Nonrepresentativeness does not automatically destroy internal validity: a causal contrast can remain valid among Biobank participants if selection is independent of the joint exposure-outcome process after conditioning on analysis variables. It does create bias if health or affluence affects participation and is also related to birth cohort or diabetes, especially when conditioning on participation opens a collider bias path.
External validity is weaker because affluent, healthy volunteers need not have the same baseline risk or sugar effect as the UK target population. Generalization requires either negligible effect modification by selection-related characteristics or standardization and inverse-probability weighting to the target population using variables measured in both sources. The unweighted estimate should otherwise be described as an effect for Biobank-like participants.
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