A different type of fat found in dairy called c9t11-CLA is linked to a higher chance of getting type 2 diabetes, which is surprising because other dairy fats are usually thought to be harmless or even helpful.
Scientific Claim
Higher plasma concentrations of the cis-9,trans-11 conjugated linoleic acid (c9t11-CLA) isomer are associated with a 39% higher risk of developing type 2 diabetes per standard deviation increase, indicating that this ruminant-derived trans fat may have a harmful metabolic effect contrary to other rTFAs.
Original Statement
“c9t11-CLA was positively associated (HR per SD 1.39; 95% CI 1.19–1.62)... c9t11-CLA was positively associated with diabetes risk (in the fully adjusted model, HR 1.39; 95% CI 1.19–1.62).”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The observational design only supports association. The authors used appropriate language ('positively associated') and reported confidence intervals. The effect size is large and robust to adjustments.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.
Systematic Review & Meta-AnalysisLevel 1aWhether the positive association between plasma c9t11-CLA and type 2 diabetes is consistent across populations and measurement methods.
Whether the positive association between plasma c9t11-CLA and type 2 diabetes is consistent across populations and measurement methods.
What This Would Prove
Whether the positive association between plasma c9t11-CLA and type 2 diabetes is consistent across populations and measurement methods.
Ideal Study Design
A meta-analysis of 12+ prospective cohorts using standardized GC-MS to measure plasma c9t11-CLA at baseline, with incident type 2 diabetes confirmed by medical records, adjusting for identical covariates (BMI, diet, activity, other FAs), and stratifying by dairy intake levels.
Limitation: Cannot determine if the association is causal or mediated by unmeasured confounders.
Randomized Controlled TrialLevel 1bWhether supplementing c9t11-CLA worsens insulin sensitivity or accelerates progression to type 2 diabetes.
Whether supplementing c9t11-CLA worsens insulin sensitivity or accelerates progression to type 2 diabetes.
What This Would Prove
Whether supplementing c9t11-CLA worsens insulin sensitivity or accelerates progression to type 2 diabetes.
Ideal Study Design
A double-blind RCT of 250 adults with prediabetes, randomized to 2g/day of purified c9t11-CLA or placebo for 12 months, with primary outcome of change in HOMA-IR and secondary outcomes of fasting glucose, adiponectin, and liver fat (MRI), measured at baseline, 6, and 12 months.
Limitation: Supplementation may not reflect natural dietary exposure; ethical concerns if adverse effects are suspected.
Prospective Cohort StudyLevel 2bIn EvidenceWhether the association between plasma c9t11-CLA and type 2 diabetes is reproducible in a different population with similar biomarker precision.
Whether the association between plasma c9t11-CLA and type 2 diabetes is reproducible in a different population with similar biomarker precision.
What This Would Prove
Whether the association between plasma c9t11-CLA and type 2 diabetes is reproducible in a different population with similar biomarker precision.
Ideal Study Design
A prospective cohort study of 9,000 adults aged 35–75 in a non-European population, measuring plasma phospholipid c9t11-CLA at baseline via GC-MS, with 8-year follow-up for incident type 2 diabetes confirmed by physician diagnosis and HbA1c.
Limitation: Still observational; residual confounding possible.
Evidence from Studies
Supporting (1)
Plasma Industrial and Ruminant Trans Fatty Acids and Incident Type 2 Diabetes in the EPIC-Potsdam Cohort
This study found that people with higher levels of a specific fat from dairy and meat (c9t11-CLA) were 39% more likely to develop type 2 diabetes, which matches the claim that this fat might be harmful.