If you’ve ever been told that eating carrots will improve your night vision, you might be surprised to know that this idea actually originated from World War II propaganda. How was it that the British Air Force was able to shoot down German aircraft in the middle of the night? To cover up their newly adopted radar technology, the British Ministry of Information pushed the message that pilots ate a bunch of carrots to improve their night vision, which coincidentally led to many people eating loads of carrots during blackouts. There is some truth to this, as carrots are high in a compound called beta-carotene, which our bodies can use to make vitamin A, a fat-soluble vitamin critical for vision in low-light conditions. 

Vitamin A also plays important roles in cell growth and division along with reproduction and immunity, and it has antioxidant properties which can protect the body from oxidative stress caused by free radicals. Although vitamin A deficiency is rare in the United States, it is still a problem in many developing countries — mainly because of diet inadequacy. Regardless, the many protective roles of vitamin A, especially those related to vision, has led researchers to study vitamin A in type 2 diabetes and other metabolic disorders. Diabetic retinopathy affects the blood vessels in the eyes, potentially leading to blindness. Here, we’ll explain the role of vitamin A in supporting human health, outline the potential roles for vitamin A in metabolic diseases, list natural sources of vitamin A, and identify safe levels in the human body.

Vitamin A signaling

Vitamin A refers to multiple compounds: the vitamin itself and its metabolites. Vitamin A is actually classified as a lipid and thus is not water-soluble. In the body, vitamin A is typically bound to proteins or found in lipid droplets. The primary function of these proteins is to transport vitamin A to areas where it can be either metabolized or stored.

There are a few mechanisms of action for vitamin A signaling:

• Rhodopsin is a key chromoprotein involved in visual processing and is made up of a protein called opsin as well as a pigmented molecule derived from vitamin A called 11-cis­-retinol.
  
• Retinoic acid is a vitamin A metabolite involved in many signal transduction pathways including stem cell differentiation and insulin secretion.
  
• Retinoic acid is also a potent transcriptional regulator and plays roles in cancer development, reproduction, and embryonic development.

Vitamin A is mainly stored in the liver in hepatocytes and adipocytes, which hints at possible roles in disorders such as obesity, diabetes, and liver disease.

What are optimal vitamin A levels?

Vitamin intake, especially for the fat-soluble vitamins, needs to be appropriately regulated. Recommended intake levels for vitamin A are not as straightforward as other nutrients because of the different bioactivities of retinol and provitamin A carotenoids. Because of this, the National Academies of Sciences, Engineering, and Medicine indicates the Recommended Dietary Allowance (RDA) for vitamin A as retinol activity equivalents (RAE), where 1 microgram (mcg) RAE is equivalent to:

• 1 mcg retinol
• 2 mcg supplemental beta-carotene
• 12 mcg dietary beta-carotene
• 24 mcg dietary alpha-carotene or beta-cryptoxanthin. 

International Units (IU) also depend on the form of vitamin A, where:

• 1 IU retinol = 0.3 mcg RAE
• 1 IU supplemental beta-carotene = 0.3 mcg RAE
• 1 IU dietary beta-carotene = 0.05 mcg RAE
• 1 IU dietary alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE 

If this seems a bit confusing, fear not. Here’s a general chart taken from the National Institutes of Health, with the IU column corresponding to the RAE of retinol rounded up to the nearest whole number. (note that the IU can only be calculated if the source of vitamin A is known, so this is a simplified example.)

Although the table might seem complicated, vitamin A deficiency in the United States is rare. However, there are groups of people who are more prone to vitamin A deficiency — including premature infants, people with cystic fibrosis, and people with gastrointestinal disorders. Clinical symptoms of deficiency include reduced ability to see at night, and chronic deficiency is associated with abnormal lung development, increased susceptibility for respiratory diseases, and mortality from infections. In addition, abnormal vitamin A levels can be a precursor to metabolic diseases.

Vitamin A and metabolic diseases

Studies examining vitamin A levels in metabolic diseases are somewhat conflicting, with some showing high serum levels associated with increased risk and others showing low serum levels correlating with pathology. Either way, vitamin A dysregulation could be an indicator of disease susceptibility. For example:

• RPB4, a liver protein primarily responsible for carrying serum vitamin A, is elevated in obesity and insulin resistance.
  
• Blocking RPB4 may be a possible therapeutic for type 2 diabetes, and there is an actively recruiting clinical trial studying the role of RPB4 metabolism in type 2 diabetes.

Vitamin A and type 2 diabetes

Type 2 diabetes (T2D) is a metabolic disorder resulting in the dysregulation of sugar as a fuel source. The long-term effects of elevated sugar and insulin resistance can lead to other problems of the circulatory, nervous, and immune systems. This multi-organ disorder has no cure, and with more than 90% of the 37 million diabetic Americans having T2D (as opposed to type 1 diabetes), scientists are taking a multifaceted approach to researching it. 

Although vitamin A is not currently mentioned in risk factors for diabetes, recent research suggests that vitamin A can play a positive role in T2D, likely through its effects on gene expression and insulin release:

• Mice that exhibit diabetic retinopathy that receive a vitamin A analog have improved eyesight.
  
• Similarly, T2D patients who have diabetic retinopathy have decreased vitamin A levels, indicating vitamin A levels may inversely correlate with disease pathology.

Is vitamin A toxic?

Before going out and stockpiling bottles of vitamin A supplements, keep in mind that excessive intake of vitamin A is associated with negative health outcomes. (Learn about other toxic vitamins in Vitamins and minerals: Can you take too many?)

Acute vitamin A toxicity occurs after someone has ingested very high doses (greater than 100x RDA) and can lead to headaches, blurred vision, nausea, aching, coordination issues, and even increased cerebrospinal fluid pressure, which can lead to coma. Chronic vitamin A toxicity can lead to muscle and joint pain, fatigue, and depression. Here are the upper limits (taken from the National Institutes of Health data, with the same caveats as above):

One of the issues with measuring vitamin A is the different bioavailabilities of the different forms. A blood test is normally used to measure retinol and carotenoid levels. However, since vitamin A is fat-soluble and can be stored in the liver and other organs, blood measurements are not the most accurate. There are specialized labs that can measure liver vitamin A levels if needed; however, in general, blood tests are sufficient.

What are the best sources of vitamin A?

Many foods contain vitamin A or beta-carotene — including vegetables, dairy products, meat, and fruit. Here are some with their associated RAE per serving and percentage of the recommended daily value (DV):

• 3 oz. beef liver: 6582 mcg RAE (731% DV)
• Sweet potato with skin, baked: 1403 mcg RAE (156% DV)
• 0.5 cup carrots: 459 mcg RAE (51% DV)
• 1 egg, hard-boiled: 75 mcg RAE (8% DV)

Our bodies convert beta-carotene to vitamin A, and supplementation should only be considered after discussing with a medical provider. 

Key takeaways

Vitamin A is a critical fat-soluble nutrient that plays important roles in health and appears to have a unique role in metabolic disorders — such as obesity, liver disease, and type 2 diabetes. However, more research is needed to determine exactly how vitamin A affects these conditions. Supplementation is currently not recommended without seeking input from your medical provider, as vitamin A toxicity is associated with severe symptoms and disorders. The good news is that appropriate levels of vitamin A can be ingested through a balanced diet full of fruits, vegetables, nuts, seeds, and healthy grains.

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