A vitamin-packed summer: when food isn't enough, supplements come to the rescue

Editorial Note

This article was previously published and has been updated according to scientific and informative criteria to reflect current literature. The text is for informational purposes only and does not replace medical advice: for therapeutic or integrative choices, consult a healthcare professional.

In brief

• Some vitamins and minerals commonly used as supplements (vitamin D, vitamin C, zinc, selenium, magnesium, potassium, multivitamin complexes, and curcumin) show variable effects: utility depends on initial status, dose, and form of administration.
• Vitamin D has clinical evidence of reducing the risk of respiratory infections, especially in deficient individuals; vitamin C and zinc may reduce the duration of cold symptoms in selected contexts.
• Supplementation is not universally necessary: effects are more often observed in people with documented deficiencies or specific conditions (intense physical activity, sweating, insufficient sun exposure).
• Clinical evidence has limitations: heterogeneity of doses, formulations, and populations makes it necessary to interpret results with caution.
• Before starting regular supplementation, assess nutritional status, ongoing therapies, and possible contraindications with a healthcare professional.

Abstract: what does science say?

Supplementation with vitamins and minerals can be useful when the diet does not meet requirements or in conditions of increased consumption/loss (sweating, intense sports activity, malabsorption, reduced sun exposure). Literature shows that vitamin D can modestly reduce the risk of respiratory infections in deficient individuals; vitamin C and zinc have limited and contextual effects on the duration of colds. Other micronutrients (selenium, magnesium, potassium) and phytocompounds like curcumin have biological plausibility and some experimental evidence, but clinical results are variable. Evidence strongly depends on dose, form, baseline nutrient status, and quality of studies; therefore, recommendations must be personalized and based on clinical and laboratory control.

Why supplementation can be useful in summer

Summer brings significant changes: increased sun exposure, but also more outdoor physical activity, intense sweating, and dietary variations. In some cases, these factors promote optimal nutritional status; in others, they can increase the need for or loss of essential elements (salts, magnesium, potassium), making supplementation useful. Furthermore, the nutritional quality of foods can be compromised by preservation and industrial processes, and some micronutrients are difficult to obtain in high quantities through diet alone. The decision to supplement must consider baseline status (blood tests when indicated), the extent of losses (e.g., significant sweating), the type of physical activity, and concomitant medical conditions. The chemical form of the supplement (e.g., vitamin D3 vs. D2, different zinc or magnesium salts) affects absorption and effect; for this reason, many guidelines and randomized studies evaluate formulations separately. In conclusion, summer supplementation can be useful when there is a documented deficiency or increased need, but it is not a "one-size-fits-all" solution.

Main section

Definition and general overview

Dietary supplementation refers to concentrated products of nutrients (vitamins, minerals, amino acids, plant extracts) administered to complement the diet. Their role is not to cure specific diseases but to support nutritional status. Epidemiological and experimental evidence shows that the actual benefit depends on the starting point: those who are deficient are more likely to achieve measurable effects, while people with adequate intake often show no significant clinical improvements. Furthermore, the effect depends on the dose, duration of intake, and form of the nutrient, with significant differences in terms of bioavailability and safety.

Quality of evidence: what do we really know?

For some nutrients, large-scale meta-analyses and randomized trials exist. For example, vitamin D supplementation has shown a modulated reduction in the risk of respiratory infections, especially in subjects with low baseline 25(OH)D levels; the effect is less evident or absent in populations with adequate levels. Similarly, systematic reviews indicate that vitamin C can modestly shorten the duration of colds in some populations and that zinc, in specific forms and doses, can reduce symptom duration when started early. For other elements (selenium, magnesium, potassium), the literature presents heterogeneous results: biological plausibility and experimental studies coexist with inconsistent clinical evidence. Even large trials on multivitamins show limited effects on chronic diseases in the general population; the greatest impact remains in correcting documented deficiencies.

The most relevant nutrients for summer: what studies say

Vitamin D: immune role and supplementation

Vitamin D plays roles in bone metabolism and immune system modulation. A meta-analysis of individual data highlighted that vitamin D supplementation significantly reduces the risk of acute respiratory infections, especially in subjects with low circulating 25(OH)D levels at the start of the study; the effect is less evident in those with adequate levels. This implies that the decision to supplement should start with an assessment of vitamin status and a dose target based on documented deficiency rather than generalized intake. The form (cholecalciferol D3) and dosage (correct dosages and monitoring) are important for efficacy and safety [1].

Vitamin C: antioxidant and cold duration

Vitamin C is an antioxidant involved in collagen synthesis and mucosal defenses. Systematic reviews indicate that regular vitamin C intake does not reduce the incidence of the common cold in the general population, but it can modestly reduce its duration and severity in some situations (for example, subjects undergoing intense physical stress). Therapeutic evidence (starting from symptom onset) is less robust and requires further studies to define effective doses and administration times. Safety is generally good at moderate doses, but very high doses should be evaluated on a case-by-case basis [2].

Zinc: evidence on cold prevention and treatment

Recent studies and reviews indicate that zinc, in particular formulations (e.g., lozenges based on acetate or gluconate) and when started within 24 hours of symptom onset, can reduce the average duration of the common cold by about a couple of days. However, the heterogeneity of doses, forms, and quality of studies, and the risk of local adverse effects (unpleasant taste, irritation) necessitate caution. Intranasal use of zinc is not recommended due to the risk of anosmia. The decision to use zinc should consider expected benefits, possible side effects, and limitations of the evidence [3].

Selenium, magnesium, and potassium: specific roles and limitations of evidence

Selenium is essential for several selenoproteins involved in immune response and oxidative stress. Recent reviews of experimental trials show heterogeneous effects: benefits on some immune functions but absence of clear clinical effects in heterogeneous trials, suggesting that the advantage is limited to deficiency states or specific doses/forms [5]. Magnesium is important for muscle contraction, energy metabolism, and fatigue reduction; reviews show conflicting results for cramps and a possible benefit in reducing muscle soreness after exercise in athletes or active individuals, but there is no robust evidence for generalized use [6]. Finally, potassium is relevant for blood pressure control; meta-analyses of clinical trials indicate that an increase in potassium intake can reduce blood pressure in hypertensive subjects, especially in the presence of high sodium consumption [7].

Curcumin: anti-inflammatory potential and bioavailability issues

Curcumin, the active ingredient in turmeric, has a solid preclinical basis as an antioxidant and anti-inflammatory. Recent reviews report favorable effects on inflammatory parameters in selected conditions, but its very poor oral bioavailability limits the clinical effect when taken in a non-optimized form. Formulations with agents that increase absorption (e.g., piperine) show greater bioavailability and are the subject of clinical studies; however, the translation into relevant clinical effects requires further controlled data [4].

Practical section

What this means in practice

In practice, considering summer supplementation means first assessing the starting point: tests (for example, 25(OH)D, electrolytes) when indicated, review of diet and losses (sweating, physical activity). For those with documented deficiencies, targeted supplementation is supported by evidence and can improve clinical parameters (e.g., vitamin D for bones and immunity). For general prevention in individuals with adequate nutritional status, the evidence does not justify systematic mass supplementation. Choose products with clear labels, biologically active formulations (e.g., vitamin D3), doses compatible with national recommendations, and monitor over time. Avoid excesses: doses above tolerable limits can cause adverse effects (e.g., excess vitamin D, long-term zinc leading to copper deficiency). Finally, before administering supplements to children, pregnant women, people with chronic diseases, or those on medication, consult a healthcare professional.

Key takeaways

• Correction of documented deficiencies is the strongest indication for supplementation.
• Vitamin D: greater benefits in deficient subjects; dosage and form matter [1].
• Vitamin C and zinc can modestly reduce the duration of the common cold in selected contexts [2][3].
• Selenium, magnesium, and potassium have important physiological roles but heterogeneous clinical evidence; greater advantages where there is a deficiency [5][6][7].
• Multivitamins have not shown clear reductions in mortality or cardiovascular events in the general population: large trials agree on limited benefits for primary prevention [8].

Limitations of evidence

It is essential to distinguish between observational associations, biological plausibility, and causal evidence obtained from randomized clinical trials. Many observational studies report associations between nutrient levels and health, but they do not establish causality. Among the most frequent methodological limitations: heterogeneity of doses and formulations, small trial sizes, short follow-up, selection of unrepresentative populations, and poor standardization of outcomes. Even in RCTs, results depend on the participant's baseline status (those who are deficient respond better), the type of administration, and the duration. Therefore, interpreting benefits as generalizable to the entire population is incorrect: personalized approaches and more high-quality studies are needed to define optimal doses and formulations.

Editorial conclusion

For summer, supplementation can be a useful tool to fill deficiencies or compensate for increased nutrient losses, but it is not a panacea. Choices should be guided by clinical evaluation, laboratory data when necessary, and knowledge of the evidence: the best practice remains a good diet combined with targeted and documented use of supplements. For personalized decisions, rely on doctors and nutrition professionals.

Editorial note (bottom of article)

The article has been updated according to criteria of scientific rigor and editorial transparency. The information provided is for informational purposes and does not replace clinical advice. Before undertaking prolonged supplementation, 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. BMJ. 2017;356:i6583. https://doi.org/10.1136/bmj.i6583 [1]
2. Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;2013(1):CD000980. https://doi.org/10.1002/14651858.CD000980.pub4 [2]
3. Ongoing Cochrane review: Zinc for the prevention and treatment of the common cold. Cochrane Database Syst Rev. 2023;CD014914. https://doi.org/10.1002/14651858.CD014914.pub2 [3]
4. Hewlings SJ, Kalman DS. Curcumin: A review of its effects on human health. Foods. 2017;6(10):92. https://doi.org/10.3390/foods6100092 [4]
5. Filippini T, Fairweather-Tait S, Vinceti M. Selenium and immune function: a systematic review and meta-analysis of experimental human studies. Am J Clin Nutr. 2023; (online) doi:10.1016/j.ajcnut.2022.11.007. https://doi.org/10.1016/j.ajcnut.2022.11.007 [5]
6. Scoping review: Role of Magnesium in skeletal muscle health and neuromuscular diseases. Int J Mol Sci. 2024;25(20):11220. https://doi.org/10.3390/ijms252011220 [6]
7. Aburto NJ, Hanson S, Gutierrez H, et al. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ. 2013;346:f1378. https://doi.org/10.1136/bmj.f1378 [7]
8. Multivitamins in the prevention of cancer and cardiovascular disease: the COcoa Supplement and Multivitamin Outcomes Study (COSMOS) randomized clinical trial. Am J Clin Nutr. 2022;115:1501–1510. https://doi.org/10.1093/ajcn/nqac056 [8]