Debunked: Vitamins and supplements won’t keep mosquitos from biting you

Debunked: Vitamins and supplements won’t keep mosquitos from biting you

This article originally appeared on The Discussion .

A long-standing medical myth states that vitamin B1, also known by thiamine, can repel mosquitoes.

A “systemic insect repellent ” which makes your entire body unattractive to biting insects sounds great. Even if you correctly reject the misinformation questioning safe and effective repellents like DEET, oral repellents would still have the benefit that you wouldn’t need to worry about covering every inch of exposed skin or carrying containers of bug spray whenever you venture into the great outdoors.

Along with thiamine, other alleged oral mosquito repellents include brewer’s yeast, which contains thiamine, and garlic, the legendary vampire repellent. Oral repellents may sound too good to be true.

As a professor of entomology in Taiwan, where the mosquito-transmitted Dengue virus is endemic, I was curious what science really says about food-based repellents. After a very deep dive into the literature and reading practically every paper ever written on the subject, I compiled this knowledge into the first systematic review of the subject.

The scientific consensus is that oral repellents are not available. Despite extensive searches, no food, supplement, medication, or condition has ever been proven to make people repellent. People with vitamin B1 deficiencies don’t attract mosquitoes as much.

Where did the myth that mosquitoes hate vitamins originate, and why is it so difficult to eradicate?

Making of a myth

In 1943, Minnesota pediatrician W. Ray Shannon gave 10 patients varying doses of thiamine, which had only first been synthesized seven years prior. They reported that it relieved itching, and prevented mosquito bites. In 1945, California pediatrician Howard Eder claimed 10 milligram doses could protect people from fleas. In Europe in the 1950s, physician Dieter Muting claimed that daily 200 milligram doses kept him bite-free while vacationing in Finland, and hypothesized a breakdown product of thiamine was expelled through the skin.

These findings attracted rapid attention and were almost immediately rejected. The U.S. Naval Medical Research Institute tried to replicate Shannon’s findings, but failed. By 1949, Californians using thiamine to repel fleas from dogs were reporting it as “completely worthless.” Controlled studies from Switzerland to Liberia repeatedly failed to find any effects at any dose. The first clinical trial in 1969 concluded definitively that “vitamin B1 is not a systemic mosquito repellent in man,” and all controlled studies since suggest the same for thiamine, brewer’s yeast, garlic, and other alternatives.

The evidence was so overwhelming that, in 1985, the U.S. Food and Drug Administration declared all oral insect repellents are “not generally recognized as safe and effective and are misbranded,” making labeling supplements as repellents technically fraud.

Medical mechanisms aren’t there

Scientists today know more about vitamins and mosquitoes than ever before.

Vitamin B1 does not break down in the body and has no known effect on skin. The body strongly regulates it, absorbing little ingested thiamine after the first 5 milligrams and quickly excreting any excess via urine, so it does not build up. It is almost impossible to overdose.

As in humans, thiamine is an essential nutrient for mosquitoes. They don’t have any reason to fear it or to try to avoid it. There is no evidence that they can smell it.

The best sources of thiamine are whole grains, beans, pork, poultry and eggs. A carnitas burrito will not repel mosquitoes.

What explains the early reports? Many used anecdotal reports from patients of less bite symptoms to indicate a reduction in biting. This was a poor design and not an accurate way to understand what was happening.

Mosquito bites are followed by two reactions: an immediate reaction that starts fast and lasts hours and a delayed reaction lasting days. These reactions are not dependent on the mosquito but on how familiar your immune system is with the species’ saliva. As we age, our bodies go from no reaction to delayed reactions to delayed reactions to both to both to both to no reaction to all. Eventually, there will be no reaction at all.

What Shannon and others thought was repellency could have been desensitization: The patients were still getting bitten, they just stopped showing symptoms.

So, what’s the problem?

Despite the scientific consensus, a 2020 survey of pharmacists in Australia found that 27% were still recommending thiamine as a repellent to patients traveling abroad: an unacceptable recommendation. Besides wasting money, people relying on vitamins as protection against mosquitoes can still get bitten, potentially putting them at risk of diseases like West Nile and malaria.

Unscrupulous dealers have been selling thiamine patches and injections to circumvent the American ban on oral repellents. Unfortunately, while thiamine is safe if swallowed, it can cause severe allergic reactions when taken by other routes. These products are thus not only worthless, but also potentially dangerous.

Not all problems can be solved by food. Long sleeves and bug spray containing DEET, picaridin or other proven repellents are still your best defense against biting pests.

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