What causes sagging of breasts?
Sagging or drooping of breasts is a natural, inevitable process that happens to all women at some point - except those with fairly small breasts. The most notable sagging happens with the process called breast involution (see below), but breasts can start drooping a little at any age, because they do NOT have muscles in them. They have ligaments and connective tissue. When the gravity pulls the breasts down, those ligaments and the skin can stretch, and so the breast then droops. This depends on the elasticity of your skin and of your ligaments, as determined by your genes and diet, and also on normal aging processes. Obviously large breasts will sag easier since the gravity is pulling them down more.
Breast involution is a process where the milk-making system inside the breast shrinks because it's not needed anymore. This happens either after weaning, or right after pregnancy if you don't breastfeed at all, or during menopause. When the tissues inside breast shrink, and the skin surrounding it doesn't, the breast can look 'empty' and saggy. Over time, some fat gets deposited back to the breast so it will look somewhat fuller but sagging usually remains.
Will bras keep the breasts from sagging?
Bras will keep your breasts from sagging while you're wearing them - but not for the remainder of time. There is no published medical literature showing that bras prevent sagging. Even the bra industry seems to know this, as the following quote illustrates:
"We have no evidence that wearing a bra could prevent sagging, because the breast itself is not muscle, so keeping it toned up is an impossibility. What it can do, particularly for larger-breasted women, is obviously to provide the comfort and the support. So, if a woman wants a particular breast profile, she will buy a particular brand, and that is what they're designed for. There's no permanent effect on the breast from wearing a particular bra. The bra will give you the shape the bra's been designed to give while you're wearing it. Of course, when you take it off, you go 'au natural.' "
executive John Dixey at Bras, Bare Facts documentary
However, there is some support for the idea that breasts in bras actually sag more (when bra is taken off). How could that be since the main idea of bras is to lift the breasts upward? The main idea is that the ligaments which support breasts in normal circumstances, would atrophy (get thinner and weaker) when breasts are immobilized within bras and don't have to bear their own weight.
There are lots of studies showing that ligaments and tendons in limbs do atrophy when the limb is immobilized. Bras obviously prevent the the natural slight up-down movement of breasts when we walk, and let shoulders bear the weight of breasts. If breast ligaments behave like limb ligaments, it may be a matter of "use it or lose it".
One study published on the subject of bras and sagging was done in 1991, in Japan. The study suggests that a bra can actually increase breast sagging rather than the opposite. The abstract says (emphasis mine):
"Eleven adult female subjects aged 22-39 years wore a certain brassiere for 3 months while anthropometry and moire fringe photographs on the anterior trunk were taken regularly once a week. After the 3 months, the brassiere was not worn for another 3 months. Then the measurements and photogrammetry were repeated for comparison using superimposed moire configurations. The results are summarized as follows. Regardless of slim or obese trunk, subjects with pendent breasts showed the highest degree of breast form "correction" from wearing the brassiere. In all subjects, after 3 months of brassiere constraint, the underbust circumference was smaller but the chest circumference became enlarged, the distance between the right and left nipples became wider, and the breasts tended to hang down. This change was more marked in obese subjects with pendent breasts. And when this type of subject wore a "well-fitted" brassiere for a long time, her breast form became developed, that is, her breasts hung down more."
Ashizawa K, Sugane A, Gunji T Institute of Human Living Sciences, Otsuma Women's University, Tokyo, Japan: Breast Form Changes Resulting From A Certain Brassiere Journal of Hum. Ergol.(Tokyo) 1990 Jun; 19(1):53-62.
The French study the sagging and bra wearing even further
Another study with similar results was conducted in 2003 by Laetitia Pierrot and Jean-Denis Rouillon. In it, 250 women who practiced sports stopped wearing a bra for one year. They were followed regularly with questionnaires and biometric measurements. At the 6-week checkup, the ladies experienced some discomfort from abandoning the bra, but this disappeared later, and by the end of the year 88% of the women reported improved comfort compared to before the study.
The measurements showed firmer, more elevated and perkier breasts. For example, the distance from the nipple to the shoulder bone was reduced, which means the breasts sagged less. They noted a better muscular development in the rotator muscles of the shoulders and the pectoralis major. Also, stretch marks on breasts became less visible.
(Pierrot L., Evolution du sein après l'arrêt du port du soutien gorge, étude préliminaire longitudinale sur 33 sportives volontaires, Thèse présentée le 19 décembre 2003 devant la Faculté de Médecine et de Pharmacie de Besançon.
[The development of the breasts after discontinuing wearing bras. Preliminary longitudinal study of 33 volunteer sportswomen. Thesis presented December 19th, 2003, Faculty of Medicine and Pharmacy, Besancon, France.]
See here for an English translation of a news article. There is also a more recent study by Jean-Denis Rouillon: Santé Le port du soutien-gorge déconseillé par un médecin bisontin (Besançon doctor advises against wearing bras). La Pays apparently published an article about this study in September 17, 2006.
Sports bras and jogging
There is one more notable study concerning sagging and bra wearing, which found that jogging caused breasts to bounce enough to sretch the Cooper's ligaments inside the breast. Naturally, stretched ligaments would cause the breast to sag more.
The lesson to take home from this study is that if you engage in jogging or some other type of very active sport, sports bras are recommendable.
To prevent or lessen the eventual sagging that will come with age, let breasts be free of bras when you can so that the ligaments inside them can be "in use"; however protect the breasts from bouncing too much doing sports by using proper sports bras.
Proper nutrition will probably also be helpful to maintain elasticity of those ligaments and of the skin.