A new study shows that you can be “highly attractive” if you have enough carboxylic acid on your skin. Highly attractive to mosquitos, that is.
Mosquitoes are an ideal vector for pathogen contraction and transmission between hosts. These pathogens include viruses such as Dengue, Yellow Fever, Zika, West Nile Virus, and Chikungunya. Parasites that cause malaria in tropical and subtropical countries are also transmitted by mosquito bites. It is therefore logical that increased exposure, or attractiveness, to mosquitos will increase the probability of contracting such pathogens. Why then do some individuals seem to be targeted by mosquitoes more than others?
A new study attempted to shed light on this age-old question of why some people are greater mosquito magnets than others. Indeed, some are victims of relentless mosquito bites and could in fact function as a “mosquito adsorbent” – a courtesy for those around them on a humid day at the park. It turns out that mosquitoes utilize their sense of smell to find food, as expected, but that those with higher levels of certain metabolites get bit more frequently than others. What is interesting is that some genes in mosquitoes allow this sensing to occur. To determine how all of this worked, researchers induced olfactory deficits by knocking out genes in mosquitoes. This caused less odor sensing to occur, but surprisingly mosquitoes were still able to distinguish between highly and weakly attractive individuals. This all indicates that attractiveness to mosquitoes is likely an amalgamation of mechanisms involving genetics and metabolomics.
The specific metabolites analyzed in this study are pentadecanoic, heptadecanoic, and nonadecanoic acids – all grouped as carboxylic acids. These are considered odd number fatty acids (ONFA), a minor group of the fatty acid repertoire in humans. Pentadecanoic and heptadecanoic acids are the most abundant ONFA, composed of 15 and 17 carbon chains, respectively. In contrast to other fatty acids, ONFA are thought to predominantly accumulate in the body from foods (e.g., dairy, meats). However, they can be synthesized in the body from straight chain fatty acids, through the mechanisms of alpha-oxidation.
The Thiamine Connection
It has long been rumored that thiamine could prevent, or at least reduce, mosquito bites. Is there research supporting this claim?
The literature is not clear, but does seem to indicate an indirect connection between thiamine (Vitamin B1) deficiency, or inadequacy, and attractiveness to mosquitos. For example, a research group studying yellow fever mosquitoes, showed that mixed with lactic acid, the carboxylic acid strongly synergizes to elicit mosquito attraction. The authors state:
Addition of lactic acid markedly increased the degree of attractiveness of formerly less attractive human odour samples and they were preferred over those which were originally the most attractive.
Indeed, lactic acid alone, acts as a sufficient mosquito attractant, but is relatively weak compared to the synergism with carboxylic acids. Another older study utilized a bioassay to identify attractants from human skin extracts. Here the mosquitoes were allowed to fly upwind toward odor sources (human skin wash extracts). They found that lactic acid was a major constituent of skin extract and that it was a necessary component for mosquito attraction.
What can cause lactic acid to build up (either in blood or tissues)? One mechanism would be the shunting of pyruvate into the lactate pathway. This can be measured in blood when someone has low thiamine. Indeed, in some conditions, the skin has been shown to carry higher amounts of lactic acid compared to blood levels, due to increased anaerobic glycolysis. Here we would expect lactic and carboxylic acids to suffuse below or above the surface of skin where it can be detected by mosquitos. In addition, a surfeit of lactic acid build up, in an overt thiamine deficiency, may not be required. This is because the mosquitos’ high sense of smell can pick small amount of odors, even when major olfactory-related genes are knocked out.
On the other hand, a potential argument against the effectiveness of thiamine in repelling mosquitoes comes from a 2005 study. The authors gave individuals over the counter B vitamins in two experiments:
- Experiment 1: B-complex containing 50 mg of B1(thiamine)/B2/B3/B5/B6, 400 µg Folate, 50 µg B12, 50 µg Biotin, and 500 mg Vitamin C. Vitamin C by itself was used as a control since the B-complex contained Vitamin C. Subjects (14 males, and 9 females) were utilized in a cross over experiments, meaning that they would start either placebo or the treatment for 7 days and then switch over to treatment or placebo, respectively. In this scheme each individual would act as his/her own paired experiment, statistically.
- Experiment 2: Used 100 mg B1, with Vitamin C as a control. Subjects (10 males, and 7 females) were studied in the same manner to experiment 1, with 100 mg B1.
The authors did not find statistically significant differences between the groups, and concluded that B vitamins were not helpful in repelling mosquitos.
Possible Problems With the Research
There may be some caveats to the protocol used in this research, that likely masked the true effects of thiamine supplementation. First, the authors may have been limited, but should have used a non-Vitamin C formulation. Although somewhat inconsistent in the scientific literature, Vitamin C supplementation can reduce blood lactate levels in certain settings, “increase in free radical production and lactate levels….were offset by vitamin C supplementation”. This, in theory, would reduce lactate in skin and therefore the strong synergistic effects of lactic with carboxylic acids would be minimized. Second, and more importantly, the method only evaluated volatile components that can be transferred from the skin into the glass bottle used to attract mosquitoes. Lactic acid is a low volatility organic compound. The other positive studies discussed above had instead extracted skin organic compounds, including lactic acid, utilizing ethanol. Overall, the 2005 study was well designed for the questions asked and provided basic preliminary experiments. Unfortunately, the design of the study negated the effects of one key compound, lactic acid, due to volatility-based evaluation. Hence, a follow up study should be performed with these concepts in mind:
- Premeasurement on skin extracts using organic acid extraction methods (ethanol or other compounds).
- Treatment with higher dose thiamine. An example would be 0.5 – 1 gram of thiamine-HCl, for 1 to 4 weeks.
- Mosquito attraction should be performed concomitantly with measurements of thiamine status (ETKA test). Similarly, measurements of lactic acid in skin extracts of treated and non-treated individuals should be performed in order to correlate quantity with phenotype.
The expectation is that individuals who are thiamine deficient would have increased lactic acid in their skin extracts and therefore would serve as a mosquito magnet.
Does Thiamine Status Play a Role in Mosquito Attraction?
The above studies combined elucidate some of the mechanisms in play in individuals who are highly attractive to mosquitos. These include interplay between genetics of mosquitos and organic compounds present in human skin. Individual differences in mosquito attraction may also be attributed in part to temperature, moisture, visual cues, and body odor. Out of these possibilities, body odor plays a major role in mediating these interactions. Specifically, both lactic and carboxylic acids play a synergistic role in attracting mosquitos, and therefore increase the odds of contracting disease. Mechanisms that result in increased lactic acid production such as exercise, and potentially low thiamine metabolism, are likely a risk factor for mosquito bites. Overall, the role for lactic acid in mosquito attraction seems to stir the arrows of mosquito-magnet theory, in-part, towards a mild/subclinical thiamine deficiency, or at least an abnormal metabolic activity. As suggested above, more studies should be performed in order to directly evaluate the effects of thiamine on lactic acid content of human skin in conjunction with effects on attracting mosquitos. In the meant time though, if you are one of those people who seems to attract mosquitos, consider looking into thiamine.
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This article was published originally on February 27, 2023.