograde cholangiopancreatography (ERCP.) Needless to say broader range of liver function tests should also be evaluated.

Parathyroid hormone level, vitamin D 25-OH level and calcium level will also be needed in order to evaluate the possible cause off calcium malabsorption as the underlying reason for elevated alkaline phosphatase by the mechanism described above.  A typical patient may have an elevated parathyroid level, low calcium and low vitamin D level.