This is a real "medical research" article just come out:
From Medical Hypotheses
Volume 72, Issue 6, June 2009, Pages 623-625
The nature of navel fluff
Hard facts on a soft matter! In their popular scientific book (Leyner M, Goldberg B. Why do men have nipples – hundreds of questions you’d only ask a doctor after your third martini. New York: Three Rivers Press; 2005), Leyner and Goldberg raised the question why “some belly buttons collect so much lint”. They were, however, not able to come up with a satisfactory answer. The hypothesis presented herein says that abdominal hair is mainly responsible for the accumulation of navel lint, which, therefore, this is a typically male phenomenon. The abdominal hair collects fibers from cotton shirts and directs them into the navel where they are compacted to a felt-like matter. The most abundant individual mass of a piece of lint was found to be between 1.20 and 1.29 mg (n = 503). However, due to several much larger pieces, the average mass was 1.82 mg in this three year study. When the abdominal hair is shaved, no more lint is collected. Old T-shirts or dress shirts produce less navel fuzz than brand new T-shirts. Using elemental analysis, it could be shown that cotton lint contains a certain amount of foreign material, supposedly cutaneous scales, fat or proteins. Incidentally, lint might thus fulfill a cleaning function for the navel.
“By the way, doctor, why do I always find fuzz in my belly button but my wife and her brother do not?” Is not it peculiar that some simple medicine-related questions from everyday life have not yet been answered? Mark Leyner and Dr. Billy Goldberg dedicated their writing efforts to answering exactly such medical questions. The result of their oeuvre was a huge success – their popular scientific medical book “Why Do Men Have Nipples”  became a global best seller. The question above attracted the attention of this article’s author, because, though simple, even Leyner and Goldberg were not able to come up with a satisfactory explanation to it:
“One question that cannot be answered, however, is why some belly buttons collect so much lint” [1, p. 57].
The accumulation of navel fluff is probably a widely spread phenomenon. Nonetheless it is an intimate question, thus indeed “you’d only ask a doctor after your third Martini” (the subtitle of ) about this topic. The question of the nature of navel fluff seems to concern more people than one would think at first glance. Even the famous journal Nature dedicated some of its precious publication space to an anonymous author’s photograph series entitled “Another matter: Navel fluff” . Three “types” of fluff were shown, from a sailor (“while at sea”), a farmer and an architect. Obviously, these photographs should only attract the reader’s attention. Coincidence or not – these are what one would regard as overwhelmingly male professions. Accordingly, and to the author’s personal experience, navel lint seems to be a phenomenon that affects primarily male adults. Maybe the answer to the introductory question is much simpler than one would expect.
The author’s hypothesis is that men’s abdominal hair collect cotton fibers from shirts and transport them into the navel by the normal body movement, supported by the direction of the abdominal hairs and their structure. After several hours, these fibers are compacted to form the typical felt-like material. According to the hypothesis, navel fluff has a cleaning function for the navel.
Material and experimental
The author first observed the accumulation of navel fluff in his early 20s. Despite thorough body hygiene including a daily morning shower, the navel filled with lint over the day. The author collected 503 pieces of navel fluff since approximately March 2005 with a total weight of almost 1 g. It was obvious that the fuzz resulted from the collection of cotton fibers from the author’s shirts, because the lint had the same color as the respective shirt. Navel lint material obtained from white T-shirts (supposedly pure cellulose) was submitted to CHNS elemental analysis for chemical information on this matter. In order to investigate the role of the abdominal hair, the author also shaved his belly for this study.
Empirical data and discussion
Questioning (male) friends, colleagues and family members, supported the hypothesis that the existence of abdominal hair is a major prerequisite for the accumulation of navel fluff.
The mass distribution of the 503 pieces of lint is shown in Fig. 2. The most abundant mass was observed for the range of 1.20–1.29 mg (29 pieces). The average value was 1.82 mg. Only few pieces with masses > 4 mg were collected. In most cases, these heavy pieces originated from brand new and soft T-shirts (and eventually polo shirts). Older T-shirts or dress shirts yielded only smaller fluff, if any. It is noticeable that five of the six heaviest pieces of lint were in the range from 7.27 to 7.42 mg, which probably reflects the maximum capacity of the author’s navel. Only one lint was heavier (9.17 mg).
The hypothesis was supported by the observation that small pieces of fuzz first form in the hair and then end up in the navel at the end of the day. The author believes that the scaly structure of hair firstly enhances the abrasion of minuscule fibers from the shirt and secondly directs the lint into one direction – the navel – where it accumulates. Thereby the hairs’ scales act like a kind of “barbed hooks”. Abdominal hair often seems to grow in concentric circles around the navel. In order to prove the role of body hair in navel fluff accumulation, the author shaved his belly and hence no more lint was deposited in his belly button. The process resumed once the hair had grown back.
Lint is characterized by a huge specific surface, where other materials can be adsorbed. It does not come as a surprise, therefore, that a previous study reported the adsorption of (precious) metals such as gold and silver onto laundry dryer lint , resulting probably from jewelry abrasion . A similar effect can be observed with navel lint, since its chemical composition (as determined in two CHNS elemental analyses) is slightly shifted away from pure cotton (cellulose). The most eye-catching deviation from the calculated values is the existence of solid nitrogen- and sulfur-compounds in the lint. Obviously, the originally pure cellulose fibers of the shirt were contaminated with a mélange of foreign materials, including house dust, cutaneous scales, fat, proteins, sweat, etc. Therefore, it would not be surprising if general practitioners reported that people who collect navel lint (i.e. people who have abdominal hair) had cleaner and more hygienic belly buttons than those who do not, because the foreign material is removed together with the lint. The author recalls that in his late childhood and youth his navel required more hygienic care to be kept clean than now that the lint helps sweep it clean.
The “investigation” of the navel fluff phenomenon allows the following concluding remarks:
1. Abdominal hair causes the accumulation of navel lint.
2. Who would like to avoid the accumulation of fluff can either shave his abdominal hair or wear old T-shirts or dress shirts.
3. The average mass of a navel fuzz collected over a three years period was found to be 1.82 mg, the most abundant individual mass was between 1.20 and 1.29 mg.
4. Assuming that an average T-shirt (180 g) can be worn up to 100 days a year, such a shirt could suffer a weight loss of up to 182 mg (0.1%) per year just by fibers that are lost into the navel.
5. Elemental analyses show that pure cotton lint adsorbs foreign materials (cutaneous scales, fat, proteins etc.) This phenomenon can be regarded as a kind of incidental cleaning function of the navel.
6. Lastly, and most importantly, we hope we have been able to provide some helpful information for GPs when they are next confronted with the simple question of “why some belly buttons collect so much lint and others do not.”;)