Vitamin D deficiency is common in older adults and may contribute to sarcopenia, but whether diabetes modifies this association and the underlying mechanisms remain unclear.

We used a population-based and experimental validation framework. In epidemiological analyses, 7,520 older adults from two nationally representative cohorts were included (HRS wave 13, n = 3,246; ELSA wave 6, n = 4,274). Sarcopenia was defined according to EWGSOP2 criteria using low grip strength and low muscle mass estimated by a validated anthropometric equation standardized by BMI. Serum 25(OH)D was categorized as low (≤50 nmol/L) or higher (>50 nmol/L). Multivariable logistic regression with multiple imputation was used to assess overall and diabetes-stratified associations, as well as multiplicative and additive interactions. For experimental validation, an aged diabetic rat model with vitamin D deficiency was established, followed by vitamin D3 supplementation (2000 IU). Glycometabolic indices, muscle function and morphology, intramuscular lipid deposition, and senescence-related markers in gastrocnemius muscle were evaluated.

Low 25(OH)D was associated with higher odds of sarcopenia overall. Among participants with diabetes, this association was stronger and reached statistical significance in ELSA (HRS: OR = 1.778, 95% CI 0.843–3.750; ELSA: OR = 2.242, 95% CI 1.055–4.764). In ELSA, the joint exposure to low 25(OH)D and diabetes was associated with increased sarcopenia odds (OR = 1.66, 95% CI 1.06–2.61), with evidence of additive interaction (RERI = 1.08, 95% CI 0.25–1.97). In aged diabetic rats, vitamin D deficiency aggravated hyperglycemia, insulin resistance, intramuscular lipid accumulation, and muscle senescence, whereas vitamin D3 supplementation improved muscle strength, myofiber cross-sectional area, and lipid infiltration.

Low vitamin D status was associated with higher sarcopenia risk, particularly in diabetes. Experimental findings further support a protective role of vitamin D against diabetes-related muscle deterioration.