Vitamin D and Strength: What a San Diego Nutritionist Wants You to Know
Vitamin D might be the most over-hyped and the most under-tested nutrient in fitness. As a Certified Nutritionist in San Diego, I see both extremes — people megadosing it like it’s a steroid, and people who are quietly deficient and wondering why their strength has stalled. Here’s what the evidence actually supports, and what it doesn’t.
What vitamin D does for your muscles
Vitamin D isn’t just a “bone vitamin.” Your muscle cells carry vitamin D receptors, and the nutrient is involved in muscle function and contraction. The NIH Office of Dietary Supplements notes that adequate vitamin D is needed to support both bone health and muscle function (NIH Office of Dietary Supplements). When levels drop too low, muscle weakness and a higher risk of falls are well-documented consequences — which is exactly why this matters for anyone training for strength.
Deficiency is more common than you’d think
Even in sunny San Diego, low vitamin D is surprisingly common — and national data backs that up. According to NHANES intake data summarized by the NIH, the vast majority of Americans take in well below the recommended amount of vitamin D from food (NIH Office of Dietary Supplements). People at higher risk include those with limited sun exposure, darker skin, older adults, and people with obesity. If you train indoors and slather on sunscreen outdoors (as you should), you’re not immune.
Does supplementing actually make you stronger?
Here’s where I have to be honest, because the research is nuanced. A large meta-analysis of 30 randomized controlled trials found that vitamin D supplementation produced a small but statistically significant improvement in overall muscle strength, with the effect most apparent in people who started out deficient or were older (Beaudart et al., 2014). In other words, correcting a true deficiency can help your strength.
But the flip side is just as important. A meta-analysis in athletes who were already vitamin-D sufficient found no significant improvement in maximal strength or power from extra supplementation (Han et al., 2023). The lesson: vitamin D fixes a deficit — it’s not a performance enhancer you stack on top of already-good levels. More is not better.
Test, don’t guess
This is the single most useful thing I tell clients: get your blood level (25-hydroxyvitamin D) tested before you supplement seriously. The NIH considers levels at or above 20 ng/mL adequate for most people, with many experts targeting higher. Knowing your number tells you whether you need a modest maintenance dose or a more deliberate correction — and prevents both under-dosing and pointless megadosing.
How much, and from where
The recommended dietary allowance is 600 IU/day for most adults and 800 IU/day after age 70, with food sources like fatty fish, egg yolks, and fortified dairy contributing modestly (NIH Office of Dietary Supplements). Many people who are deficient need more than the RDA to get their blood level up — which is exactly why testing matters. If you do supplement, vitamin D3 is the preferred form, and it’s best absorbed with a meal containing fat.
I stock practitioner-grade vitamin D3 (often paired with K2) through my Designs for Health supplements — third-party tested and properly dosed. Use code for.
The bottom line
Vitamin D is foundational for muscle function, deficiency genuinely blunts strength, and correcting it helps — but loading up when you’re already sufficient won’t make you stronger. Test your level, dose to your needs, and build the rest of your results on consistent training and nutrition. Want a plan built around your numbers? Start with a free BMR & macronutrient report, train with a San Diego personal trainer, or work with a San Diego sports nutritionist who builds your supplementation around the science.
These statements have not been evaluated by the Food and Drug Administration. This article is for educational purposes and is not medical advice. Consult a qualified health-care provider before starting any supplement or making major dietary changes, especially if you have a medical condition or take medication.
References
1. NIH Office of Dietary Supplements. Vitamin D — Fact Sheet for Health Professionals. ods.od.nih.gov.
2. Beaudart C, et al. The effects of vitamin D on skeletal muscle strength, muscle mass, and muscle power: a systematic review and meta-analysis of randomized controlled trials. J Clin Endocrinol Metab. 2014.
3. Han Q, et al. Effects of vitamin D supplementation on maximal strength and power in athletes: a systematic review and meta-analysis of randomized controlled trials. Front Nutr / PMC. 2023.