What Is the Oxygen Delivery Score?

The capacity to carry oxygen in the blood is one of the most critical determinants of cardiorespiratory fitness and endurance. The level of hemoglobin in the blood determines how much oxygen can be carried to the working muscles, which then directly affects performance metrics like Critical Power and VO₂Max.^1,2 Iron status, B vitamin levels, and genetic differences in hemoglobin production are common causes of lower hemoglobin that can affect performance. The Oxygen Delivery Score examines critical markers of oxygen carrying capacity, and if needed triggers additional scores to explore potential causes of lowered oxygen delivery.

Keywords: Oxygen, Endurance, Anemia

Associated Biomarkers

Experienced Physiological Effects:

• Fatigue
• Underperformance
• Low cardiorespiratory fitness

Physiology Deep Dive:

During aerobic exercise in trained individuals, the primary limiter of oxygen uptake—one of the critical determinants of endurance capacity/performance—is the ability of the cardiorespiratory system to deliver oxygen to the exercising muscles.³ As a result, oxygen delivery is the primary limiting factor for VO₂max, which is the maximum rate at which one’s body can uptake, transport, and utilize oxygen during intense exercise. The oxygen carrying capacity of the blood is primarily determined by levels of hemoglobin, a protein in red blood cells that binds to oxygen to iron. VO₂max tends to increase linearly with increases in hemoglobin,² and both anemia (hemoglobin levels <11.7 g/dL in women, <13.2 g/dL in men) and low iron status (even within the normal range) are associated with loss of performance, greater fatigue, and reduced energy efficiency. Therefore, iron status and measures of red blood cell and hemoglobin concentrations in the blood are critical blood-based measures of someone’s oxygen delivery capacity. The oxygen delivery score combines these metrics to identify potential constraints on endurance performance. In addition to hemoglobin and red blood cell counts, this includes hematocrit (the percentage of blood volume taken up by red blood cells) and ferritin—a measure of the body’s primary iron stores and capacity for hemoglobin production. In endurance athletes, mild anemias or mild decreases in hemoglobin concentration may be seen due to hemodilution, where plasma expansion outpaces red blood cell production, with no negative effect on performance.⁵ However, in addition to low iron, anemia can also be caused by a range of conditions including deficiencies in B vitamins, chronic diseases or inflammation, and genetic conditions.⁶ If anemia is present these other conditions will be considered based on well-established diagnostic algorithms. However, any anemia or evidence of iron deficiency must be discussed with your physician.

Constraint Zones:

Green:

Blood markers associated with oxygen delivery are within expected performance thresholds. Nutrient status is likely adequate for red blood cell production, and normal responses to training are expected.

Yellow:

Blood markers associated with oxygen delivery are slightly outside ideal performance thresholds but not deficient. If these changes are new, check nutrient status and consider the training stress and low energy availability scores for additional possible causes. Consider hemodilution if changes are seen in the setting of endurance training.

Red:

Blood markers associated with oxygen delivery suggest anemia and/or iron deficiency. Check additional scores for potential causes including nutrient deficiencies and inflammation. Consider the training stress and low energy availability scores for additional possible causes. If anemic, this should be discussed with your physician.