Vitamin D: The Deficiency No One Notices That Affects Everything in Your Body

By Vanessa Manfredini | Biomedical Scientist and Future Nutritionist

Constant fatigue. Low immunity. Muscle aches without apparent cause. Unstable mood. Difficulty losing weight. More hair loss than usual.

These symptoms have many possible causes, but there is a silent nutritional deficiency that appears as a suspect in almost all these conditions and affects over 1 billion people worldwide: vitamin D deficiency.

And the most striking part? Most people with the deficiency don't know they have it. Symptoms are vague, the test is rarely requested in routine consultations, and the solution, when identified, is often simpler than it seems.

Today I'm going to explain what vitamin D truly is, why it's so much more important than just bones, what the symptoms of deficiency are, and what to do to resolve it.

"Vitamin D is not just a vitamin. It's a hormone that regulates over 200 genes in the human body, and most people have low levels of it."

Vitamin D: Vitamin or Hormone?

Technically, vitamin D is classified as a vitamin, but in practice, it acts like a steroid hormone. It is produced in the skin from sun exposure, circulates through the blood, is activated in the liver and kidneys, and acts on receptors spread throughout almost all body tissues.

There are two main forms:

• Vitamin D2 (ergocalciferol) - obtained from plant-based foods and some fungi
• Vitamin D3 (cholecalciferol) - produced in the skin through sun exposure and found in animal-based foods. It is the most efficient form for raising blood levels

The test that measures vitamin D in the blood is 25(OH)D 25-hydroxyvitamin D. Reference values vary slightly between laboratories, but in general:

• Below 20 ng/mL - deficiency
• Between 20 and 29 ng/mL - insufficiency
• Between 30 and 100 ng/mL - sufficiency
• Above 100 ng/mL - potential toxicity

Many functional and integrative medicine specialists consider the ideal level for optimal health to be between 40 and 60 ng/mL, above the minimum recommended by conventional medicine.

Why is Vitamin D Deficiency So Common?

Even though Brazil is a tropical country with abundant sun, vitamin D deficiency is surprisingly prevalent. The reasons are several:

• Increasingly indoor lifestyle - office work, home office, shopping malls, indoor gyms
• Constant use of sunscreen - necessary to prevent skin cancer, but blocks vitamin D synthesis
• Pollution in large cities - reduces UVB ray penetration
• Diet poor in vitamin D sources
• Obesity - adipose tissue sequesters vitamin D, reducing its availability
• Aging - skin loses efficiency in synthesis with age
• Darker skin - higher melanin content reduces cutaneous synthesis
• Conditions that impair intestinal absorption - such as dysbiosis and leaky gut

What is Vitamin D For, Far Beyond Bones

Most people associate vitamin D only with bone health. This is correct, but it's just the tip of the iceberg.

Immune System

Vitamin D is one of the most important regulators of the immune system. It modulates both innate and adaptive immunity, reduces the production of inflammatory cytokines, and plays a protective role against respiratory infections, which became very evident during the Covid-19 pandemic, when studies showed that people with vitamin D deficiency had more severe forms of the disease.

Bone and Muscle Health

Vitamin D is essential for the intestinal absorption of calcium and phosphorus, the minerals that form bones. Without it, even if you consume a lot of calcium, it will not be well absorbed. In muscles, it participates in muscle contraction and the prevention of falls in the elderly.

Mental Health and Mood

Vitamin D receptors are present in the brain, especially in areas linked to mood and cognition. Studies show an association between vitamin D deficiency and depression, anxiety, and cognitive decline. Supplementation in deficient individuals can improve mood, especially during winter months with less sun.

Metabolism and Body Weight

Vitamin D influences insulin sensitivity, adipose tissue function, and appetite regulation. Studies show that obese individuals tend to have lower vitamin D levels and that supplementation can contribute to weight loss as part of a broader protocol.

Cardiovascular Health

Adequate vitamin D levels are associated with a lower risk of hypertension, heart attack, and stroke. It regulates the renin-angiotensin system, which controls blood pressure, and has an anti-inflammatory action on blood vessel walls.

Autoimmune Diseases

Here, the connection is especially important and directly linked to what we discussed in the posts about Hashimoto's and PCOS. Vitamin D plays a fundamental regulatory role in the immune system, and its deficiency is associated with a higher risk of developing autoimmune diseases such as multiple sclerosis, lupus, rheumatoid arthritis, Crohn's disease, and yes, Hashimoto's thyroiditis.

Gut Health and Microbiota

Vitamin D receptors are present in intestinal cells. It participates in maintaining the intestinal barrier, regulates the local immune response, and modulates microbiota composition. Vitamin D deficiency is associated with increased intestinal permeability and dysbiosis, creating a cycle, as dysbiosis, in turn, impairs vitamin D absorption.

"Vitamin D doesn't just take care of your bones. It takes care of your immune system, your hormones, your mood, your gut, and your heart."

Symptoms of Vitamin D Deficiency

Symptoms are varied and often confused with other conditions:

Most Common Symptoms

• Persistent fatigue and tiredness even after sleeping well
• Bone pain, especially in the back, hips, and legs
• Diffuse muscle aches and muscle weakness
• Low immunity, frequent infections, and slow recovery
• Depression and mood changes, especially in winter
• More hair loss than normal

Less Common Symptoms

• Slow wound healing
• Frequent headaches
• Difficulty concentrating and brain fog
• Recurrent urinary tract infections
• Worsening of autoimmune conditions
• Difficulty losing weight even with diet

How to Get Vitamin D

Sun: The Most Efficient Source

Sun exposure is responsible for about 80 to 90% of the vitamin D your body produces. For efficient synthesis:

• Expose arms and legs; the larger the exposed area, the more vitamin D is produced
• Ideal time in Brazil: between 10 AM and 3 PM when UVB rays are most intense
• Required time: 15 to 30 minutes per day for fair-skinned individuals; more time for darker skin
• No sunscreen during this period; apply afterward
• Glass blocks UVB rays; sun through a window does not produce vitamin D

Food Sources of Vitamin D

Foods contribute only 10 to 20% of the requirements, which is why sun exposure and supplementation are so important. The best food sources:

• Canned sardines - one of the richest and most accessible sources in Brazil
• Fresh or canned salmon
• Canned tuna
• Egg yolk - especially from free-range chickens
• Sun-exposed mushrooms - shiitake, shimeji, the only relevant plant source
• Fortified milk and yogurt - check the label

Supplementation

For most people with confirmed deficiency, supplementation is necessary because it is very difficult to achieve adequate levels with just sun and diet. Some important points:

• Prefer vitamin D3 (cholecalciferol) - more efficient than D2
• Combine with vitamin K2 - essential for directing calcium to the bones and not to the blood vessels
• Take with a meal containing fat - vitamin D is fat-soluble and needs fat to be absorbed
• The dose and duration of supplementation should be determined based on your test; there is no universal dose
• Monitor levels every 3 months during supplementation

Vitamin D and Women's Health: Special Attention

For women, vitamin D deserves attention at specific times in life:

• PMS and menstrual cycle - adequate vitamin D levels reduce cramps and PMS symptoms
• Pregnancy - deficiency during pregnancy increases the risk of pre-eclampsia, gestational diabetes, and premature birth
• Menopause - fundamental for bone health and reducing the risk of osteoporosis
• Hashimoto's and PCOS - as we saw in previous posts, vitamin D plays an important modulating role in both conditions

Conclusion

Vitamin D is one of the most important nutrients for human health and one of the most deficient in the Brazilian population, despite our generous sun. It doesn't just take care of bones. It takes care of the immune system, hormones, mood, gut, and heart.

If you don't know your vitamin D level, this is a simple test worth including in your next consultation. Identifying and correcting a deficiency can be a game-changer for your health, especially if you live with fatigue, low immunity, mood changes, or autoimmune conditions.

Take care of your sun exposure, take care of your diet, and, when necessary, supplement with guidance. Your body will thank you. 🌞

"It's not just a vitamin. It's a hormone your body needs to produce every day, and modern life is preventing it."

Did you like this content?

Leave your comment below: have you measured your vitamin D level? Were you surprised by the result? Tell me! And follow me on Instagram @vanessamanfredini.nutri for more content on women's health and evidence-based nutrition! 🌿

Shall we learn a little more? Read about Hashimoto's and Diet here.

With care and science, Vanessa 🌹

This content is based on scientific studies and guidelines on vitamin D. Main references:

1. Holick MF. Vitamin D deficiency. New England Journal of Medicine, 2007.
2. Pludowski P, et al. Vitamin D supplementation guidelines. The Journal of Steroid Biochemistry and Molecular Biology, 2018.
3. Brazilian Society of Endocrinology and Metabolism (SBEM). Position Statement on Vitamin D.
4. Amrein K, et al. Vitamin D deficiency 2.0: an update on the current status worldwide. European Journal of Clinical Nutrition, 2020.

The information is for educational purposes and does not replace individualized professional guidance. The dose and need for supplementation should be evaluated with tests and professional guidance.