Is Skin the Largest Organ of the Human Body?

Well, yes! I was under the same impression until a peer reviewer suggested a revision for my submitted article, with a specific question – is skin really the largest organ of the body?

Though many would agree, it has actually been a debatable topic in the world of dermatology semantics. Though the term “largest” has a clear definition, when talking about the same set of items, how you determine something to be the largest versus how someone else does this could differ depending on the logic used. For example, largest based on surface area? Largest based on weight? Largest based on biological function? Each scenario could give a different answer.

When speaking about the body, human skin, the lungs, and the gut are generally considered to be the largest organs.  However, based on the classical perspective of their functions, skin has been considered to have the smallest surface area. Estimates of these surface areas, based on height, weight, and other assumptions, have led to widely accepted surface area values of 2 m² for the skin, 30 m² for the gut, and 50 m² for the lung.

Interestingly, the 2 m² surface area of the skin is mostly agreed upon in the medical and scientific community. This value is based on the assumption that the skin is a flat surface, but this assumption is misleading if the functions of the skin microbiome are to be incorporated into describing the skin structure. We have considered these surface area calculations at face value for decades, without a doubt. But in reality, human skin is not a flat surface. The presence of approximately 5 million appendages, like hair follicles and sweat ducts, greatly increases the epithelial surface area accessible to the microbiome, making the skin surface area around 25 m²! The skin provides the largest surface for housing a wide diversity of microbiomes, most of which are harmless or even beneficial for their host.

This new estimate of skin’s surface area shows how the skin has potentially a large interface with the microbiome. This is a special interface that is not easily accessible to topical products as most of the epithelial surface is protected below the follicular opening. Therefore, the old estimate of 2 m² is more appropriate for calculations of heat and water loss, and for topical application of products that cannot penetrate into the follicle. However, for the world of microbiome research, the new surface area approach has its own significance.

At the end of the day, while scientists might find this intriguing, what does this mean for consumers? Frankly, it may not hold much relevance at the moment. However, keep your eye out for products marketed around prebiotic/probiotic/postbiotic microbiome benefits as this new paradigm for skin surface area including the microbiome, exponentially increases the potential for such products. Don’t be surprised if you see too-good-to-be-true skin microbiome claims down the road (there are already plenty out there).

References:

1. Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317:1098 [Pubmed]
2. Goldsmith LA. My organ is bigger than your organ. Arch Dermatol (1990) 126:301–2 [Pubmed]
3. RD Sontheimer, Skin Is Not the Largest Organ, Journal of Investigative Dermatology (2014) 134, 581–582 [Pubmed]
4. Richard L. Gallow, Human Skin Is the Largest Epithelial Surface for Interaction with Microbes, Journal of Investigative Dermatology (2017) 137, 1213e1214. [Pubmed]
5. Hasleton PS. The internal surface area of the adult human lung. J Anat. 1972;112(Pt 3):391–400 [Pubmed]
6. Helander HF, Fandriks L. Surface area of the digestive tract—revisited. Scand J Gastroenterol. 2014;49:681–9. [Pubmed]
7. Photo credit – fauxels

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