Q: WHY is the combination of high intensity and/or long duration activity a mistake when calories are being severely restricted?
A: Here's a precis on some of what's going on:
As someone else pointed out, the body seeks homeostsasis and it is very good at fighting back in terms of fat loss.
Moreso for women's bodies than men.
So what's up with the high intensity thing and fat loss especially in the context of hardcore caloric restriction?
A few things to consider
1. What is burned (calorically or energetically) during activity is only part of the equation, of more relevance is that happens the other 23-24 hours of the day. Sometimes, when people try to train intensely and this is magnified on low calorie diets, it means that they compensate elsewhere during the day for their activity. Why? Because they are tired.
And you tend to be more tired from high intensity stuff. So you do less later in the day. If training too intensely for that 30-45 minute span (and say you burn 25% more calories) means that you sit around more for the other hours that you're awake (burning 30% less calories), that can more than compensate for what you did in training.
The problem is that folks are overfocusing only on the calorie burn of that activity itself when there are other important factors at work. Who cares if you burned 200 more calories during activity if it means that you sit around so much later in the day that you burn 300 less?
Note: numbers are being used for illustration, don't read too much into them.
2. There are also clear hormonal effects. I've talked about leptin endlessly on the site so I'm not retyping it here. Just note that in the original PSMF + lots of cardio study,they noted a larger drop in metabolic rate with the addition of lots of cardio compared to without and that's why I made the suggestions that I made in the book.
Essentially, the body senses caloric availability which is simply intake - output and that determines a lot of what's going on. Women's bodies seem to respond generally more greatly to shifts in this dynamic with negative adaptations. So the combo is probably relatlively worse for them than for men (although neither group does particuarly well with it, women just do worse).
Cortisol is another biggie. Excessive activity, and this is magnified with large caloric deficits, raises cortisol and this has a number of potentially negative effects.
Here are two:
a. Water retention: cortisol binds to the mineralocorticoid receptor (the receptor involved in water retention, well one of them). And although cortsiol has like 1/100th of the effect on water balance of the primary hormones (aldosterone and a cople of others), since there is like 8000 times as much of it it can cause a major effect.
I strongly believe that a lot of the 'metabolic magic' that some people are reporting (e.g. weight loss not scaling with anything logical) is simply water retention. Why do I think this? Because invariably when you get those folks to chill out,and either raise calories or cut activity, the problem goes away. There can also be subclinical thyroid problems present in a lot of women and that too causes water retention.
And, as you'd expect, some women are relatively more or less prone to this.
Consider that, for women under normal circumstances, water shifts over the cycle can be up to 10 pounds. I'm sure some woman go through more while some clearly go through less. That alone can mask fat loss. A woman who should be losing 2 lbs/week would have that fat loss masked for 5+ weeks. On a more moderate 1 lb/week, it would take 10+ weeks for the fat loss to show up against that water balance issue.
Now add to that the stress of hard dieting and excessive high-intensity training. The problem is magnified because this will raise cortisol that much more. I ranted and raved about this in the interval vs. steady state series on teh main site, I'd suggest reading that. But I see dieters trying to follow training programs that no elite athlete could recover from. And the elite athlete is eating enough.
As well, some people have a personality type that can only be described as 'wound a little tight'. They are chronic stress cases under normal circumstances, they are the ones that tend to respond to weird things like a lack of weight loss by 'getting really stressed out about it' and trying to work harder. These folks already tend to overproduce cortisol and it JUST GETS WORSE when they try to do too much activity with too little food.
b. Excessive cortisol, especially chronic elevations cause other problems not the least of which is leptin resistance. Which only magnifies the drop in leptin from dieting. This could be another mechanism behind the greater drop in metaoblic rate for the study I mentioned above.
The bottom line, simply is this: the combination of excessive deficits and either too much or too intense activity doesn't work for the majority. The why is interesting, do'nt get me wrong; at the end of the day, the practical implications are what's important here IMO.
