Albumin: what the test shows and why it matters for athletes

Albumin is the most abundant protein in blood plasma and one of the basic indicators of a biochemical analysis. Athletes often perceive it as an «indicator of whether there is enough protein in the diet», but in reality albumin says much more about the liver, kidneys, water balance and inflammation than about the amount of protein on the plate. The editorial team explains what the test measures and how to interpret it correctly.

What albumin is

Albumin is a small water-soluble protein synthesized exclusively by the liver. It makes up the greater part of all the proteins in blood plasma — approximately half or a little more of the total protein. The rest is accounted for by globulins, in particular immunoglobulins, and fibrinogen.

Every day the liver of a healthy person produces several grams of albumin per kilogram of liver mass, and the protein itself lives in the body for several weeks: the half-life is usually estimated at approximately 3 weeks. Because of this the albumin concentration responds to changes slowly, and an acute decrease in synthesis is not immediately visible on the test.

Only part of the body's albumin is in the vessels. The greater share is in the intercellular space of the skin, muscles and other tissues and is constantly exchanged with the blood through the walls of the capillaries. This exchange depends on the permeability of the vessels, which changes with inflammation, injury, physical exertion.

The reference ranges of serum albumin for adults in most laboratories are approximately 35–50 g/L. The exact values depend on the method: common dye methods (with bromcresol green or purple) can give somewhat different results, so tests from different laboratories should be compared with caution.

The main functions of albumin

Albumin is often called a «transport and water-holding» protein. Its properties are described in detail in reviews by Fanali and co-authors (2012) and Levitt and Levitt (2016). These functions explain why a decrease in albumin can have consequences for the whole body.

For athletes the role of albumin in the transport of fatty acids is especially interesting. During prolonged aerobic work fat stores release fatty acids, and it is precisely albumin that delivers them to the working muscles. Without it, fats that are insoluble in water could not be transported efficiently by the blood.

The connection with calcium has direct practical significance. If albumin is reduced, the total calcium on the test also looks low, although the active ionized calcium may be normal. Therefore doctors calculate «corrected» calcium or directly measure ionized calcium.

Similarly, albumin binds part of the testosterone and other hormones. Some formulas for calculating «free» testosterone use the albumin value, so an error in it can also distort the hormonal profile.

Albumin and training

Regular endurance training increases the volume of blood plasma, and albumin plays a key role in this. A review by Convertino (1991) described that the expansion of plasma in the first weeks of training is to a large extent associated with an increase in the total amount of albumin in the vascular bed: the protein holds additional water.

An interesting consequence: since the plasma volume increases, the albumin concentration on the test of a trained person may remain the usual one or even be slightly lower, although the total amount of protein in the vessels has increased. This is physiological dilution, similar to «sports pseudoanemia».

Acute exertion acts in the opposite way. During intense or prolonged work part of the plasma leaves the vessels into the tissues and is lost with sweat, and the albumin concentration temporarily rises. In observations of marathon runners after the finish, an increase in the concentrations of many protein indicators has been described (Kratz et al., 2002).

Thus, a single albumin value in an athlete should always be considered in context: was the person dehydrated, did they train the day before, in what phase of preparation are they. Without this it is easy to mistake physiological changes for pathology.

Why the test matters for athletes

The most common mistake is to consider albumin a marker of the adequacy of protein in the diet. The ASPEN position stand (Evans et al., 2021) directly emphasizes that albumin and prealbumin should not be used as indicators of nutritional status: they are far more strongly affected by inflammation, hydration and the work of the liver and kidneys than by protein intake.

A person who undereats protein for weeks may have an entirely normal albumin, because the body conserves it and redistributes it between tissues and the blood. And conversely, with an acute infection or injury albumin decreases even with impeccable nutrition — the liver «switches over» to synthesizing acute-phase proteins.

Instead, albumin is useful as part of a basic biochemical screening. Together with other indicators it helps to assess:

• liver function— a persistent decrease in albumin synthesis occurs in chronic liver diseases;
• kidney function— loss of albumin in the urine is an early sign of kidney damage;
• hydration status— an increase together with other indicators points to thickening of the blood;
• the presence of inflammation— a decrease together with an increase in C-reactive protein;
• the correctness of other indicators— of calcium, free testosterone, some drugs.

For athletes who take drugs with potential hepatotoxicity, in particular oral anabolic steroids, albumin is one of the indicators of the synthetic function of the liver. However, it changes late, so for early assessment doctors look first of all at transaminases, bilirubin and GGT.

How to take the test and what affects the result

Albumin is a relatively stable indicator, but it is noticeably affected by body position, hydration and the technique of drawing blood. Following simple rules makes the results comparable over time.

1. Give blood in the morning on an empty stomach, after a day without intense training.
2. Drink water as usual; avoid both dehydration and drinking a large volume of fluid immediately before the test.
3. Sit for 10–15 minutes before blood is drawn: in an upright position part of the plasma moves into the tissues, and the protein concentration rises.
4. Warn the nurse if the tourniquet was applied for too long: prolonged compression of the vein raises the concentration of proteins.
5. Compare results from a single laboratory.

Taking protein shakes or protein food the day before has virtually no effect on albumin. Therefore it is impossible to «improve» the test by eating more protein before giving it — and this is another confirmation that albumin does not reflect current nutrition.

The result obtained is interpreted together with total protein, creatinine, liver indicators, a urine test and the clinical picture. A single albumin value without context rarely gives an answer to the question.

Editorial conclusions

Albumin is the main plasma protein that holds water in the vessels and transports fatty acids, hormones, calcium and drugs. It is synthesized by the liver and lives in the blood for several weeks.

In athletes the albumin concentration depends on hydration and training: acute exertion temporarily raises it, while long-term adaptation to endurance can slightly dilute it at the expense of plasma expansion.

Albumin is not a reliable marker of protein intake. Its value lies in screening the liver, kidneys and inflammation and in the correct interpretation of calcium and hormones.

We also recommend reading our materials on the causes of albumin deviations from normal, on total protein in a blood test, and on liver tests for athletes.

References

1. Levitt DG, Levitt MD. Human serum albumin homeostasis: a new look at the roles of synthesis, catabolism, renal and gastrointestinal excretion, and the clinical value of serum albumin measurements. Int J Gen Med. 2016;9:229–255.
2. Fanali G, di Masi A, Trezza V, et al. Human serum albumin: from bench to bedside. Mol Aspects Med. 2012;33(3):209–290.
3. Evans DC, Corkins MR, Malone A, et al. The use of visceral proteins as nutrition markers: an ASPEN position paper. Nutr Clin Pract. 2021;36(1):22–28.
4. Convertino VA. Blood volume: its adaptation to endurance training. Med Sci Sports Exerc. 1991;23(12):1338–1348.
5. Kratz A, Lewandrowski KB, Siegel AJ, et al. Effect of marathon running on hematologic and biochemical laboratory parameters, including cardiac markers. Am J Clin Pathol. 2002;118(6):856–863.
6. Sanchis-Gomar F, Lippi G. Physical activity – an important preanalytical variable. Biochem Med (Zagreb). 2014;24(1):68–79.
7. Rifai N, Horvath AR, Wittwer CT (eds). Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier.