Vitamin D: the silent deficiency affecting 75% of French people

Vitamin D holds a unique place among micronutrients. Unlike other vitamins, it does not simply act as an enzyme cofactor or antioxidant: it behaves as a steroid hormone. Once synthesized in the skin under the action of ultraviolet rays or brought in through food, it is converted in the liver and then the kidneys into its active form, calcitriol, which regulates the expression of more than 200 genes in the body.

This characteristic explains the breadth of its biological actions. Vitamin D is essential for the intestinal absorption of calcium and phosphorus, two minerals critical for bone and dental strength. It regulates cell differentiation, plays a role in the prevention of certain cancers, modulates the immune system, and is involved in regulating mood, sleep, and blood sugar metabolism.

Two main forms exist: vitamin D2 (ergocalciferol), of plant origin, and vitamin D3 (cholecalciferol), synthesized by the skin or provided by animal products. D3 is significantly more effective than D2 at raising blood levels and is now considered the reference form for supplementation.

France sits between the 41st and 51st north parallels. At these latitudes, especially north of Lyon, the angle of solar rays is insufficient between October and April to allow vitamin D synthesis in the skin, even with prolonged exposure. For nearly six months each year, the French sun is simply not powerful enough to trigger vitamin D production in our skin.

But geography is not the only factor. Even during summer months, most French people do not get enough sun exposure to meet their needs. Indoor work, wearing covering clothing, systematic use of high-SPF sunscreen, and concerns about sun-related risks all drastically reduce effective exposure time. A study published in the European Journal of Clinical Nutrition estimated that 75% of French adults had vitamin D levels below the optimal threshold of 30 ng/mL.

Skin color also plays a role. People with darker skin need two to six times more sun exposure than lighter-skinned individuals to synthesize the same amount of vitamin D, putting populations of African, Caribbean, or North African origin living in France at greater risk of severe deficiency.

On the dietary side, vitamin D is naturally present in very few foods: mainly fatty fish, eggs, and certain organ meats. Unlike countries such as the United States or the Nordic countries, France does not have a systematic policy of fortifying foods with vitamin D. As a result, for the vast majority of the French population, diet alone cannot compensate for insufficient sun exposure.

Vitamin D deficiency is often called the invisible illness, and for good reason: its symptoms are vague, gradual, and easy to attribute to other causes. Many deficient individuals complain of persistent fatigue that does not improve despite adequate sleep, without ever connecting it to low vitamin D levels. This fatigue is real and documented: vitamin D plays a role in mitochondrial function, the energy machinery of our cells.

Diffuse muscle and bone pain is another frequent sign. Deficient individuals often describe a heaviness in the legs, aches without particular physical effort, or bone tenderness to the touch in the shins or sternum. These pains can easily be confused with fibromyalgia or simple overexertion.

Increased susceptibility to infections is also characteristic. Vitamin D stimulates the production of antimicrobial peptides by immune cells. When deficient, this first line of defense is weakened, and respiratory infections, colds, and flu episodes become more frequent and prolonged. Several meta-analyses have shown that vitamin D supplementation significantly reduced the risk of respiratory infections, especially in people with severe deficiency.

Finally, in some people, deficiency manifests as mood disturbances: unusual irritability, a depressive state that intensifies in winter, loss of motivation. This is linked to vitamin D's role in regulating serotonin, a key neurotransmitter for emotional balance. Seasonal affective disorder, the mild depression affecting many people in autumn and winter, is partly associated with falling vitamin D levels.

When vitamin D deficiency persists over time, its effects on the body extend far beyond temporary fatigue. The most well-known consequence involves bone strength. Vitamin D is essential for intestinal calcium absorption: without it, even adequate calcium intake cannot be properly utilized. In adults, prolonged deficiency contributes to osteoporosis and increases fracture risk, which is particularly dangerous in older individuals.

The link between vitamin D and the cardiovascular system is increasingly well documented. Vitamin D receptors are present in heart muscle and blood vessel cells. Observational studies associate low vitamin D levels with a higher prevalence of high blood pressure, heart failure, and cardiovascular events.

On the metabolic front, growing research suggests that vitamin D plays a role in insulin sensitivity and blood sugar regulation. People with vitamin D deficiency have an increased risk of developing type 2 diabetes and often display a more pronounced metabolic syndrome. The mechanism appears related to gene expression involved in insulin signaling in pancreatic cells and adipose tissue.

A link with excess weight has also been identified. Vitamin D is a fat-soluble vitamin: it is stored in adipose tissue. In overweight individuals, vitamin D becomes sequestered in body fat and less available to other tissues, creating a vicious cycle where excess fat worsens the deficiency and the deficiency, by disrupting metabolism and mood, can itself complicate weight loss.

Vitamin D has the peculiarity of being present in a very limited number of foods, making it difficult to cover needs through diet alone. The most concentrated sources are undoubtedly fatty fish. Salmon, mackerel, herring, sardines, and tuna top the list, with contents ranging from 300 to 1,200 IU per 100 g depending on the species and preparation method. Eating two to three servings of fatty fish per week is one of the best dietary strategies for supporting vitamin D status.

Cod liver oil deserves special mention: it is the most concentrated food source of vitamin D3, with approximately 10,000 IU per 100 ml. Used for centuries in Nordic countries to prevent rickets, it remains one of the most potent dietary sources today, though it should be consumed in moderation due to its high vitamin A content.

Eggs are a more modest but easily integrated daily source: one whole egg provides around 40 to 50 IU of vitamin D, concentrated in the yolk. To maximize this contribution, opt for eggs from free-range hens exposed to sunlight, which can contain two to three times more vitamin D than those from caged hens.

Certain mushrooms, particularly shiitake, can synthesize vitamin D2 when exposed to ultraviolet rays. A simple tip: leaving your mushrooms to dry in the sun for a few hours before cooking can multiply their vitamin D content by five to ten. Finally, some fortified foods such as milk, yogurt, or breakfast cereals can contribute to daily intake for those who consume them regularly.

Given the near-impossibility for most French people to cover their vitamin D needs through sun exposure and diet alone, supplementation is often unavoidable, at least during the autumn-winter period. French health authorities recommend systematic supplementation for children, the elderly, pregnant and breastfeeding women, and individuals with darker skin or limited sun exposure.

The recommended form is vitamin D3 (cholecalciferol), significantly more effective than D2 at raising blood levels of 25-OH vitamin D, the reference marker. Daily supplementation at moderate doses (800 to 2,000 IU) is generally considered more physiological and more effective than a monthly bolus. High-dose ampules are also available by medical prescription to correct severe deficiencies.

Since vitamin D3 is fat-soluble, it is absorbed alongside dietary fats. Taking your supplement with your main meal, the one richest in healthy fats, significantly optimizes its absorption. This practical recommendation is often overlooked but can make a real difference in supplementation effectiveness.

An important point concerns the combination with vitamin K2. Vitamin D promotes calcium absorption, but it is vitamin K2 that directs this calcium toward bones rather than arteries. For those supplementing at high doses, a D3 and K2 combination (as MK-7) is recommended by many experts to prevent calcium deposits in blood vessels. Safe doses for a healthy adult generally range from 800 to 4,000 IU per day, the upper tolerable limit set by European authorities.

For people who practice intermittent fasting, vitamin D deserves particular attention. Like all fat-soluble vitamins, it is absorbed alongside dietary fats and stored in fatty tissues. Its bioavailability is therefore directly linked to the quality and composition of meals eaten during the eating window.

A short eating window, typical of the 16/8 protocol, concentrates all nutritional intake into 8 hours. This is both a constraint and an opportunity. A constraint because it is important to include vitamin D sources and healthy fats in each meal. An opportunity because this concentration encourages greater attention to the nutritional quality of each plate.

On the metabolic level, intermittent fasting improves insulin sensitivity and reduces systemic inflammation, two mechanisms that interact positively with vitamin D. Preliminary studies suggest that the improvement in insulin sensitivity induced by fasting may enhance cellular utilization of vitamin D, amplifying its beneficial effects on blood sugar regulation and lipid metabolism.

The loss of body fat associated with regular intermittent fasting practice offers another indirect advantage: by reducing adipose tissue, it decreases the sequestration of vitamin D in fat stores and increases its availability to other organs. This is one mechanism by which weight loss is often accompanied by a spontaneous improvement in vitamin D status, even without changing intake. To make the most of this synergy, consume your vitamin D sources alongside healthy fats such as olive oil, avocado, or nuts at your main meal.

The information in this article is provided for educational and informational purposes only. It does not constitute medical advice and does not replace consultation with a doctor or qualified healthcare professional.

If you think you may have a vitamin D deficiency, a blood test measuring 25-OH vitamin D levels is the only examination that can establish a reliable diagnosis. Consult your doctor before starting high-dose supplementation, as vitamin D overdose, though rare, can cause hypercalcemia with serious effects. Certain medical conditions and medications can affect vitamin D metabolism. Intermittent fasting is not suitable for everyone, particularly those with eating disorders, certain chronic conditions, or specific medical treatments.