Alkaline phosphatase in athletes

The assay measures the total activity of alkaline phosphatase in blood, in units per litre (u/l). Different tissues each make their own variant of the enzyme, an isoenzyme, and the standard assay adds those variants together without separating them. Some reports use the Dutch abbreviation AF, others the international ALP; it is the same test either way.

Two variants matter to an athlete. The bone isoenzyme comes from osteoblasts, the cells that build new bone. The liver isoenzyme comes from the lining of the small bile ducts. A single alkaline phosphatase value does not show which of the two has risen, and that is where the confusion on a training report begins.

On such a report raised values of AST, CK and alkaline phosphatase sometimes sit side by side, and that looks like one story. It is not. AST and CK leak from loaded muscle fibres and respond strongly and quickly to heavy training. Alkaline phosphatase does not, because muscle tissue does not make this enzyme; at most it responds indirectly, through faster bone turnover. Gamma-GT, the fourth enzyme on such a report, comes from liver and bile ducts and does not respond to training at all.

The reference bands come from NUMBER, the harmonisation project of the Dutch Society for Clinical Chemistry. For men aged 19 to 50 the band runs from 45 to 128 u/l, for women in that group from 38 to 123 u/l. The limits on your own laboratory report remain decisive.

A raised alkaline phosphatase in a trained athlete asks exactly one question: did this come from bone or from the bile ducts? You answer it with gamma-GT, because that enzyme sits in the liver and not in bone. If gamma-GT stays normal while alkaline phosphatase is raised, the rise probably comes from bone tissue.

In an athlete that is often well explained. A healing stress fracture raises the value for weeks, because the osteoblasts are then working at full capacity; the same is true after an ordinary fracture. There is also a slower route, and it is less innocent: a low vitamin D speeds up bone turnover and with it lifts this enzyme. That is a real pattern in athletes who train indoors or see little daylight. A long-standing energy intake that is too low for a high training volume disturbs bone metabolism in the same quiet way.

The conclusion this page wants to prevent is the reflex to schedule a rest week. A deload lowers CK, because CK comes from muscle. On a bone-driven alkaline phosphatase, rest does not have that effect: you change nothing about the number, and the real question stays open.

If gamma-GT does rise alongside, the conversation shifts to bile flow. Read the result then together with AST and direct bilirubin, because with obstructed bile flow that bilirubin rises too.

Alkaline phosphatase is a standard part of a liver panel, so most athletes meet the value without having asked for it. Measuring it deliberately makes sense in three situations: after a stress fracture or an ordinary fracture, to follow recovery; with persistent bone or shin pain that does not fit a known injury; and with a known low vitamin D, where bone turnover may already be raised.

Always request the test together with gamma-GT. Without that second enzyme a raised result cannot be placed, and the measurement then yields nothing usable.

For this enzyme itself a hard session is no problem, because muscle is not a source. If the test is done alongside CK and AST it is: those two rise sharply after loading, so schedule the draw 48 hours after your last hard session.

Fasting is usually not required for this test, but it helps. In people with blood group O or B the intestinal form of the enzyme rises for some hours after a fatty meal, and that can push a borderline value just over the line.