My Fitness Pal made me gain weight...

What’s Going On: Let’s Talk About Cortisol
Cortisol is one of those hormones that I imagine everyone reading this has heard about and about which a lot of misinformation exists. Simply cortisol is a stress hormone, released by the body in response to nearly all kinds of stress. In the fitness/bodybuilding world, cortisol has gotten an almost exclusively negative reputation (cortisol is ‘bad’ in the way that testosterone and thyroid are ‘good’) although this is simplistically incorrect.
Rather, whether cortisol does good things or bad things in the body depends on how it’s released. Simply (and I’d simply, ha ha, refer folks to Robert Sapolsky’s amazing book Why Zebras Don’t Get Ulcers for a detailed look at this; I also talk about cortisol in The Stubborn Fat Solution), acute pulses of cortisol tend to do good things and be adaptive and chronic elevations in cortisol tend to be bad and be maladaptive.
For example, the morning cortisol pulse helps to promote fat mobilization. In contrast, a chronic elevation of cortisol (especially in the face of high insulin levels) tend to promote visceral fat accumulation. As a non-fitness related topic, acute pulses of cortisol tend to be good for memory (why we often remember stressful situations in such detail) while chronic elevations (as often seen in depression) make memory go down the toilet. And there are endless other examples of where acute cortisol pulses are good and chronic elevations are bad; again see Sapolsky’s book for details.
In any case, dieting in general is a stress. And of course training is a stress. And the more extreme you do of each, the more of a stress occurs. And I suspect that a lot of what is going on when folks try to combine excessive caloric deficits with massive amounts of activity is that cortisol just goes through the roof (there’s another issue I’ll come back to at the end that relates to this). Simply, you get these massive chronic elevations in cortisol levels.
Tangentially, this is also one reason I suspect that various types of cyclical dieting help with some of this issue. For at least brief periods, when calories are raised to maintenance or above, you break the diet/training induced elevations in cortisol. This of course assumes that the person isn’t mentally stressed to the nines by raising calories like that but I’m getting ahead of myself.
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So Why is This Bad?
As noted above, chronic elevations in cortisol can cause a lot of bad things to happen. One of them is simply water retention and I’ve mentioned in previous articles that water retention can mask fat loss, sometimes for extremely extended periods. I talked about this in some detail in The LTDFLE and suspect that some of the ‘fat loss’ is actually just water loss when calories are raised and cortisol mediated water retention dissipates. Reducing total training (volume, frequency, intensity or some combination) does the same thing.
But that’s probably not all of what’s going on. Another effect of chronically elevated cortisol levels is leptin resistance in the brain. I’m not going to talk about leptin endlessly here again, you can read the Bodyweight Regulation Series for more information. When the normal leptin signal to the brain is blocked, a lot of things can go wrong metabolically and I suspect that this is part of the problem.
In this vein, although not necessarily related to cortisol per se, at least one study found that the addition of 6 hours per week of aerobic activity to a very low calorie diet (in this case a protein sparing modified fast) caused a larger decrement in metabolic rate than the diet alone. The body appears to monitor caloric availability (simplistically caloric intake minus output) and if it gets too low, bad things can happen.
This is why I so strongly suggested AGAINST the inclusion of much cardio in The Rapid Fat Loss Handbook; it causes more harm than good. Invariably, the biggest source of failure on that plan is when people ignore my advice and try to do a bunch of cardio. And fat loss stops.
In any case, there are several different plausible mechanisms by which the combination of excessive caloric deficits an large amounts of activity can cause problems. Whether it’s simply cortisol related water retention, a drop in metabolic rate due to leptin resistance or something else, something is going on. From a more practical standpoint, for a lot of people, the combination simply doesn’t work. Mind you, some seem to get away with it but not all.
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An Additional Variable
There is another variable that I have noticed over the years in looking at this issue. As odd as it sounds, it has to do with personality. In discussing this, for example, I’ve often noted that the people who seem to have the biggest issues with the whole lots of cardio/big caloric deficit tend to be a little bit ‘tightly wound’ (to put it politely). A bit less politely they are stress cases.
You can almost ‘hear’ the stress in their typing. Every post has lots of exclamation points and there is this undercurrent of “I MUST LOSE FAT NOW!!!!!!” in their posts. When fat loss stalls for a day, they freak out and want to cut calories or go add another hour of cardio. You can almost ‘see’ the tension in them as they sit hammering at the keyboard looking for solutions.
And this is an issue because these types of folks already over-secrete cortisol. As a true oddity, there is the issue of amenorrhea (loss of menstrual cycle). Typically it’s been thought to be related to body fat levels or caloric intake and this is a general cause. But there is often a type of amenorrhea seen in women without any of the normal predisposing factors. In this case, it’s all due to mental stress.
Basically, there is a subset of folks who are already high-level stress cases. They tend to be drawn to harder is better in the first place, tend to be resistant to change (like my client from my early 20′s) and their already high level of cortisol production is simply amplified by the combination of too much activity and too few calories. And suggestions to raise calories and/or reduce activity are invariably met by resistance (again, like my client from ages ago). What they really need is to just chill the hell out.