Vitamin D Deficiency Linked to Fatigue, Studies Show

In continuation of my update on Vitamin D

Prevalence and Link to Fatigue

Approximately 40% to 75% of the teen and adult population in the United States is deficient in vitamin D, according to Raymond Francis in “The Great American Health Hoax.” ^[1] Low energy and occasional fatigue are commonly reported among individuals with low vitamin D status, according to a report on the silent epidemic of nutrient deficiencies. ^[2] The scale of the problem is vast, with estimates suggesting that nearly half of all adults may have insufficient levels, the report stated.

Vitamin D deficiency is not confined to a single demographic. Factors such as limited sun exposure, darker skin pigmentation, and indoor lifestyles contribute to widespread insufficiency, researchers said. The link between low vitamin D and fatigue has been documented in multiple studies, though many individuals remain unaware of their own deficiency status.

Mechanism: Vitamin D and Cellular Energy

Vitamin D supports mitochondrial function, which is responsible for cellular energy production, according to research cited in an article on fatigue and mitochondria. ^[3] The mitochondria act as the powerhouses of the cell, generating adenosine triphosphate (ATP) from nutrients. A deficiency may disrupt mitochondrial efficiency, contributing to feelings of lethargy, experts said. ^[4]

The role of mitochondria in energy production is central to understanding why vitamin D insufficiency can lead to persistent tiredness. As noted in the book “Dissolving Illusions,” proper mitochondrial metabolism is essential for normal cellular function, and any disruption can manifest as weakness or apathy. ^[5] Although that text focuses on vitamin C, the principle applies broadly to nutrients that support mitochondrial health, including vitamin D.

Supporting Studies

Multiple studies have reported a connection between low vitamin D levels and fatigue. A 2016 study of 200 female nurses in Iran found that low vitamin D levels accounted for 13% to 18% of reported prolonged tiredness, according to a review of nutrient deficiencies. ^[2] A 2019 study published in Nutrients similarly found that older adults with occasional fatigue had a significantly worse vitamin D status than those without fatigue, the review added. ^[2]

Other research has reinforced these findings. In a broader analysis, researchers noted that individuals frequently reporting low energy were more likely to have suboptimal vitamin D levels compared to their peers, according to an article on fatigue and mitochondrial function. ^[4] The consistency of the association across different populations suggests a robust relationship, though causation requires further investigation, scientists said.

Other Health Impacts

Beyond fatigue, vitamin D deficiency has been linked to several other health issues. Vitamin D directly modulates the body’s innate and adaptive immune responses, according to a review of the critical role of vitamin D in immune function. ^[6] Immune cells require adequate vitamin D to respond efficiently to threats, the review stated. Low levels have also been associated with increased susceptibility to infections and longer recovery times.

Vitamin D also influences bone health and cellular aging. Raymond Francis wrote that vitamin D is essential for calcium metabolism, supports bone health, helps control blood sugar, and protects against cancer. ^[1] The nutrient significantly increases the production of telomerase, an enzyme that repairs telomeres and slows aging, according to the same source. ^[1] These wide-ranging effects underscore the importance of maintaining optimal levels.

Solutions: Increasing Vitamin D Levels

Optimal vitamin D levels are considered to be above 50 nanograms per milliliter, and supplementation with 5,000 IU daily is recommended by some nutritionists, according to an interview on health testing and supplementation. ^[7] At-home blood tests can help individuals assess their current levels, the interview stated. ^[7] Testing allows for personalized dosing rather than generic recommendations.

The safety of higher vitamin D doses has been affirmed by recent reassessments. A news report noted that a “schoolboy error” in earlier guidelines led to overly cautious upper limits, and the Academy of Medicine has since acknowledged that higher intakes are not toxic for most people. ^[8] Dietary sources such as fatty fish, egg yolks, and fortified foods can also contribute, though supplementation is often the most reliable way to correct a deficiency, experts said.

Conclusion

Individuals experiencing unexplained fatigue may consider testing their vitamin D levels, clinicians advised. ^[7] Supplementation and dietary changes can help restore levels to the optimal range, though consulting a healthcare provider is recommended before starting any regimen, according to the interview. ^[7]

The evidence linking vitamin D deficiency to fatigue is supported by prevalence data, mechanistic studies, and clinical observations. Given the high rates of insufficiency in the general population, testing and appropriate supplementation offer a straightforward strategy for addressing a common but often overlooked contributor to low energy, researchers concluded.

https://en.wikipedia.org/wiki/Vitamin_D

Vitamin D Deficiency Linked to Fatigue, Studies Show 

Alternative sweetener sorbitol linked to liver disease

Sweeteners such as aspartame, found in Equal packets, sucralose (Splenda), or sugar alcohols are often seen as healthier alternatives to food with refined sugar (glucose). But that assumption is being challenged with new research, including the recent finding that the sugar alcohol sorbitol is not as harmless a sugar substitute as once thought.

The study, published recently in Science Signaling, follows a line of research detailing the harmful effects of fructose on the liver and other systems from the lab of Gary Patti, at Washington University in St. Louis.

Patti, the Michael and Tana Powell Professor of Chemistry, in Art & Sciences, and of genetics and medicine, at WashU Medicine, has previously published research about how fructose processed in the liver can be hijacked to supercharge cancer cells. Previous research has also found that fructose is a key contributor to steatotic liver disease, affecting 30% of the adult population worldwide.

Sorbitol's effects on the body

The most surprising finding from the current work is that because sorbitol is essentially "one transformation away from fructose," it can induce similar effects, Patti said.

The research involved experiments with zebrafish demonstrating that sorbitol, often used in "low-calorie" candy and gum, and commonly found in stone fruits, can naturally be made by enzymes in the gut and eventually converted into fructose in the liver.

Patti's team found there are many roads to fructose in the liver, and potential detours, depending on a person's sorbitol and glucose consumption patterns, along with the bacterial populations colonizing their gut.

The role of gut bacteria

For starters, although most of the research on sorbitol metabolism has focused on its production due to glucose overload in pathological settings such as diabetes, sorbitol can be naturally produced in the gut from glucose after eating, Patti said.

The enzyme that produces sorbitol has a low affinity for glucose, so glucose levels must be high for it to take effect. That is why sorbitol production has primarily been associated with diabetes, where blood glucose levels can become elevated. But, even in healthy settings, glucose levels in the gut become high enough after feeding to drive sorbitol production within the intestine, according to the team's zebrafish experiments.

"It can be produced in the body at significant levels," said Patti. "But if you have the right bacteria, turns out, it doesn't matter."

Sorbitol-degrading Aeromonas bacterial strains convert the sugar alcohol into a harmless bacterial byproduct. "However, if you don't have the right bacteria, that's when it becomes problematic. Because in those conditions, sorbitol doesn't get degraded and as a result, it is passed on to the liver," he said.

Once in the liver, it is converted to a derivative of fructose. It's important to determine if alternative sweeteners are providing a healthy alternative to table sugar, since people with diabetes and other metabolic disorders may be relying on them as "sugar-free" products.

Dietary habits and health implications

Gut bacteria do a good job of clearing sorbitol when it is present at modest levels, such as those found in fruit. But problems arise when sorbitol quantities become higher than what gut bacteria can degrade. This can occur when excessive amounts of glucose are consumed in the diet, which lead to high levels of glucose-derived sorbitol, or when dietary sorbitol itself is too high.

The more glucose and sorbitol consumed, then, even if someone has the friendly bacteria that clears it, those gut microbes may be overwhelmed with the task.

Avoiding both sugar and alternative sweeteners is increasingly complicated, as many foods are packed with multiple varieties of all the above. Patti was bemused to discover his own favorite protein bar was chock full of sorbitol.

The lab will need to do more research to understand the specific mechanisms for how bacteria clears sorbitol, but the basic idea that these sugar alcohols, called polyols, are harmlessly expelled, may not hold true. "We do absolutely see that sorbitol given to animals ends up in tissues all over the body," he said.

Bottom line: it's becoming more apparent that "there is no free lunch" when trying to find sugar alternatives, with many roads leading to liver dysfunction.

https://en.wikipedia.org/wiki/Sorbitol

Madelyn M. Jackstadt et al, Intestine-derived sorbitol drives steatotic liver disease in the absence of gut bacteria, Science Signaling (2025). DOI: 10.1126/scisignal.adt3549

Alternative sweetener sorbitol linked to liver disease