In 1912, British biochemist Harriette Chick shaped nutritional research with her field studies on rickets prevention through diet and sun exposure – an early, groundbreaking observation for later vitamin D research. This work marked a turning point: health is not merely the absence of disease but the precise management of nutrients, light, and lifestyle. Today, in the jungle of supplements, Chick's doctrine is more relevant than ever: knowledge protects – from deficiency, from excess, and from marketing myths.

Supplements are concentrated nutrients meant to fill specific gaps. Crucial is bioavailabilitythe proportion of a nutrient that reaches the body and is utilized, as it determines whether a preparation actually has an effect. When it comes to vitamin D, the distinction is worth noting: cholecalciferolvitamin D3, classic supplement form versus calcifediol25(OH)D, the circulating storage form in the blood; often acts faster when taken as supplements. The chemical form also makes a difference with minerals: zinc in the form of zinc picolinateorganic compound with high solubility is usually better available than zinc oxide, which is absorbed poorly. Additionally, the matrix and timing influence the uptake: fat improves the absorption of fat-soluble vitamins; dietary fibers and phytatesplant compounds that bind minerals can block minerals. Finally, medications, the gut microbiome, and age act as bioavailability "gatekeepers" – they either open or close the door to efficacy.

The improper use of supplements can be harmful. For vitamin D, overdoses have been documented with hypercalcemia, hypercalciuria, and soft tissue calcifications; moderate daily doses in the range of 800–1000 IU are considered safe for correcting deficiency, while high long-term or bolus doses pose unnecessary risks ^[1]. Conversely, preparations without bioavailable forms or appropriate intake conditions tend to be ineffective: food, matrix, and gut situation measurably determine how much is absorbed ^[2]. Interactions are also clinically relevant – calcium, for instance, can slow the absorption of certain medications; combinations such as antithrombotics with ginkgo carry increased risk of side effects, particularly in older, polymedicated individuals ^[3]. Positively, controlled UV exposure reliably increases serum vitamin D without a rise in inflammation or evident skin damage at short, standardized doses – a natural lever that can complement or replace supplements ^[4].

An overview of bioavailability shows: the form makes the function. Calcifediol is more bioavailable than cholecalciferol; methylfolate is better utilized than folic acid; fat increases the absorption of fat-soluble vitamins, while phytates from plant foods bind minerals. Host factors – from the microbiome to life phases – additionally modulate nutrient utilization, and several medications lower vitamin status. The implication is pragmatic: those who supplement should consider form, matrix, and context to optimize efficacy and safety ^[2]. Specifically for zinc, an in vitro digestion model shows that bioaccessibility varies depending on form and diet: zinc picolinate achieves the highest, zinc oxide the lowest values; fiber-rich diets significantly reduce availability. This confirms that chemical form and dietary environment are crucial ^[5]. For vitamin D, a safety analysis underscores that the therapeutic range may be narrower than previously thought. While 800–1000 IU/day is considered safe for the prevention and correction of deficiency, long-term high or interval dosing increases risks beyond calcium-phosphate homeostasis; age, gender, and baseline status further modulate risk ^[1].

- Only supplement what has been tested as deficient: specifically determine 25(OH)D, ferritin, B12, folate, iodine/TSH, or zinc and orient doses based on lab results. For vitamin D in cases of deficiency, typically dose moderately (usually 800–1000 IU/day) and avoid high doses and bolus strategies unless medically indicated ^[1].
- Pay attention to bioavailability: take vitamin D with a fat-containing meal; consider calcifediol instead of cholecalciferol for quicker increases if necessary ^[2]. For zinc, prefer zinc picolinate; fiber-rich meals may reduce absorption ^[5]. For folate, consider methylfolate when bioavailability is critical ^[2].
- Check for interactions: openly disclose all supplements to your doctor/pharmacist. Maintain distance between minerals like calcium/magnesium and levothyroxine or certain antibiotics; be cautious with antithrombotic medication and plant compounds like ginkgo. Older individuals with polypharmacy should utilize standardized supplement screening ^[3].
- Activate natural vitamin D: 3–4 short sun exposures per week (e.g., 5–10 minutes on larger skin areas, depending on skin type and UV index) measurably raise 25(OH)D levels without an increase in inflammation in controlled settings. Combine this with regular outdoor activity for a double effect on energy and bone health ^[4].
- Critically assess sources: question the economic motives and evidence base of social media content; rely on trusted professional sources and medical advice rather than marketing promises ^[6].

The next wave of supplement research will be more personalized: bioavailability optimization through formulation, microbiome interactions, and age-dependent absorption will come into focus ^[2]. At the same time, better safety data on vitamin D dosing regimens and clear guidelines for high-risk groups, as well as robust models for supplement-drug checks in clinical practice are needed ^{[1] [3]}.