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Alkaline phosphatase (AP)

A composite value derived from bile ducts, bones, the gut and the placenta. Only when considered alongside gamma-GT does it indicate the cause of the increase. The common assumption that an elevated level indicates a vitamin D deficiency does not hold water, and hardly anyone looks at the lower values, even though there is information there.

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What is being measured

Alkaline phosphatase is not a single substance, but a group of related enzymes. They are located in the cells of the bile ducts, in the bones, in the intestines and in the placenta. The standard finding measures the sum, and therefore an increased value by itself only says that there is more of it somewhere.

The task in the bone is known. The form found there locally increases the inorganic phosphate and breaks down pyrophosphate, an inhibitor of mineralization. Both together are involved in the storage of minerals in the bone matrix. An increase in the bone indicates increased remodeling; He doesn't say whether it's normal or pathological.

Reference range

  • Men around 40 to 130 U/l
  • Women around 35 to 105 U/l
  • Significantly higher in growing adolescents, and that is normal

Conventions of laboratory medicine, not study findings. They differ depending on the measurement method, the area depends on your own findings.

Where the value comes from

In adults, two serious sources come into question: the bile ducts and the bone. Keeping them apart is the real task, and a second value based on the same finding helps enough in everyday life.

The trick is this Gamma-GT. It is located in the bile duct cells and not in the bone. A study of AP subtypes in liver disease found a high degree of concordance between the biliary tract subtype and gamma GT. So where both rise together, it points to the bile ducts; If AP alone increases, a source outside the liver becomes more likely. A proof is neither of these, and where it matters, the sub-forms can be specifically determined.

  • AP high, Gamma GT high: speaks for the bile ducts. The guidelines for the investigation of abnormal liver values ​​cite a disproportionately increased AP in relation to AST and ALT as the definition of cholestatic damage.
  • AP high, Gamma GT normal: makes a source outside the liver more likely, usually the bone. This has not been proven, and the intestine and placenta are also possible.
  • AP normal, Gamma GT high: atypical for cholestasis. This does not exclude it, see the Gamma GT page.
  • Both normal: can even then OLD and AST be increased, namely from the muscle.

What increases the value

The bile ducts

Anabolic androgenic steroids can cause cholestatic liver injury. In the series of 44 young men who developed liver injury after bodybuilding supplements, all had clinical signs of such injury; the median time to injury was 73 days, 71 percent were hospitalized, and no one died or needed a new liver; The paper does not report individual AP values. In addition to cholestasis, a review article mentions peliosis hepatis as well as benign and malignant liver tumors and notes that liver function usually normalizes after discontinuation with supportive treatment, but individual long-term effects remain. The connection to the overall picture is below Liver and Bilirubin.

The bone

Increased bone remodeling can raise the shape of the bone, for example during growth or in the event of a healing fracture; The cause behind this cannot be seen from the alkaline phosphatase alone. The fact that the level of the value is related to the condition of the bones is shown by an evaluation of 6,333 people, in which an index formed from AP and creatinine was related to the risk of reduced bone density, with an odds ratio of 2.09 in the top compared to the bottom group.

And sometimes a medication

An experiment with rifampicin shows that an increased value does not automatically mean liver or bone disease. 600 milligrams daily for one week significantly increased alkaline phosphatase in the plasma compared to placebo, particularly bone shape. Rifampicin binds the nuclear receptor PXR; How the induction of alkaline phosphatase comes about is unclear.

The vitamin D reflex doesn't last

The common rule of thumb is: AP alone is increased, i.e. vitamin D deficiency. This was tested on 110 people at an orthopedic consultation with measured deficiency, 26 of whom were mild, 61 moderate and 21 severe. In all three groups, alkaline phosphatase was within the normal range and there was no relationship between it and vitamin D levels (r = 0.05). The work concludes that alkaline phosphatase is not suitable as a screening test for vitamin D deficiency.

This is a cross-sectional study and all values ​​were within the normal range, which mathematically reduces the connection. It does not refute the fact that a very advanced deficiency with softening of the bones increases alkaline phosphatase. It refutes the reverse conclusion, which is what is drawn in everyday life: from an increased value to a vitamin D deficiency, or from a normal value to a sufficient one Vitamin D status. This investigation does not support either of these.

A low value is also a finding

Hardly anyone looks down, and that's a mistake. Among 865 consecutive adults at a bone-specializing clinic, 56 had persistently low alkaline phosphatase, or 6.5 percent. Two carried variants in both copies of the responsible gene, 29 others had variants in one copy. A model consisting of AP below 35 U/L as well as elevated urinary phosphoethanolamine and elevated plasma pyrophosphate showed high diagnostic performance.

The second observation is more interesting for strength athletes. In a study of 68 people, low muscle strength was significantly more common in those affected than in comparison people, 30 percent compared to 6 percent, and handgrip strength was lower. Regardless of the diagnosis, low alkaline phosphatase activity was associated with a higher risk of low muscle strength.

A cross-sectional study with 68 participants, and the authors specifically note that the causal role of alkaline phosphatase in muscle strength remains an open question. What the finding bears is attention: a value below the reference range is no reason for relief.

However, alkaline phosphatase alone is not enough for the diagnosis. In a study of 69 people with hypophosphatasia or antiresorptive-treated osteoporosis, the bone remodeling markers CTX and P1NP as well as four ratios formed from them were superior to alkaline phosphatase alone in this distinction; P1NP achieved an area under the curve of 0.962 and the ratio of AP to P1NP reached 0.964. The control group there only consisted of women.

What this means in strength training

  • The training did not move her in the present study. In a study of 15 healthy men accustomed to moderate exercise without resistance training, AST, ALT, LD, CK, and myoglobin increased significantly after one hour of weight lifting and remained elevated for at least seven days. Bilirubin, gamma-GT and alkaline phosphatase remained within normal limits. Muscle damage is greater in those who are unfamiliar than in those who are trained, which rather supports the null finding for these three values; However, it is not included for other forms of training and durations of exertion.
  • The value cannot be interpreted without gamma GT. The two belong to the same diagnosis, otherwise it remains unclear whether the sender is the bile duct or the bone.
  • A baseline value before the start saves the guesswork. If you had 118 U/l in the spring, you don't have to calculate if it's 124 in the fall to see if that means anything.
  • In terms of growth, the AP is higher. In a young adult who is still growing, an elevated value is therefore not necessarily a liver sign; You still have to classify it together with the Gamma GT.
  • A low value is considered. It occurs more frequently in the cohort mentioned than expected and was associated with lower muscle strength.

Classification

Alkaline phosphatase is a collective value, and that is not a weakness, but rather its peculiarity. It doesn't answer a question on its own, and in conjunction with Gamma GT it answers a very useful one: is the increase coming from the bile ducts or from the bone. So she belongs next to it Bilirubin and gamma-GT to the values that remained unaffected by muscle strain in the study mentioned, while AST and ALT temporarily increased from the muscle there. Elevated transaminases should therefore be read together with CK, the course and the other findings and not on their own.

It cannot withstand two common short circuits. One is the jump from an elevated level to a vitamin D deficiency, for which the present study finds no connection. The other is the assumption that there is nothing to see below.

It remains unclear what a low value means for a healthy strength athlete. The present works come from bone consultations, not from sport, and they are cross-sections. They have not investigated whether low alkaline phosphatase can be influenced and whether this changes muscle strength.

Sources

  1. Vimalraj S. Alkaline phosphatase: Structure, expression and its function in bone mineralization. Gene. 2020. PubMed
  2. Kwo PY et al. ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. Am J Gastroenterol. 2017. PubMed
  3. Burlina A et al. Studies on alkaline phosphatase isoenzymes in hepatic diseases. Relation to gamma-glutamyltransferase. Clin Chim Acta. 1978. PubMed
  4. Stolz A et al. Severe and protracted cholestasis in 44 young men taking bodybuilding supplements: assessment of genetic, clinical and chemical risk factors. Aliment Pharmacol Ther. 2019. PubMed
  5. Petrovic A et al. Anabolic androgenic steroid-induced liver injury: An update. World J Gastroenterol. 2022. PubMed
  6. Nabil H et al. Rifampicin induces the bone form of alkaline phosphatase in humans. Basic Clin Pharmacol Toxicol. 2022. PubMed
  7. Shaheen S et al. Serum alkaline phosphatase screening for vitamin D deficiency states. J Coll Physicians Surg Pak. 2012. PubMed
  8. Hidaka N et al. Integrated Genetic and Biochemical Approach to Patients with Bone Disorders Exhibiting Low Alkaline Phosphatase. Calcif Tissue Int. 2026. PubMed
  9. Andreo-López MC et al. Alkaline Phosphatase as a Potential Biomarker of Muscle Function: A Pilot Study in Patients with Hypophosphatasia. Int J Mol Sci. 2025. PubMed
  10. Bertoldo F et al. Possible role of bone turnover markers in the diagnosis of adult hypophosphatasia. J Bone Miner Res. 2024. PubMed
  11. Cai Z et al. Serum alkaline phosphatase/creatinine ratio serving as a screening tool for low lumbar bone mineral density in patients with degenerative lumbar scoliosis. J Orthop Surg Res. 2025. PubMed
  12. Pettersson J et al. Muscular exercise can cause highly pathological liver function tests in healthy men. Br J Clin Pharmacol. 2008. PubMed

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