Journal

Key micronutrients. Vitamin D, vitamin C, zinc, omega-3: understanding their real biological role.

Introduction — From nutrient intake to cellular regulation

For a long time, micronutrients were approached primarily through a quantitative lens: meeting recommended daily intakes, preventing deficiencies, correcting measurable shortfalls. This approach has delivered major public-health gains, yet it is no longer sufficient to explain a now well-documented reality: persistent fatigue, increased susceptibility to infections, chronic low-grade inflammation or metabolic disturbances can persist despite a diet considered “adequate” and theoretically sufficient nutrient intakes [1–4].

Findings from cellular biology, immunometabolism and ageing research have fundamentally shifted this perspective. A micronutrient is not merely an “input”, but a biological regulator whose effectiveness depends on how it is integrated into complex cellular networks, its synergies with other nutrients, and the broader physiological context (inflammation, oxidative stress, digestive function, mitochondrial activity).

Vitamin D — An immuno-metabolic regulatory hormone

Vitamin D cannot be reduced to its skeletal role. Its active form behaves like a steroid hormone through the vitamin D receptor (VDR), which is expressed in many tissues—including immune, muscle, intestinal and brain cells [1–3].

Experimental and clinical evidence indicates that vitamin D participates in modulating innate and adaptive immunity, regulating cytokine production, maintaining systemic inflammatory balance, and supporting certain metabolic functions [1–3]. Molecular biology studies have shown that VDR regulates the expression of hundreds of genes involved in these processes, confirming its central role in cellular signalling [2].

Where to find vitamin D

Cutaneous synthesis (primary source)
Exposure of the skin to UVB radiation enables endogenous vitamin D synthesis. This production depends strongly on latitude, season, age, skin pigmentation and the use of sun protection. In Europe, synthesis is insufficient for several months of the year.

Dietary sources
Dietary intake is generally modest but contributes to overall status:
fatty fish (sardines, mackerel, herring, salmon), cod liver and cod-liver oil, egg yolk, fortified dairy products, and certain UV-exposed mushrooms.

The high prevalence of vitamin D insufficiency highlights that biological availability depends not only on intake, but also on the body’s capacity to activate and utilise this hormonal signal.

Practical box — Vitamin D

At-risk profiles and common situations of deficiency

• Low sun exposure (autumn–winter, indoor work, higher latitudes).
• Darker skin* or older age (reduced cutaneous synthesis).
• Overweight and obesity (sequestration in adipose tissue).
• Digestive or hepato-biliary disorders affecting absorption and activation.
• Chronic low-grade inflammation, which reduces the effectiveness of vitamin D signalling.

*With the same level of sun exposure:

• lightly pigmented skin synthesises vitamin D more quickly,
• heavily pigmented skin requires longer exposure to produce an equivalent amount of vitamin D.

Vitamin C — Antioxidant, enzymatic cofactor and tissue support

Vitamin C is an essential water-soluble micronutrient involved in multiple biological functions. It acts as an antioxidant, but also as an enzymatic cofactor in key metabolic pathways—particularly those related to collagen, catecholamines and the stress response [4].

Available evidence suggests it contributes to neutralising oxidative stress, supporting innate and adaptive immunity, and maintaining the integrity of connective tissues and biological barriers [4].

Where to find vitamin C

Main plant sources
Fresh fruits and vegetables are the primary source: citrus fruits, kiwi, strawberries, blackcurrants, peppers, broccoli, cabbages, parsley, spinach.

Factors influencing real-world intake
Vitamin C is sensitive to heat, oxidation and prolonged storage. Cooking, refining and time significantly reduce its content, which helps explain why “theoretical” intakes may translate into insufficient biological availability.

Recent epidemiological data also remind us that severe deficiencies can still occur in contexts of deprivation or highly unbalanced diets [12,13].

Practical box — Vitamin C

At-risk profiles and common situations of deficiency

• Diet low in fresh fruits and vegetables.
• High consumption of ultra-processed foods.
• Smoking (increased requirements).
• Chronic stress, recurrent infections, high-intensity physical activity.
• Food insecurity or intestinal malabsorption [12,13].

Zinc — Enzymatic cofactor and immune regulator

Zinc is involved in several hundred enzymatic reactions and plays a structural role in cellular signalling. It is essential for the maturation of immune cells, regulation of inflammation, wound healing, and maintenance of mucosal integrity [5–7].

Even a moderate deficiency is associated with impaired immune response and increased susceptibility to infections [6]. Subclinical situations are common and often undiagnosed.

Where to find zinc

Animal sources (high bioavailability)
Oysters and seafood, red meat, organ meats, poultry, eggs, dairy products.

Plant sources
Legumes, seeds (pumpkin, sesame), nuts, whole grains.
The bioavailability of plant-based zinc is lower due to phytates, which limit absorption.

Zinc status therefore depends not only on intake, but also on dietary quality, digestion and mineral interactions.

Practical box — Zinc

At-risk profiles and common situations of deficiency

• Vegetarian or vegan diets that are not properly optimised.
• Chronic digestive disorders (malabsorption, dysbiosis).
• Chronic stress and persistent inflammation.
• Recurrent infections, slow wound healing, hair loss.
• Older adults (reduced intake and absorption).

Omega-3 — Structural lipids and mediators of inflammation resolution

Long-chain omega-3 fatty acids, EPA and DHA, are essential components of cell membranes and precursors of lipid mediators involved in modulating and resolving inflammation [8,9].

They influence membrane fluidity, cellular communication, and cardiovascular, brain and immune function. Clinical findings vary by population, dose and metabolic context, which helps explain the sometimes heterogeneous results seen in intervention studies [10,11].

Where to find omega-3

Marine sources (EPA and DHA)
Fatty fish: sardines, mackerel, herring, anchovies, salmon.
Seafood to a lesser extent.

Plant sources (ALA, precursor)
Flax seeds, chia seeds, walnuts, rapeseed oil and flaxseed oil.
Conversion of ALA to EPA and DHA in humans is limited, making marine sources particularly relevant for these active forms.

The quality, freshness and oxidative stability of lipid sources strongly determine their biological value.

Practical box — Omega-3

At-risk profiles and common situations of deficiency

• Low intake of fatty fish.
• Marked omega-6/omega-3 imbalance (Western diets).
• Chronic low-grade inflammation.
• Higher cardio-metabolic risk terrain.
• Pregnancy and breastfeeding (increased DHA needs).

Synergies, biological context and real-world effectiveness

Vitamin D, vitamin C, zinc and omega-3 never act in isolation. Their effectiveness depends on their functional interactions, the state of low-grade inflammation, the integrity of the microbiota and intestinal barrier, and mitochondrial function.

A strictly additive approach does not restore biological balance in a lasting way. Cellular nutrition depends on the body’s capacity to integrate these micronutrients into coherent, functional networks.

The METHODE ESPINASSE approach

At METHODE ESPINASSE, micronutrients are approached as levers of cellular regulation—integrated into coherent strategies grounded in scientific evidence and an understanding of the underlying biological mechanisms.

The goal is not to artificially increase intake, but to restore the cell’s capacity to use what it receives, taking into account chronic inflammation, oxidative stress, digestive function and overall metabolic balance.

Conclusion — From nutritional intake to cellular function

Vitamin D, vitamin C, zinc and omega-3 are not standalone solutions, but powerful biological levers when understood and used within a coherent framework.

The central question is not only where to find them, but whether the cellular environment allows their effective use. This shift—from nutrient intake to real cellular function—is the vision carried by METHODE ESPINASSE.