Economists’ Inequality Statistic Reveals Which Metabolites the Body Truly Controls

A statistic invented more than a century ago to measure the gap between rich and poor is now helping biochemists answer a surprisingly different question: which molecules in our blood does the body actually bother to control? In a new study published in the journal Metabolomics, a team led by Douglas Kell of the University of Liverpool shows that the Gini coefficient, the same non-parametric measure of inequality used by economists to compare income distributions, can serve as a remarkably effective surrogate for how tightly a metabolite is regulated, and by extension, whether it originates inside the body or arrives from outside sources such as drugs, food, or the diet-derived antioxidant ergothioneine.

The Gini coefficient takes a value between zero and one. In economics, a value of zero would mean everyone earns exactly the same income, while a value approaching one means a single individual holds nearly all the wealth. The researchers reasoned that the same logic applies to metabolite concentrations measured across a population of samples. If a molecule is homeostatically regulated by cells, tissues, or the organism as a whole, its concentration should be similar from person to person, yielding a low Gini coefficient. Conversely, a molecule that is exogenous, such as a pharmaceutical drug that only some individuals have ingested, should show wildly unequal concentrations across a cohort, producing a Gini coefficient close to one.

To test this idea, the team mined publicly available metabolomics datasets, including a large study of more than 200 identified plasma metabolites measured in 1,125 individuals with chronic obstructive pulmonary disease, available through the Metabolomics Workbench. The results were striking. Endogenous metabolites, which the authors call endogenites, peaked in their Gini distribution at around 0.2, while exogenous molecules such as drugs and food-derived compounds peaked above 0.95. The median Gini coefficient across the entire dataset was 0.263. When the researchers classified molecules by origin, 73 percent of exogenous molecules had Gini coefficients above 0.5, compared with just 2.9 percent of molecules considered endogenous or regulated.

Among the most tightly controlled molecules were the amino acids. Essential and non-essential amino acids had identical average Gini coefficients of just 0.14, and nine of the 25 lowest Gini values in the dataset belonged to amino acids including methionine, arginine, proline, serine, phenylalanine, asparagine, tryptophan, lysine, and glutamine. Glutamine itself recorded the lowest value of all, a Gini coefficient of 0.066 with a 95 percent confidence interval of just 0.063 to 0.068, a figure even lower than any observed in the team’s earlier transcriptomics analyses. Given that glutamine is a major hub of nitrogen metabolism, such extreme uniformity makes biological sense and suggests the molecule could even serve as a normalisation standard in metabolomics studies where sample volumes are uncertain.

At the opposite extreme sat pharmaceutical compounds. The anticonvulsant lamotrigine, for example, posted a Gini coefficient of 0.99, meaning its presence in plasma was almost entirely confined to the small subset of participants taking the drug. In a second dataset of 681 serum metabolites from 340 individuals studied in the context of tuberculosis, the highest values belonged to metabolites of paracetamol and aspirin. The researchers also examined vitamins, which occupy an interesting middle ground: they are essential and therefore physiologically important, yet exogenous in origin. Their Gini coefficients fell in an intermediate range of roughly 0.2 to 0.4, consistent with partial regulation as cofactors, though the values varied more than threefold across vitamins, likely reflecting differences in diet, absorption, supplementation, and microbiome interactions.

The study’s second focus was ergothioneine, a sulfur-containing amino acid derivative with the formula C9H15N3O2S and an exact monoisotopic mass of 229.0885 Da. Humans cannot synthesise this compound; it comes entirely from the diet, most notably mushrooms, and is transported into tissues by a dedicated transporter. Growing evidence links higher ergothioneine levels to reduced risks of cardiovascular disease, cognitive decline, dementia, and frailty, and previous work by the same group showed that women with high plasma ergothioneine were far less likely to develop pre-eclampsia. Because ergothioneine is exogenous but clearly physiologically important, the team predicted it would show an intermediate Gini coefficient, and the data confirmed this. Across multiple independent studies, ergothioneine’s Gini coefficient clustered consistently between 0.3 and 0.4: 0.38 in the COPD dataset, 0.373 in the tuberculosis cohort, 0.325 in a whole-blood dementia study, 0.457 in an ageing study, and 0.37 to 0.4 in a large dementia cohort from Singapore.

The analytical chemistry behind these measurements is itself noteworthy. Ergothioneine’s protonated form has a mass-to-charge ratio of 230.0958 in positive electrospray ionisation mode, and no other biologically relevant molecule lies within even 10 parts per million of this value, making database searches for the compound unusually straightforward. In new experimental work reported in the paper, the team measured ergothioneine in 40 antenatal serum samples from a pilot study at Liverpool Women’s Hospital, using ultra-high performance liquid chromatography coupled to an Orbitrap Exploris 240 mass spectrometer at a resolution of 120,000, with calibration solutions spanning 0.01 to 500 micromolar.

The Liverpool pilot delivered two surprises. First, the median ergothioneine concentration was just 180 nanograms per millilitre, far below the 261 nanograms per millilitre median seen in the earlier SCOPE study of 432 pregnant women; in fact, 180 nanograms per millilitre corresponds only to the ninth percentile of the earlier cohort. Second, women who went on to develop pre-eclampsia did not show the expected lower ergothioneine levels. The authors suggest this apparent contradiction dissolves once the population’s very low baseline is recognised: when nearly everyone is deficient, the protective relationship with concentration is obscured. Intriguingly, the Gini coefficient in the Liverpool cohort was lower than in almost all other ergothioneine studies, hinting that a depressed Gini value, even without absolute concentrations, might flag a population with inadequate ergothioneine exposure and a likely need for supplementation.

The researchers propose rough interpretive thresholds: metabolites with Gini coefficients below about 0.25 are subject to significant homeostasis or show low variation in exogenous supply, while those above about 0.75 are likely exogenous and largely unregulated. Molecules in between, like most vitamins and the nutraceuticals ergothioneine and kynurenic acid, are probably exogenous but partially regulated. The authors caution that a high Gini coefficient could sometimes reflect analytical error, missing values, or variable pharmacokinetics, making the metric best viewed as hypothesis-generating. A low value, however, is hard to explain away, and reliably indicates tight biological control. The team also notes that urinary metabolomes did not show systematically higher Gini coefficients than plasma, and that applying the approach to gut microbiome-derived metabolites awaits raw data that are not yet publicly available.

Beyond its technical contribution, the work carries a broader message about health inequality. The Liverpool findings, with median ergothioneine levels sitting at the ninth percentile of a comparable cohort, echo documented patterns of socioeconomic disparity in British health and mortality statistics. If a simple statistic borrowed from economics can simultaneously identify which molecules the body defends, expose hidden dietary deficits, and strengthen the case for targeted nutritional intervention, the Gini coefficient may prove to be one of the most versatile imports metabolomics has ever received from the social sciences.

Subject of Research: Using the Gini coefficient as a surrogate measure of metabolite regulability and homeostasis, with a focus on the diet-derived antioxidant ergothioneine

Article Title: The Gini coefficient as a surrogate for the regulability or homeostasis of metabolite concentrations: focus on ergothioneine

Article References: Kell, D. B., Dunn, W. B., Winder, C. L., Anand, K., Greenfield, B., Kenny, L. C., Merriel, A., Moore, J. B., & Waitt, C. (2026). The Gini coefficient as a surrogate for the regulability or homeostasis of metabolite concentrations: focus on ergothioneine. Metabolomics, 22(5), Article 151. https://doi.org/10.1007/s11306-026-02534-1

Image Credits: AI Generated

DOI: 10.1007/s11306-026-02534-1

Keywords: Gini coefficient, metabolomics, ergothioneine, homeostasis, metabolite regulation, nutraceutical, pre-eclampsia, mass spectrometry, amino acids, vitamins, dietary antioxidants, health inequality