Fig. 6

Independent U-shaped interrelated tendency between health decline score, multi-disease score or multi-symptom score at the optimally fitted DMM in the older adults by multilevel regression. A multilevel modelling was fitted with substantial adjustments by age, sex, setting, residency, education attainment, personal income (level-1) and region variation (level-2). After multiple fitting iterations, an optimization by various interval of DMM interval addressed DMM = 0.02 as optimal, based on which an intriguingly independent U-shaped interrelated tendency was shown between health decline score, multi-disease score and multi-symptom score. Compared to the reference subgroup (DMM at-0.02 ~ 0, health decline β = 0, MDS β = 0, MSS β = 0), the Level-1 subgroup demonstrated insignificantly yet mildly reduced health decline (β = 0.0497, P = 0.77) by a moderate increase in multi-diseases score (β = 0.2063) and a significant reduction in multi-symptom score (β = -0.2413). Significantly decreased health decline (β = -0.6824, P = 0.002) is shown in the subgroup Level-2 with multi-disease score and multi-symptom score in parallel; Level-3 subgroup addressed the significantly increased health decline (β = 0.8771, P < 0.001) with the highest multi-symptom score (β = 0.7570, P < 0.001) and unchanged multi-disease score comparing with the reference (P = 0.471)