I'll be the first to admit that there were some pretty bizarre forms of medical treatment in the 16th century (just wait until you get to my post for R). But although we of the 21st century might not agree with our 16th century peers about the causes of obesity, most of the recommended remedies would strike us as quite familiar.
In early human history, it was virtually impossible for anyone to be overweight. Having to hunt for one's food required a tremendous amount of physical activity, and food itself often came in the form of large animals fully capable of killing and eating you first.
But eventually, based on the urgent need of ensuring an adequate food supply, humans learned both to farm crops and to domesticate food animals, and the activity of getting your next meal became much less fraught with peril. In fact, people got so good at producing food for themselves that eventually most societies had a comfortable abundance (at least for those who could afford it). Simultaneously, lifestyles were becoming more sedentary, and by Roman times, physicians were treating excess weight as a public health problem. What treatments did they suggest? For the most part, exercise and a healthy diet.
Since the time of Hippocrates (460-370 B.C.), through the 16th century, and for at least 100 years after it, most people believed that good health was maintained by a proper balancing of the four humors in the body: blood, phlegm, yellow bile and black bile. Obesity, like many other physical ailments, resulted from an imbalance in the system, and the goal was to figure out ways to rebalance.
The Roman physician Galen (A.D. 131-201?), whose theories governed medical practice up to and throughout the Middle Ages, described his treatment for obesity as follows:
I have made any sufficiently stout patient moderately thin in a short time,
by compelling him to do rapid running, then wiping off his perspiration
with very soft or very rough muslin, and then massaging him maximally
with restoratives, and after such massage leading him to the bath after
which I give him nourishment immediately but bid him rest for a while
or do nothing to which he was accustomed, then lead him to a second
bath and then give him abundant food of little nourishment so as to
fill him up but distribute little of it to the entire body.
Similarly, Avicenna (A.D. 980-1037), a highly influential physician of the Arab world, wrote:
The regimen which will reduce obesity. Produce a rapid descent of the
food from the stomach and intestines, in order to prevent completion
of absorption by the mesentery. Take food which is bulky but feebly
nutritious. Take the bath before food, often. Hard exercise...
The early 16th century marked the beginning of the so-called Age of Science, during which people began to use hypothesis and experiment as methods to discovering solutions. Antonio di Paolo Benivieni (1443-1502) was the author of a pathology textbook published posthumously in 1507, in which he provided the first known pathological descriptions (case studies) of obese individuals, as well as discussions of treatments.
Luigi Cornaro (1467-1566) was an Italian layman who published a short book, Discourses on a Sober Life, in 1558 describing his own struggles to conquer obesity and his eventual success. He made no secret of what he believed to be the cause of his excess weight; his book began, "O wretched, miserable Italy! Does not thou plainly see that gluttony deprives us of more soul years than either war or the plague itself could have done?" Cornaro came from a wealthy Venetian family and had freely indulged himself in food and drink until about the age of 40, when his health problems began to catch up with him. He consulted with his doctor, who advised him to eat only wholesome foods, and only in small quantities. Cornaro immediately followed this advice and lost all of his excess weight, and his health problems all disappeared. He became a zealot for healthy living, published his book at age 80, and lived to see the fourth edition published when he was 99.
But, as we know in the 21st century and as physicians probably knew in the 16th, prescribing diet and exercise to counteract obesity doesn't always work.