It's a bit of a tricky question honestly.
Do you ever see these social media posts from fitness influencer types going "oh ho ho ho, starvation mode isn't real! Let's not be silly! Here's what's really going on..." and then they go on to explain and describe... uhh... pretty much what you thought starvation mode was to begin with, am I right?
Here's the thing.
For a while starvation mode was a hypothesis for which there wasn't a lot of clinical evidence. Therefore many self professed "evidence based" fitness types rejected it outright, regardless of anyone else's personal experiences or observations. Because "that's just anecdotal".
Over the past few years though, we've started to understand a little more about Adaptive Thermogenesis, about the Constrained Model Of Energy Expenditure, and so on.
So... you'll see a few posts out there these days explaining "Starvation mode isn't a real thing that exists, but Adaptive Thermogenesis is", but from your perspective there's no fucking difference between one and the other. These people just don't want to own up to having put out shit content and shit advice for years. That's my take.
What IS Adaptive Thermogenesis, though?
First... humour me for a moment and imagine the following.
You haven't really exercised in a few years. You sign up for group fitness or you buy a home workout DVD or whatever. 15 minutes in... you're out of breath, heart pounding through your chest, sweaty, and feeling a little faint from low blood sugar.
Easy to imagine, right? Possibly something you've experienced.
You keep doing this workout 3x per week though and before long, you can make it through the whole thing and you actually enjoy it rather than feeling like you might puke, pass out, or die.
That's kinda what "getting fit" is.
At first, you're not terribly fit and it takes a LOT of oxygen and energy to get through a little of that workout.
But after a while... not so much.
As you get used it, you can perform the same amount of activity with less energy expenditure.
A reasonably acceptable hypothesis, right? The amount of energy expenditure required to perform a certain activity is not a fixed amount.
OK so let's continue.
One of the things that we now understand a little bit better about maintaining weight loss is... it's harder when you have a lower percentage of fat free mass than when you have a higher percentage of fat free mass. We also know that significant weight regain is more likely when energy intake has been low, compared to when "energy flux" is high. AKA high energy in, high energy expenditure. We also know that beyond a certain level, more activity does not equate to higher energy expenditure.
There are various mechanisms at play here but like we talked about and agreed upon above, energy expenditure is variable. So when you participate in exercise and activity while restricting to insufficient levels of intake, what you're training the body to do is to get better at performing that activity with that amount of energy to draw upon, and eventually to do it while drawing less and less from what is available.
But, you're training the body to cope with that level of exercise. Not so much to benefit from it.
Theoretically if you "eat less calories than you burn", you should lose weight.
In practice that may happen to begin with, but rather than drawing more energy from fat stores to make up the shortfall, the body starts to conserve energy instead. Rather than a reduction in body weight, you have a reduction in Resting Metabolic Rate. You have a reduction in the Thermal Effect of Food (aka the energy cost of digesting and metabolising food) as you're not eating much of it, and the energy that you would otherwise be expending at training is simply absent and unavailable, so your workouts kinda suck, you don't make consistent improvements in performance like you otherwise might, your enthusiasm runs out out sooner, you start to cut your sessions shorter, and you're less likely to be energetic and active throughout the rest of the day as well.
This Non Exercise Activity Thermogenesis is likely to be the biggest loss of expenditure to compensate for starvation... and I'd simply describe it as... you know, the difference between kinda dragging your feet doing what you have to do throughout the day, or having a bit of an extra spring in your step as you move around, dancing around a little bit as you're doing the housework, maybe feeling more inclined to do a little extra outside in the garden as well... vs just being fucken knackered all the time lacking the will to get up off the couch. Don't get me wrong though, I love my couch too and couch time is also valuable.
While you're over exercising and underfueling, what you're also doing is NOT making energy and resources available to add to your muscle and bone mass, which as we discussed earlier is what sets you up for greater weight gain when you run out of enthusiasm and gravitate back towards more sedentary habits.
So... call it what you want.
"Adaptive Thermogenesis" is the more scientifically valid term but it's pretty much the same difference.
What it does NOT mean though is "if you're late for a meal you go into starvation mode and get fat" sort of thing as you might have read at some point in time somewhere else.
How "Calories In / Calories Out" works is... if your energy intake is in EXCESS of your requirements, you get fatter. If your energy intake is IN DEFICIT (aka you take in less than you burn) you'd expect to lose weight, but if you're too severely underfueled for too long, your body reduces expenditure to compensate. It's a seemingly paradoxical situation of "no longer being in deficit due to insufficient energy provision, and needing to increase energy intake to get back into deficit by giving the body relief from the need to compensate and conserve expenditure".
For results we want to be fueled at a BENEFICIAL level where the body has the luxury of taking up all that we put in, and putting it all to good use in improving performance, replenishing glycogen stores, adding muscle mass, and improving bone density. Obviously to also see fat loss this needs to be short of the point of excess... but we want to train the body to productively utilise MORE and more fuel via foods, so as we become more proficient at exercise, as we progress to more challenging choices of exercise, more demanding training routines, and from performing at beginner level to intermediate level training enthusiasts, that point of excess gets higher and higher.
