From A to D: the vitamin alphabet for a healthy spring

In brief

• Vitamins and trace elements play diverse roles in energy production, muscle maintenance, and immune response.
• There is evidence of efficacy for some substances in specific contexts (e.g., vitamin D and respiratory infections), but the effect depends on nutritional status, dosage, and population.
• A varied diet remains the primary strategy; targeted supplementation may be appropriate if there are limitations to dietary intake or following clinical evaluation.
• Available evidence has methodological limitations: distinguishing observational studies from RCTs and integrating recommendations with individual assessments is crucial.

Abstract: what does science say?

Vitamins (A, B, C, D, E) and certain trace elements like zinc and selenium participate in biological functions relevant to facing seasonal changes: energy production, tissue integrity, and modulation of immune defenses. Systematic reviews and meta-analyses indicate that the benefit of supplementation varies greatly depending on the initial state (e.g., deficiency vs. sufficiency), dosage, type of population, and measured outcome. Some interventions have shown favorable results in specific outcomes (for example, reduction of respiratory episodes with vitamin D supplementation in deficient populations), but there is no general evidence that systematic intake of supplements improves the health of those with adequate nutritional status. The main limitations include heterogeneity of studies, different measurements of clinical outcomes, and rare information on long-term doses; therefore, practical recommendations require caution and individual evaluation.

Key vitamins for energy, immune system, and tissues

This paragraph introduces the main biological functions in which vitamins are involved: energy metabolism, immune support, and tissue integrity. These are essential nutrients that act as enzymatic cofactors, antioxidants, or hormonal regulators; their practical role depends on the initial nutritional status, the chemical form taken, and the interaction with other nutrients. The most robust clinical evidence refers to populations with documented deficiency or insufficiency; for the general population, the approach based on diet and clinical evaluation remains a priority. Below is an overview by vitamin groups and trace elements with references to relevant reviews and studies.

Vitamin B6 and B group: metabolism and energy

B vitamins, and in particular vitamin B6 (pyridoxine), are involved in numerous metabolic reactions that convert macronutrients into energy, in the synthesis of neurotransmitters, and in the formation of amino acids. Review literature highlights the role of B6 as a cofactor in essential reactions for the metabolism of carbohydrates, fats, and proteins, and in the synthesis of bioactive compounds relevant to the nervous system. In conditions of insufficiency or increased need (for example, during certain growth phases or in inflammatory states), targeted supplementation can correct biochemical deficits; however, for those consuming a varied and complete diet, clinical deficiency is rare, and the additional effects of supplementation are not clearly established. [5]

Vitamin D: bones, muscles, and defense against infections

Vitamin D regulates calcium homeostasis and acts on muscle tissues and the immune system. Meta-analyses of randomized trials have reported a modest reduction in the risk of respiratory infections with vitamin D supplementation, especially in individuals with low baseline levels. [1] In contrast, the effect of supplementation on muscle function is less consistent: more recent reviews show conflicting results and suggest limited or no benefits in non-deficient populations. [8] For these reasons, the recommended clinical approach involves assessing the status (laboratory test) and targeted interventions when necessary, rather than generalized supplementation without preliminary measurement.

Vitamin C: immune defenses and collagen

Vitamin C plays multiple roles: antioxidant action, support for collagen synthesis, and a role in the functioning of the immune system. Meta-analyses and systematic reviews indicate that regular supplementation does not significantly reduce the incidence of the common cold in the general population, but it can reduce its duration and severity in some contexts and improve specific clinical outcomes in selected groups. [3] Furthermore, studies and reviews on skin physiology document the crucial role of vitamin C in collagen synthesis and epidermal health, supporting tissue function. [4]

Vitamins A and E: antioxidants and tissue integrity

Fat-soluble vitamins A and E participate in protecting cells from oxidative stress and maintaining tissues. In preclinical studies and some reviews, vitamin E is described as an important biological antioxidant with roles in the stability of lipid membranes; vitamin A is involved in cell differentiation processes and immunomodulation. Clinical effects observed in populations vary depending on nutritional status: in conditions of nutritional sufficiency, the additional benefits of supplementation are not uniformly demonstrated, and some studies suggest caution in the use of high doses. [7]

Zinc and other trace elements: role in immune response

Zinc is an essential trace element for the development and function of immune cells. Reviews and experimental studies document its role in regulating the immune response and modulating inflammation; in contexts of deficiency, replenishment can improve immune parameters and reduce the risk of infections. [6] As with vitamins, the best approach is to identify and correct documented deficiencies, as an excess of some micronutrients can have adverse effects.

What it means in practice

From a practical point of view, the evidence suggests a gradual and personalized approach. The priority is improving nutrition: a varied diet, rich in fruits, vegetables, legumes, whole grains, lean protein sources, and fatty fish provides many of the necessary vitamins and minerals without resorting to supplements. In cases where the diet is insufficient, or in the presence of risk factors for deficiencies (advanced age, malabsorption, dietary restrictions, very limited sun exposure), it is appropriate to assess nutritional status with laboratory tests and discuss with a healthcare professional the advisability of targeted supplementation. For vitamin D, for example, the review of RCTs shows benefit mainly in subjects with low baseline levels; for vitamin C, the advantages on the duration and severity of some episodes are documented in meta-analyses, but do not justify indiscriminate intake in the well-nourished population. [1][3][4][2]

Furthermore, some simple seasonal behaviors can support well-being: regular and moderate sun exposure when possible to promote vitamin D synthesis, regular consumption of colorful fruits and vegetables for antioxidants and carotenoids, attention to hydration and recovery after physical exertion to maintain electrolyte balance. In the presence of persistent symptoms, marked fatigue, or chronic conditions, consulting a doctor for diagnostic evaluations and personalized therapy is the correct path.

Counteracting "waste products" (oxidative stress)

The term "waste products" is often used to refer to free radicals and reactive molecules generated by oxidative metabolism. Endogenous and dietary antioxidants contribute to neutralizing these compounds and protecting biomolecules and cell membranes. Literature shows that a diet rich in natural antioxidants (vitamins C, E, carotenoids, polyphenols) is associated with favorable markers of reduced oxidative stress and cellular health in observational studies; however, supplementation studies with high doses of single antioxidants have produced variable and sometimes counterintuitive results. [7][2]

For this reason, editorial recommendations prioritize dietary intake of antioxidants through fruits and vegetables rather than the systematic use of high-dose supplements. Supplements can be useful in specific and documented situations, but the evidence does not support generalized use for the prevention of chronic diseases in the population with adequate nutritional status. [2]

Key points to remember

• Vitamins are essential cofactors for energy, immune function, and tissue integrity; their effect depends on the initial state and context.
• Vitamin D shows benefits on some respiratory outcomes in deficient subjects; laboratory evaluation is recommended before supplementing. [1]
• Vitamin C supports collagen synthesis and can reduce the duration of some infectious episodes in particular conditions. [3][4]
• A balanced diet remains the main strategy; targeted supplements make sense based on clinical indication. [2]
• Zinc and B6 are important for immune function and energy metabolism; correcting documented deficiencies is a priority. [6][5]

Limitations of evidence

It is fundamental to distinguish between observational studies and causal evidence provided by randomized clinical trials. Observational studies document associations useful for generating hypotheses, but do not prove causality. Reviews and meta-analyses of RCTs provide stronger evidence, but they too can suffer from heterogeneity in doses, formulations, duration, and populations studied. Furthermore, much research does not adequately differentiate between deficient and non-deficient subjects, and long-term data on the effects and safety of high doses are lacking. [1][2][3]

Finally, individual variability (age, health status, medications, intestinal absorption) necessitates personalized evaluation. For these reasons, any interpretation requires caution and, when possible, confirmation with diagnostic tests and clinical consultation.

Editorial conclusion

Vitamins from A to D are essential biological tools to support energy, muscle function, tissue integrity, and immune responses during seasonal changes. Scientific evidence indicates that intervening on documented deficiencies can improve specific health outcomes, while the indiscriminate use of supplements in people with adequate nutritional status is not supported by solid evidence. The practical message for the public is clear: prioritize a varied and balanced diet, evaluate personal factors that increase the risk of deficiencies, and consult a healthcare professional for tests and targeted advice. This approach maximizes benefits and reduces unnecessary risks and expenses.

Editorial note

This article was published in a previous version and is updated according to scientific and divulgative criteria to reflect recent evidence. The text is for informational purposes only and does not replace personalized medical advice; for supplementation choices or therapies, consult your doctor or a qualified healthcare professional.

SCIENTIFIC RESEARCH

1. Martineau AR, Jolliffe DA, Hooper RL et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. https://doi.org/10.1136/bmj.i6583. [1]
2. Gombart AF, Pierre A, Maggini S. A Review of Micronutrients and the Immune System — Working in Harmony to Reduce the Risk of Infection. https://doi.org/10.3390/nu12010236. [2]
3. Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. https://doi.org/10.1002/14651858.CD000980.pub4. (historical review). [3]
4. Hemilä H, Chalker E. Vitamin C reduces the severity of common colds: a meta-analysis. BMC Public Health. https://doi.org/10.1186/s12889-023-17229-8. [3]
5. Pullar JM, Carr AC, Vissers MC. The Roles of Vitamin C in Skin Health. https://doi.org/10.3390/nu9080866. [4]
6. Ueland PM, McCann A, Midttun Ø. Vitamin B6 and Its Role in Cell Metabolism and Physiology. Annual Review of Nutrition. https://doi.org/10.1146/annurev-nutr-071714-034330. [5]
7. Wessels I, Maywald M, Rink L. Zinc as a Gatekeeper of Immune Function. https://doi.org/10.3390/nu9121286. [6]
8. Traber MG, Atkinson J. Vitamin E Regulatory Mechanisms. https://doi.org/10.1146/annurev.nutr.27.061406.093819. [7]
9. Bislev LS, Grove-Laugesen D, Rejnmark L. Vitamin D and muscle health: a systematic review and meta-analysis of randomized placebo-controlled trials. https://doi.org/10.1002/jbmr.4412. [8]

Note: the sources listed above have been selected for relevance and verified by DOI. If some bibliographic details are missing or updated, they are clearly indicated as [placeholder] in the text.