Mysterious body mass gain: could it be water storage due to heat adaption?

Question

Over the past two weeks, I've noticed about a 3-4 pound gain in body mass, without any noticeable change in appearance (like muscle definition, or what veins are visible where). I do seem to have the vague impression that I have slightly bigger muscles. (Not explained by resistance exercise: I have not been doing any weights at all for two months due to a fractured radius in my left arm.) I haven't changed my caloric intake or reduced my activity. Could this be storage of water due to an upswing in temperature?

I exercise intensely outside, five days a week or more: it's seven miles each way to cycle to work and back, plus I go running. Recently the weather has gotten hotter. Weather reports are saying "heat wave", yet I'm not feeling it at all (my reaction has been, huh? what heat wave?) In fact, I'm feeling more comfortable outside than, say, a month ago.

For instance, I went for an 8 mile run in the heat of the early to mid afternoon, carrying a mere 750 ml bottle of water. Only a month ago, in spite of cooler weather, my mouth and throat would have been going dry, especially after that water ran out, but even before that. Today I noticed that my mouth never dried up, even over the last two miles when I had no more water in the bottle. I sweated profusely; I could easily wring dripping water out of my shorts by the end of the run. Yet I didn't feel thirsty at all during or after. It didn't even occur to me to drink anything when I returned to the office!

Do people turn to camels when they are used to heat, carrying an extra liter or two of water somewhere?

There is no sign of any humps, though, so if this is really the case, I suppose we will never settle the question whether I'm dromedary or bactrian.

Answer 1

People don't ever turn to camels and store massive amounts of water for later. It makes more sense one of the following is happening:

• increased sodium intake through diet causing water retention

• Irregular bowel movements causing apparently weight gain without change in overall body composition

• weighing yourself at different times of the day. You weigh the least in the morning af

Answer 2

In addition to the options provided by JSimonsen's answer, I have another possibility:

• Muscle gain: You might think that you haven't been performing some resistance exercises; however, there's are at least 2 resistance exercises you've been performing:
  • Biking. 70 miles biking every week will certainly increase your thigh and calves muscles (especially if high resistance is used). Since the legs comprise some of the largest muscles in the body, a 4-lb increase in the legs won't be easily noticeable.
  • Running. Depending on the intensity and duration, there should be a noticeable increase in your calves and thighs (if sprinting).

Contrary to popular opinion, weight training the upper body isn't the only way to gain mass; you can strengthen your lower body muscles (resulting in some mass increase) with biking, sprinting, and jumping.