The study, led by Marianne Nygaard of the University of Southern Denmark together with colleagues at the University of Bristol and other institutions, drew on the Danish GEMINAKAR cohort, which was established between 1997 and 2000 through the population-based Danish Twin Registry. Participants, aged 18 to 67 at baseline, underwent detailed examinations including fasting blood sampling, and were followed up more than a decade later, in 2010 to 2012. From this resource, the team assembled 132 complete monozygotic twin pairs with food frequency questionnaire data, genome-wide DNA methylation measurements, and covariate information at both time points, yielding a dataset of 264 individuals tracked across roughly twelve years of adult life.

The technical machinery of the study reflects the current state of the art in epigenetic aging research. DNA was extracted from whole blood, bisulfite converted, and profiled on Illumina Infinium MethylationEPIC BeadChips, with stringent quality control and fixed effects functional normalization to minimize batch artifacts. From these methylation data, the researchers computed principal component-based versions of four widely used epigenetic clocks, PCHannum, PCHorvath1, PCPhenoAge, and PCGrimAge, along with DunedinPACE, a measure trained on longitudinal data that estimates the pace of aging rather than a static biological age. Age acceleration measures were derived as the residuals of each clock regressed on chronological age, and all outcomes were standardized to allow comparison across clocks.

Dietary intake was assessed with validated semiquantitative food frequency questionnaires covering 192 foods and recipes, originally developed for the Danish Diet, Cancer and Health study. Adherence to national dietary guidance was quantified with the Danish Dietary Index, a score from zero to six built on the official Danish food-based dietary guidelines and the Nordic Nutrient Recommendations. The index rewarded higher intakes of fruits and vegetables, fish, whole grains, and fiber, and penalized high intakes of meat, saturated fat, and added sugar. Each component was scored as the ratio of actual to recommended intake, with a score of one indicating full compliance for that item.

The design’s decisive feature was the co-twin control approach. Monozygotic twins share practically all of their segregating DNA and, in most cases, a highly similar childhood environment. When twins within a pair differ in their diet scores, those differences cannot be explained by their genes or by the family they grew up in. Comparing individuals across the whole sample captures both the effect of diet and everything else that makes healthy eaters different, including inherited tendencies. Comparing twins within pairs strips away the familial confounding. Because both dietary habits and epigenetic aging are themselves partly heritable, the authors argued that genetic confounding was a plausible and testable threat to the observational literature.

The initial results looked reassuringly familiar. In cross-sectional, individual-level analyses at baseline, every beta coefficient was negative, meaning that people who adhered more closely to the dietary guidelines showed decelerated epigenetic aging. The signal was strongest for the second-generation clocks: each one-unit increase in the Danish Dietary Index was associated with roughly 0.55 to 0.67 years lower PCGrimAge and a 3.1 percent lower pace of aging on DunedinPACE, corresponding to standardized effect sizes of about 0.16 to 0.23 standard deviations. The same directional tendency appeared in the follow-up data, echoing previous cross-sectional studies that linked higher diet quality to younger epigenetic ages, particularly for clocks trained to predict morbidity and mortality rather than chronological age.

Then the picture changed. When the same association was tested within twin pairs, the strong signals for PCGrimAge and DunedinPACE attenuated dramatically, leaving no robust association between dietary adherence and epigenetic age acceleration. The implication is uncomfortable: the individuals who eat well may simply be people whose genes and upbringing favor both better diets and slower biological aging, rather than people whose diets are actively slowing their clocks. Because the cohort contained only monozygotic pairs, the study could not disentangle genetic confounding from shared environmental confounding, but either way, the familial factors appear sufficient to explain much of the apparent benefit.

The longitudinal analyses added a further twist. When the researchers modeled the change in diet score from baseline to follow-up against the change in epigenetic age measures, the estimates were primarily positive, unexpectedly suggesting that improvements in dietary adherence were associated with increased, not decreased, age acceleration, although the confidence intervals were wide and the authors caution against overinterpreting this pattern. Subgroup analyses that might have clarified the result, for example comparing people who stayed at low diet scores with those who improved from low to high, were impossible because the relevant subgroups contained too few participants, around seventeen individuals, to support meaningful inference.

The authors are careful about what their findings do and do not show. The study remains observational, so reverse causality and confounding by non-shared factors cannot be excluded. Blood cell composition was deliberately not adjusted for, since doing so could remove part of the biological signal of interest, meaning that variation in cell mixture may contribute to the results. Dietary data were self-reported and retrospectively collected, which may attenuate estimates, particularly within twin pairs where the differences between co-twins are small and measurement error from both twins compounds. The sample, drawn from a cohort selected to be free of diabetes and cardiovascular disease at baseline, is healthier than the general population and relatively small, limiting statistical power and generalizability. Nearly half of eligible monozygotic pairs were excluded for missing data, although participants and non-participants were broadly comparable.

Even so, the study’s strengths are substantial, and its conclusion is a sobering one for the epigenetic anti-aging boom. No strong independent association between dietary adherence and epigenetic aging survived the twin comparison, and the authors conclude that apparent benefits of healthy eating on epigenetic aging may partly reflect genetic and shared environmental factors rather than diet itself. This does not mean diet is irrelevant to aging; randomized trials of dietary interventions have reported slowing of Horvath clock and GrimAge measures, and the broader evidence that high dietary quality reduces cardiovascular disease, cancer, and all-cause mortality remains intact. What the twin data challenge is the specific causal story written into methylation-based age estimates. As epigenetic clocks move from research labs toward commercial wellness products, the Danish twins offer a reminder that correlation, even molecular correlation measured across hundreds of thousands of CpG sites, is not automatically causation, and that the family you come from may shape both your plate and your methylation patterns in ways no questionnaire can fully untangle.

Subject of Research: The association between adherence to Danish dietary guidelines and epigenetic aging measures in monozygotic twins

Article Title: Dietary pattern and epigenetic aging: a longitudinal twin study

Article References: Nygaard, M., Elliott, H. R., Smith, G. D., Relton, C., Kyvik, K. O., & Dalgård, C. (2026). Dietary pattern and epigenetic aging: a longitudinal twin study. GeroScience. https://doi.org/10.1007/s11357-026-02550-y

Image Credits: AI Generated

DOI: 10.1007/s11357-026-02550-y

Keywords: epigenetic aging, DNA methylation, epigenetic clocks, twin study, dietary patterns, Danish Dietary Index, DunedinPACE, GrimAge, co-twin control design, confounding, GeroScience, longitudinal study