Can Sardines Twice Weekly Lower Prediabetes Risk?

Eating about two servings of sardines a week improved several diabetes-risk markers in older adults with prediabetes in one randomized trial. That makes sardines a reasonable food to add to a Mediterranean-style diet. It does not prove that sardines prevent type 2 diabetes.

The distinction matters. The trial measured insulin resistance, blood lipids, blood pressure, and a diabetes-risk score over 12 months; it did not measure who ultimately developed diabetes. Still, sardines are inexpensive, widely available, and pack marine omega-3 fats and taurine into one whole food. For someone building a diabetes-prevention diet, that is a more useful proposition than treating a fish-oil capsule as a tiny can of fish.

What the sardine trial actually found

In a 2021 randomized controlled trial, researchers in Barcelona assigned 152 adults aged 65 or older with prediabetes to one of two 12-month programs. Both groups received counseling built around a Mediterranean dietary plan for type 2 diabetes prevention. The intervention group, 75 participants, also added 200 grams of sardines per week. The control group, 77 participants, followed the same counseling plan without the sardines.

At the end of the study, the sardine group had lower HOMA-IR, a calculated marker of insulin resistance, than the control group (p=0.032). More participants also moved out of the "very high risk" category on the FINDRISC diabetes-risk score (p=0.035). HDL cholesterol and adiponectin rose, while triglycerides and blood pressure fell; each of those results had p<0.05. HbA1c, the standard long-term blood-glucose marker, did not change significantly.

The study also documented a biological effect of eating the fish. Erythrocyte omega-3 fatty-acid subtypes increased and omega-6 subtypes decreased, each with p<0.001 across the reported subtypes. Circulating taurine increased as well. Those measurements show that the intervention changed participants' nutrient exposure. They do not show which nutrient caused the changes in insulin resistance or risk score.

The result deserves interest, but not a victory lap. About 20% of participants dropped out: 12 in the sardine group and 18 in the control group. The trial was unblinded, so participants assigned sardines may have changed other habits, and investigators knew which group each participant joined. The two headline p-values sit close to the usual 0.05 cutoff, several outcomes were tested, and the published abstract does not provide effect sizes or 95% confidence intervals for the HOMA-IR or FINDRISC changes. Without those numbers, readers cannot judge whether the statistically significant difference was clinically large, small, or fragile.

Sardines fit a dietary pattern with better evidence

The case for the Mediterranean diet does not depend on this one fish trial. A 2023 analysis from the randomized PREDIMED trial found that each yearly increase in Mediterranean Diet Adherence Score (MEDAS) was independently associated with lower incidence of type 2 diabetes. In that Spanish trial population, the Mediterranean diet plus extra-virgin olive oil arm had about a 40% lower relative risk of diabetes than the control group at the highest adherence. Relative risk is not absolute risk: the finding does not tell every individual how many cases per 100 people a diet will prevent.

A dose-response meta-analysis of 14 prospective cohorts also found that greater Mediterranean-diet adherence was associated with lower type 2 diabetes incidence. Prospective cohorts can account for many confounders, but they cannot fully separate diet from income, activity, health care, and other habits that travel with it. The analysis also reported high heterogeneity (I²=82%), meaning the size of the association differed substantially among studies with different populations, background diets, adherence scores, and adjustment methods.

Put together, these studies support Mediterranean-style eating as the foundation. The sardine study asks a narrower question: whether a defined amount of one oily fish can improve risk markers when added to that foundation. It offers a positive but preliminary answer for adults 65 and older with fasting glucose of 100–124 mg/dL.

Why sardines could plausibly affect metabolism

Sardines provide EPA and DHA, the marine omega-3 fatty acids. In people with type 2 diabetes, a 2022 systematic review and meta-analysis found omega-3 supplementation changed glycemic markers and cardiovascular risk markers, though supplement effects were generally modest. A separate randomized trial in people with type 2 diabetes found that marine EPA/DHA, unlike plant-derived ALA, changed erythrocyte fatty-acid composition. That supports the interpretation that the membrane-fat changes in the sardine trial came from marine omega-3 exposure.

Human trials also suggest omega-3 fats can affect metabolic markers outside established diabetes. A 2025 double-blind trial in healthy middle-aged and older adults found fish oil altered novel early type 2 diabetes-risk biomarkers. Another 2025 randomized trial of EPA improved HOMA-IR and reduced inflammatory cytokines in people with burns. The latter is mechanistically relevant but clinically indirect: burn patients are not adults with prediabetes.

Taurine is the other plausible contributor. Sardines contain it, and the sardine trial found higher circulating taurine in the intervention group. A 2025 taurine review describes observational links between higher plasma taurine and lower type 2 diabetes risk, plus preclinical evidence involving Akt/GLUT4 signaling, hepatic glucose production, and beta-cell oxidative stress. Those pathways make biological sense. They remain hypotheses for prevention because dedicated taurine trials in people with prediabetes are absent.

Vitamin D should not carry much of this argument. Sardines contain vitamin D, but an updated 2024 meta-analysis found inconsistent effects of isolated vitamin D supplements on glycemic control in type 2 diabetes. Food can behave differently from a supplement, yet current evidence does not identify vitamin D as a reliable driver of the sardine result.

Why the signal may not translate into fewer diabetes diagnoses

HOMA-IR and FINDRISC predict future diabetes risk, but improving a predictor does not guarantee fewer diagnoses years later. The sardine trial was not powered or designed to test incident type 2 diabetes, and its 12-month follow-up cannot close that gap. The absence of a significant HbA1c effect adds restraint: the trial did not show improvement across every glycemic measure.

A 2022 network meta-analysis of nutritional supplements found modest and often uncertain effects on HbA1c and HOMA-IR in established type 2 diabetes. That does not refute the whole-food trial; a whole fish changes more than one nutrient at once. It does mean that a small, unblinded study with near-threshold p-values cannot establish clinically meaningful insulin sensitization on its own.

Prediabetes also describes more than one metabolic state. A 2025 systematic review and meta-analysis found that lifestyle-intervention responses differ among impaired fasting glucose, impaired glucose tolerance, and combined phenotypes. The sardine trial enrolled people by fasting-glucose criteria and did not report phenotype-stratified results. Its findings may therefore not apply evenly to people identified by an oral glucose-tolerance test or to younger adults.

For adults 65 or older with prediabetes, 200 grams of whole sardines per week is a low-downside addition to a Mediterranean-style eating pattern. Treat it as a food choice with a promising surrogate-marker trial behind it, not as a proven diabetes-prevention treatment.

How to use this evidence

Choose canned or fresh sardines twice weekly if you enjoy them and they fit your medical and dietary needs. The tested dose was 200 grams per week for 12 months, alongside Mediterranean-diet counseling. That context matters: the study did not compare sardines against a typical Western diet, nor did it test fish oil capsules as a substitute.

Keep the bigger levers in view. Sustained adherence to a Mediterranean dietary pattern has the stronger evidence base for lower diabetes incidence. Physical activity and caloric management remain part of established prevention practice. Sardines can make that pattern easier to eat well; the available evidence does not show that they can do the job alone.