Perspiration: what is the origin of bad odors?

Contrary to a very widespread misconception, sweat is not inherently foul-smelling.

When it has just been secreted, it is generally odorless. Unpleasant odors appear later, mainly at the surface of the skin, when certain microorganisms in the skin microbiota break down the compounds present in the secretions. Body odor therefore results less from sweat alone than from the interaction between sweat, the skin, and the bacteria that live there.

Not all sweat glands contribute to this phenomenon in the same way. Eccrine glands, distributed over almost the entire surface of the body, secrete a clear, watery sweat that is primarily intended for thermoregulation. This sweat consists mainly of water, mineral salts, and a few small molecules, and it contributes little to unpleasant body odors. Apocrine glands, on the other hand, play a much more important role in unpleasant body odor. Located mainly in the armpits, around the nipple (areola), and in the ano-genital region, they become active at puberty under the influence of androgens and produce a more turbid secretion, richer in lipids and proteins. It is precisely these organic compounds that provide skin bacteria with a favorable substrate for odor formation.

Note : The sebaceous glands can also play a role, but a more secondary one. By secreting sebum into the hair follicle, they supply lipids that can also be transformed by certain bacteria into odorant molecules. Nevertheless, in terms of contributions to body odor, apocrine secretions are clearly predominant, followed by eccrine secretions, while sebum appears to play a more subtle role.

The role of the microbiota is central in the emergence of unpleasant odors.

The bacteria most often involved belong in particular to the genera Corynebacterium and Staphylococcus, for example Staphylococcus hominis or Staphylococcus epidermidis. Starting from odorless precursor compounds present in sweat, they produce volatile molecules responsible for different odor notes. Isovaleric acid, derived in particular from leucine metabolism, is classically associated with a sweat or “locker room” smell. Other compounds, such as certain volatile fatty acids, give sour, rancid, or cheesy notes. Finally, sulfur-containing molecules, especially thioalcohols, are involved in the strongest and most persistent underarm odors.

Note that not all areas of the body smell in the same way, because they do not have the same glands, the same skin environment, or the same microbiota. The armpits are the reference area when discussing body odor, because they combine several favorable factors: an abundance of apocrine glands, the presence of hair, a warm and humid environment, and a high bacterial density. The feet are another area frequently involved, but here the odor depends more on eccrine sweat, skin maceration, and bacterial proliferation in an environment that is often enclosed. The scalp or certain genital areas can also develop specific odors, related to their own combination of secretions and micro-organisms.

There are also significant individual variations. Sex, age, and genetics influence the composition of skin secretions. For example, men on average have larger apocrine glands, which can contribute to a stronger body odor. Age also plays a role: before puberty, apocrine activity is low, which explains why the typical unpleasant underarm odors mainly emerge during adolescence. Genetics can also modulate body odor, particularly in the axillary (underarm) region, notably via the ABCC11 gene. Certain variants of this gene are associated with a much lower production of odorant precursors in the armpits, and therefore with a markedly reduced body odor. This genetic variation is especially common in East Asia.

Unpleasant odors result not so much from the amount of sweat produced as from its composition.

Furthermore, emotions can influence body odor. Under the effect of stress, fear, or anxiety, the apocrine glands are more strongly stimulated, in connection with adrenaline and activation of the sympathetic nervous system. This phenomenon explains why emotional sweating, especially in the underarm area, is often perceived as stronger or more unpleasant than sweating caused by heat.

Diet can also modify body odor. Alcohol, certain spices, and garlic can influence the composition of secretions or alter the skin microbiome. Hygiene also plays an obvious role: the longer secretions accumulate on the surface of the skin, the more time bacteria have to break them down. The presence of underarm hair can also increase the retention of odorous secretions. Finally, certain metabolic or infectious diseases can characteristically change body odor, although these are more specific situations.

Limiting odors related to perspiration is based above all on a simple principle: reducing the buildup of sweat and limiting the bacterial transformation of secretions.

Maintaining good hygiene is therefore the first step. Washing regularly, especially after physical activity, helps eliminate both sweat and some of the microorganisms present on the surface of the skin. It is also important to dry yourself thoroughly, because residual moisture promotes bacterial growth.

The choice of clothing can also help. Synthetic materials tend to trap heat and moisture, creating an environment that favors odor formation. In contrast, natural fibers such as cotton, linen, or wool allow better evaporation of sweat and limit maceration. The same principle applies to the feet: choose breathable shoes, avoid styles that are too tight, and alternate pairs to reduce moisture and therefore bacterial growth.

Beyond these measures, deodorants and antiperspirants can be a solution.

Their mechanisms of action differ. Deodorants work primarily by limiting the growth of bacteria responsible for forming odor-producing compounds, or by neutralizing these molecules. Antiperspirants, in contrast, aim to reduce sweat production by partially blocking the sweat ducts, most often through the use of aluminum salts. By decreasing the amount of sweat available, they indirectly reduce the substrate used by bacteria. The use of aluminum salts is, however, a subject of debate. Their ability to block sweat glands is well documented, but their long-term safety remains under discussion, particularly because of their potential to accumulate in the body. To date, the available data do not allow a definitive conclusion that there is a proven risk at the doses used in cosmetics, but this uncertainty is driving the search for alternative solutions.

Recently, some scientists have shifted their focus from simply suppressing sweat to modulating the skin microbiota. One study evaluated antiperspirant formulations enriched with lysates ofLactobacillus ferment, combined with plant extracts such as hops (Humulus lupulus) and sage (Salvia officinalis). In a model of the axillary microbiota, the addition of these compounds significantly reduced the microorganisms involved in odor formation, while promoting a more stable microbial balance. Formulations containing these postbiotics were found to be more effective than those without them, suggesting that targeting the bacterial ecosystem could be an interesting alternative to aluminum salts.

Finally, certain lifestyle adjustments can complement these measures. Limiting the consumption of alcohol or very spicy foods, which can alter the composition of secretions, as well as learning to manage stress more effectively, which can increase apocrine sweating, may help reduce everyday body odors.

• LAZAREVIC V. & al. Mapping axillary microbiota responsible for body odours using a culture-independent approach. Microbiome (2015).

• NAGARAJAN N. & al. Understanding the microbial basis of body odor in pre-pubescent children and teenagers. Microbiome (2018).

• SMEETS M. & al. Intrinsic and extrinsic factors affecting axillary odor variation: A comprehensive review. Physiology and Behavior (2023).

• CELEBI L. & al. Postbiotics cosmetic formulation: In vitro efficacy studies on a microbiome friendly antiperspirant. Journal of Research in Pharmacy (2023).