Study shows no weight gain on birth control
In another study with a fairly high dose pill (50 mcgm) from 20 years ago, investigators found an 11.4% weight increase of over 4.4 lbs (2.0 kg) but also a 14.3% incidence of over a 4.4 lbs weight loss on the same pill (1).
In other words there was just as much weight loss as weight gain. This finding could be interpreted as the pills cause no overall weight change in woman on the average, but an alternative explanation is that some women get nauseated from pills and have a net weight loss while the others who did not get nauseated gained a substantial amount of weight.
Do oral contraceptives cause water retention and how much?
Estrogen in high doses is known to cause weight gain especially that due to fluid retention (1). The more estrogen there is in a pill, the more tendency to gain weight such that a 50 mcgm pill will result in more weight gain than a 35 mcgm pill (1).
The mechanism of action is probably direct stimulation by the estrogen in pills of kidney substances called renin-angiotensin that cause water retention. The water retention then causes sodium (salt) retention. A lower estrogen level pill, e.g., 20 mcgm, will help reduce weight gain due to fluid retention.
This was confirmed in a another study with 30 mcgm pills in which there was essentially no difference in weight gain or weight loss between the placebo group and the oral contraceptive group (1).
Again, however, 30% of these women had a weight gain of more than 1 lb but the net result was the same in the control group. Those who did have weight gain had increased fat but no difference in fluid retention amounts indicating that fluid weight gain may be less of a problem with the newer, lower estrogen pills.
Do birth control pills stimulate your appetite?
There have been reports through the years, especially with the older, higher dose pills, of adverse effects on insulin resistance (1). Even recent studies seem to indicate that current pills can raise insulin levels (1, 1).
Insulin resistance is a condition in which insulin levels rise in response to carbohydrates and drive all energy into the fat cells and essentially prevent weight loss even with dieting (1).
Not all women are susceptible to insulin resistance and thus not all women gain weight using oral contraceptives. Those that have a tendency to abnormal glucose metabolism, however may be the ones who gain weight.
If a woman gains weight upon starting oral contraceptives and there are not other explanations, she should be checked out for possible insulin resistance.