<p>Frailty is a global public health issue associated with multiple adverse outcomes in later years, however, studies have shown exercise, diet, and vitamin D supplements can help reverse frailty. There is a lack of further risk stratification and evidence for secondary interventions in populations that have undergone primary prevention. This was a prospective cohort study involving 53106 physically active vulnerable individuals from the UK Biobank. We combined vitamin D with cumulative dietary risk scores to evaluate its predictive value. Correlation and predictive power were assessed using restricted cubic spline (RCS) analysis, corrected Cox models, stratified analysis, Kaplan–Meier analysis, and Harrell's c-index. During 12.7&#xa0;years of follow-up, there were 870, 1317 new-onset metabolic dysfunction-associated fatty liver disease (MAFLD) and mortality. RCS analysis revealed a J-shaped negative or positive correlation in vitamin D and dietary risk score respectively. The hazard ratios (95% confidence interval) for MAFLD and mortality in the vitamin D deficiency and high dietary risk groups were 1.80 (1.45–2.24) and 1.99 (1.67–2.38), compared to those with no vitamin D deficiency and low dietary risk. Kaplan–Meier analysis suggested vulnerable individuals with vitamin D deficiency and a poor diet were at a higher risk of disease in the future. Further, separate analyses of vitamin D and dietary risk scores did not demonstrate better efficacy. The combination of vitamin D and dietary risk score offers a more effective method for assessing disease risk in vulnerable populations. This provides evidence for secondary guidance following exercise interventions, which is significant for older population.</p> Graphical abstract <p></p>