G6PD deficiency is the most common enzyme deficiency in humans, affecting an estimated 400 million people worldwide. How common it is depends heavily on your ancestry: prevalence in males ranges from about 12% in people of African descent to less than 0.3% in people of European descent. In the United States, roughly 1 in 10 African American males has it.
Prevalence by Ethnicity and Region
G6PD deficiency is not evenly distributed across populations. The highest rates appear in people of African, Mediterranean, Middle Eastern, and Southeast Asian descent. In males, estimated prevalence breaks down roughly like this:
- African descent: around 12.2%
- Asian descent: 2.7% to 3.5%
- Middle Eastern descent: about 2.1%
- European descent: less than 0.3%
Data from the UK illustrates these disparities in sharp detail. Approximately 1 in 7 Black males and 1 in 63 Asian males carry a G6PD deficiency gene variant, compared to fewer than 1 in 10,000 White males. Two specific genetic variants account for much of this: one is common in people of African ancestry (carried by about 11.6% of African Americans), and another is found in roughly 1.6% of South Asian males.
These numbers mean that in diverse countries like the United States, the United Kingdom, and Australia, G6PD deficiency is far more common than many people realize. It is also significantly underdiagnosed, particularly in communities where routine screening isn’t standard.
Why Males Are Affected More Often
G6PD deficiency is an X-linked genetic condition, which is the key reason it shows up far more often in males. Boys inherit a single X chromosome from their mother. If that one copy carries a defective G6PD gene, they have the deficiency with no backup. Girls inherit two X chromosomes, one from each parent, so a working copy from one parent can compensate for a faulty copy from the other. A girl would need to inherit the defective gene from both parents to be fully affected.
Women who carry one defective copy are typically asymptomatic but can pass the gene to their children. This carrier status is common in the same populations where the deficiency is prevalent, meaning a carrier mother has a 50% chance of passing it to each son.
The Malaria Connection
The reason G6PD deficiency is so common in certain populations, rather than being a rare genetic accident, comes down to malaria. G6PD deficiency is concentrated in regions where malaria is or historically was endemic: sub-Saharan Africa, the Mediterranean basin, the Middle East, and parts of South and Southeast Asia. Carrying the deficiency appears to offer a survival advantage against malaria parasites, similar to the way sickle cell trait does. Over thousands of years, natural selection favored people who carried the gene in these regions, which is why prevalence remains high today even in descendants who have migrated far from malaria zones.
What the G6PD Enzyme Actually Does
G6PD is an enzyme inside red blood cells that plays a protective role. It helps produce a molecule that acts as an antioxidant shield, neutralizing harmful oxygen byproducts (free radicals) that would otherwise damage the cell membrane and hemoglobin. Think of it as a built-in defense system that keeps red blood cells intact under stress.
When G6PD activity is too low, red blood cells lose that protection. Under oxidative stress, from certain foods, medications, infections, or even physical stress, the unprotected hemoglobin breaks down and forms clumps called Heinz bodies. The cell membrane stiffens and the red blood cells are destroyed faster than the body can replace them. This is hemolysis, and it’s the core problem in G6PD deficiency.
Severity Varies Widely
Not all G6PD deficiency is the same. The WHO classifies it into levels based on how much enzyme activity remains. People with severe deficiency (less than 10% of normal enzyme activity) can experience chronic anemia even without obvious triggers. Those with moderate deficiency (10% to 60% of normal) typically feel fine day to day but are vulnerable to hemolytic episodes when exposed to a trigger. Many people with mild deficiency (60% or more of normal activity) never know they have it.
The specific genetic variant matters too. The variant most common in African Americans tends to cause moderate deficiency, while the Mediterranean variant often causes more severe enzyme loss. This is why two people with G6PD deficiency can have very different experiences.
What Triggers a Hemolytic Episode
Most people with G6PD deficiency feel perfectly healthy until something overwhelms their red blood cells’ limited defenses. The best-established triggers are fava beans (also called broad beans), naphthalene (found in mothballs), and certain medications. Infections and metabolic stress like diabetic ketoacidosis can also set off an episode.
A comprehensive review found that fava beans and naphthalene are the most consistently documented dietary and chemical triggers. Some evidence also points to certain food coloring agents, unripe peaches, fenugreek seeds, and menthol products, though these are less well established. The list of problem medications is longer and includes some antimalarials, certain antibiotics, and a few other drug classes, which is especially relevant in malaria-endemic areas where people with undiagnosed G6PD deficiency may be prescribed these drugs.
What a Hemolytic Episode Feels Like
After exposure to a trigger, hemolysis typically begins within 24 to 72 hours. The onset can be faster and more dramatic with fava beans, especially in children. Early signs include fatigue, weakness, and irritability. As more red blood cells are destroyed, jaundice develops (a yellowing of the skin and eyes), the heart rate rises to compensate for dropping red blood cell counts, and urine may turn dark brown or cola-colored from the released hemoglobin passing through the kidneys.
The good news is that acute episodes are usually self-limiting. The body ramps up production of new red blood cells, which have high levels of G6PD enzyme, and the crisis typically resolves within 8 to 14 days. Severe episodes can require medical support, but most people recover fully once the trigger is removed.
Many Cases Go Undiagnosed
Because G6PD deficiency causes no symptoms between episodes, many people carry it without knowing. In the United States, newborn screening for G6PD deficiency is not universal, and routine testing in adults is uncommon. This means a first hemolytic episode, sometimes triggered by a new medication or an unfamiliar food, may be the first sign that someone has the condition. In diverse populations where the deficiency is prevalent, this diagnostic gap contributes to health inequalities. A UK study found that undiagnosed G6PD deficiency in Black and Asian individuals affected the accuracy of certain diabetes blood tests, since ongoing low-level red blood cell turnover can skew results.
A simple blood test can confirm G6PD deficiency. If you have ancestry from a high-prevalence region and have ever experienced unexplained anemia, jaundice, or dark urine after illness or medication, testing is straightforward and widely available.