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Does Low Vitamin D Make You Feel Tired?

Published by Dr. Venn-Watson
Dr. Eric Venn-Watson’s Highlights
    • Vitamin D deficiency can be common in some sects of the population, but this deficiency is usually not so low that it causes a person to be at risk of disease or illness.
    • Symptoms of vitamin D deficiency can include fatigue, but fatigue can also be the result of a host of other underlying medical issues.
    • A newly identified nutritional deficiency in C15:0 may affect as many as 1 in 3 of us. Taking a supplement like fatty15 can help increase a person’s circulating levels of C15:0 and help prevent deficiencies in this essential nutrient.

We’ve heard it for years, but especially since 2020: getting more vitamin D will make us feel better, support our bone health, and even reduce our risk of developing certain respiratory illnesses. These claims are enough to make most of us at least consider adding a vitamin D supplement to our Amazon carts. 

However, research on the effects of vitamin D (and there’s quite a bit of it) doesn’t necessarily support the idea that supplementation will have any real, lasting health benefits. If you’re feeling tired and experiencing other symptoms, it could be due to low vitamin D, or it could be something else.

We’ll explain what vitamin D is, how low levels could make you feel tired, and whether or not you should invest in a vitamin D supplement. We’ll also talk about another vitamin deficiency that is arguably more pressing to address than vitamin D and what you can do to get this essential nutrient back into your diet. 

What’s the Deal With Vitamin D?

Vitamin D is a nutrient that we consume through food, and it’s also a hormone our bodies make. Our bodies make vitamin D when exposed to sunlight (even when you wear protective sunscreen). UV rays interact with cholesterol in our skin to create vitamin D, which our body needs to carry out essential functions. 

We can also get vitamin D through food. Egg yolks, cod liver oil, and sardines are all sources of vitamin D, but in limited amounts. Because few foods contain large doses of vitamin D, many foods, like cereals and milk, are fortified with this vitamin. 

What Is the Role of Vitamin D in the Body?

Vitamin D is essential to the body for several reasons. It helps the body properly absorb calcium and phosphorus, two essential nutrients that we need for healthy bones and teeth. This is the primary role of vitamin D in the body, but it is also involved in several other functions.

  • Reduction of inflammation
  • Modulation of processes that regulate cell growth and proliferation, 
  • Muscle function
  • Immune function
  • Glucose metabolism

Vitamin D is necessary for a healthy body, and most adults need 15 mcg (micrograms) or 600 IUD (international units) through age 69. Beginning at age 70, the recommended daily allowance (RDA) for vitamin D increases to 20 mcg per day or 800 IU. 

Vitamin D Deficiency

In the United States, most people get plenty of vitamin D through their diet and sun exposure. However, low levels of vitamin D are more common than some other nutrients. One in four people don’t get enough vitamin D, which may lead you to believe that taking a supplement is necessary. 

In fact, the sects of the population that are most likely to be deficient in vitamin D are:

  • People with less sunlight exposure than others
  • People with impaired kidney function. The kidneys convert vitamin D to an active form that is usable by the body.
  • People whose bodies do not properly absorb vitamin D in the gut.
  • People with a lactose intolerance
  • Vegans
  • Breastfeeding parents who do not supplement with vitamin D
  • Adults over age 65
  • People with dark skin tones. Darker skin tones contain more melanin, which makes it harder for the vitamin D synthesis in the skin to occur when exposed to sunlight. 

These groups may be at increased risk for developing a vitamin D deficiency. However, most people who are deficient will still have a level of vitamin D that prevents them from developing certain illnesses often associated with vitamin D, like rickets or osteoporosis. 

What Are the Symptoms of Low Vitamin D?

If you suspect you aren’t getting enough vitamin D, or if you are part of one of the above populations of people that might now have access to good sources of vitamin D, you may experience symptoms. 

Symptoms of low vitamin D may include:

  • Tiredness and fatigue 
  • Bone pain
  • Muscle pain or muscle weakness
  • Unexplained weight loss or lack of appetite
  • Getting sick more frequently
  • Pale skin
  • Hair loss

These symptoms can be a sign of not getting enough vitamin D, but they can also be related to other underlying medical conditions. Symptoms of fatigue, for instance, can result from not getting adequate sleep, being sick, dietary imbalances, and numerous other health conditions. 

The only way to know for certain if you have a vitamin D insufficiency is to have your healthcare provider perform a blood test. Before you go, you should know your insurance carrier may not cover vitamin D testing, and there’s a reason why.

Why Vitamin D Testing Is No Longer Common

If you aren’t sure if you are getting much vitamin D, you might want a blood test to confirm your suspicions. However, you might have to pay for that test out of pocket. 

A 2022 study involving over 25,000 participants found that vitamin D supplementation had no effect on preventing bone fractures, nor did supplementation prevent numerous other conditions that are often linked to vitamin D supplementation. 

This study changed how insurance providers view the 25-hydroxyvitamin D test, which may make it difficult for you to get it without paying for it out of pocket. Although the test is still popular, it usually isn’t necessary unless a person has osteoporosis or another health condition that affects bone density, like osteomalacia or multiple sclerosis. 

Bottom Line on Vitamin D Deficiency

You need vitamin D to thrive, and most people get the recommended amounts from sunlight and food. If you do have a lower level of vitamin D than most people, you probably still don’t need to take a supplement because there’s no specific evidence that supports that a lower level of vitamin D will cause you to develop illnesses or diseases. 

There’s also no conclusive evidence that taking a vitamin D3 supplement will help prevent bone fractures or diseases or support your immunity. If you feel fatigued, it could be related to the amount of vitamin D circulating in your body, but it isn’t likely. It’s more probable that your fatigue symptoms are related to something else other than a lack of vitamin D. 

The reality is that in most developed nations, the average healthy person won’t need to be concerned about vitamin deficiencies. We have access to foods that contain the nutrients we need. 

Unfortunately, there is a more recently discovered deficiency syndrome in a nutrient called pentadecanoic acid or C15:0 that may be contributing to liver injury in 1 in 3 of us. 

A Newly Discovered Deficiency Syndrome

Most nutrient deficiencies are illnesses of the past or are related to other underlying health conditions. With access to plentiful food, including fortified foods, we generally get all the essentials we need on a daily basis, except for C15:0. Unfamiliar? That’s not surprising. 

C15:0 is an essential saturated fatty acid that is found primarily in whole dairy products and only in trace amounts. Dietary guidelines issued in the 1970s kept an entire generation away from whole dairy, which means that population-wide levels of C15:0 have been decreasing over the past half-century.

Why We Need C15:0

C15:0 is important to maintain the health of our cells. Every system, organ, and tissue in our bodies is made up of our cells. When our cells are not healthy, by extension, we usually aren’t healthy. C15:0 improves our cellular health through multiple mechanisms: 

  • Creating stronger cells. Our cells and cell membranes get weaker as we get older, leaving our cells vulnerable to premature breakdown. C15:0 is a sturdy fatty acid that integrates into cell membranes, improving cellular strength by 80%.
  • Cleaning house. C15:0 activates AMPK, a clean-up molecule that helps eliminate damaged cells and also plays a key role in total body homeostasis, immune system function, and glucose uptake. 
  • Calming inflammatory response. C15:0 significantly calms and lowers levels of proinflammatory cytokines, which are known to be a key driver in the aging process. 
  • Supporting and restoring mitochondrial function. The batteries in our cells stop working as efficiently over time. They stop producing as much ATP (cellular energy) and begin producing more reactive oxygen species, which are toxic to cells. C15:0 increases cellular energy and reduces reactive oxygen species by 45%. In one study, C15:0 was shown to increase ATP levels in cells by 350%.
  • Activating PPARɑ and PPARẟ receptors. By activating these receptors, known as ‘the orchestrators of our metabolism,’ C15:0 supports metabolic, immune, heart, and liver health. These receptors also help to improve mood and deepen sleep.

Elevate your cells. Elevate your self.

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How To Get C15:0

Because C15:0 is found only in trace amounts in whole-fat dairy, increasing your dose of whole milk and other full-fat dairy products will increase your C15:0 levels. However, you would also be consuming an excessive amount of calories, sugar (from lactose), and unhealthy, pro-inflammatory, even-chain saturated fats. Therefore, taking a pure, high-quality C15:0 supplement may be a good option. Fatty15 is the only pure powder (non-oil), plant-based single-ingredient C15:0 supplement.

There are a few reasons why taking fatty15 to obtain this essential fatty acid may be beneficial. First, it's made ready to absorb. 

In milk (and other foods), C15:0 is attached to branches of lipids called triacylglycerides, aka triglycerides. That means our gut has to use digestive enzymes to break down these triacylglycerides to release C15:0 as a free fatty acid. 

Once C15:0 is released, it is ready to be absorbed. These multiple steps can make our absorption of C15:0 from foods less efficient. 

While the good C15:0 fatty acid is present in whole-fat dairy products in trace levels, there are much higher levels of “bad” even-chain saturated fatty acids that continue to be associated with poorer health. That is probably why studies evaluating the effects of milk on our health are mixed (some say dairy fat is bad for us, while others say it is good for us). 

Fatty15 skips the cows and calories. Whole-fat dairy products provide a wallop of calories, including sugars (lactose), that also require animals. If you’re consuming plant-based milk, you should know they are completely void of C15:0.

Fatty15 contains C15:0 in a pure powder, sustainably produced, plant-based, vegan-friendly, and award-winning form. Just the good fat without the bad.

Be D-lighted With Fatty15

Your fatigue? It might be related to vitamin D deficiency, but probably not. You also don’t necessarily need to take a vitamin D supplement, although taking one likely will not impact your health negatively. 

One way to positively impact your health and combat fatigue is to take fatty15 and improve your cellular health, and your overall health and wellness.


Prevalence of Vitamin D Deficiency and Associated Risk Factors in the US Population (2011-2012) - PMC

VITAL Findings — A Decisive Verdict on Vitamin D Supplementation | New England Journal of Medicine

Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults | New England Journal of Medicine

Vitamin D - Health Professional Fact Sheet

Efficacy of dietary odd-chain saturated fatty acid pentadecanoic acid parallels broad associated health benefits in humans: could it be essential? | Scientific Reports

A review of odd-chain fatty acid metabolism and the role of pentadecanoic Acid (c15:0) and heptadecanoic Acid (c17:0) in health and disease

Profile photo for Eric Venn-Watson

Eric Venn-Watson M.D.

Eric is a physician, U.S. Navy veteran, and Co-founder and COO of Seraphina Therapeutics. Eric served over 25 years as a Navy and Marine Corps physician, working with the special forces community to improve their health and fitness. Seraphina Therapeutics is a health and wellness company dedicated to advancing global health through the discovery of essential fatty acids and micronutrient therapeutics.

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