The most fascinating look at "starvation mode" was from a recent study interpreting data from the old Minnesota Starvation Experiment (MSE). Healthy, normal (not overweight) men were basically imprisoned for months, over 6 months IIRC, while they went through the following calorie phases:
- ad lib calories were recorded (eating whatever they wanted in the cafeteria)
- calories were severely restricted for several months. Body fat was shed initially in a sharp drop, but then completely plateaued as LBM continued to be shed.
- calories were raised, but not quite to the previous ad lib level. The body rapidly restored fat BUT NOT LBM as soon as calories were raised.
- the subjects were allowed to eat ad lib again. They ate MANY more calories than their previous ad lib, and their bodies continued to add fat until body fat reached its previous level from the beginning of the study. THEN the body proceeded to add back LBM, BUT 1) it continued to add fat while adding back LBM, and 2) at the end of the study, LBM was still slightly less (97% I think) than baseline, while fat mass was higher (like 110% or something).
So, body comp was worse after a period of severe calorie restriction.
I've also read informal discussion (not published scientifically, so don't know if it is true) that subjects suffered from a variety of long-lasting ill effects from the experiment.
The recent analysis of the data tried to look at mechanisms in the body for this apparent memory for fat mass and LBM and what exactly happens as the body fights to maintain it or regain it.
The experiment also matches exactly what I have seen happen in many obese friends and family members who have "crash dieted," and that includes many hospital-sponsored PSMFs, of which I have a very low opinion. Yes the weight comes off -- as it did in the MSE -- and it goes back on even faster than it came off.
Regarding "a calorie is a calorie," it would REALLY HELP if people could realize that it is JUST A MODEL of energy metabolism and expenditure. It describes pretty well (not perfectly: no model is perfect, nor should they be) what happens, on average over a period of time, in a person in a "normal" metabolic mode. The model has its limits of conditions under which it can make a useful prediction. The MSE described above shows exactly that severe calorie restriction DOES CHANGE the body's energy expenditure. You could call this "starvation mode."
It is also a near-universal truth of dieting that after a period of calorie restriction, your calorie expenditure will fall. I have been curious about this, so I bought a GoWear Fit calorie expenditure device and tracked my calorie expenditure while dieting. I decided to try severe calorie restriction. My expenditure remained stable for 3 weeks. Then it dropped like a rock. A lot of the drop comes from less movement and activity. Without the graphs from this device, I would never have been aware of this. It FELT like I was just as active as usual. Part of energy conservation is slowing and reducing movements. I also noticed a drop in BMR (calories burned while sitting or sleeping). You might think it would be easy to overcome the movement reduction by purposefully moving more. And to some extent, you can, especially with feedback from the device. BUT, even while you move, your energy expenditure goes down. So for the SAME activity or SAME exercise with the SAME perceived exertion (or greater!), you burn fewer calories.
Now some people interpret this drop in expenditure as proof your thyroid is crashing or leptin or whatever -- you're killing your hormones. They might be involved in this energy conservation somehow, but I don't think that's really the concern. I didn't have my thyroid tested, but I DID NOT notice any drop in my body temperature (morning, daytime, or activity) even while my calorie expenditure dropped significantly.
Still, the calorie model holds here for fat loss. Calories in was the same, but calories out dropped a lot, so weight/fat loss slowed. You have to either drop calories further, or increase expenditure through more activity.
So for short-term dieting, I think that conservation mode is a better term than starvation mode. Starvation mode is to be avoided, through a more moderate calorie deficit. Conservation mode is unavoidable, and the only solution is eating less and moving more.
I also firmly believe that most overweight people, and even many athletic types or bodybuilders, will have the best results losing fat at a fairly slow rate. I think the pound-per-week typical recommendation has a lot going for it. I firmly believe that you can MINIMIZE the LBM-wasting panic mechanisms seen in the MSE by taking a slower, more patient route. By "the best results losing fat," I mean minimal loss of LBM, minimal loss of perceived energy (which leads to less EE anyway), AND least rebound hunger that drives you to eat like a pig till all your fat is restored.
As far as people who believe that they are dieting but not losing weight, I find the answer is always simple. It's not crashed hormones. It's just that normal, natural conservation mode that is INEVITABLE with dieting: you WILL expend less energy, and you WILL have more hunger and drive to eat, even if you are not conscious of it. That's how the body works. And if it didn't work that way, people could die of starvation from an ill-advised low-calorie diet. The body protects itself from this extremely well. You cannot change this.
So if someone claims to be eating very little but not losing fat, but then I require them to measure every single molecule that goes into their mouths, and record it (say in Fitday), they find that ON AVERAGE, over say a week, they are eating many more calories than they claimed. I've had people PM me, then refer me to their blog...in their PM, they list a diet that should have them rapidly wasting away, like plain lettuce, chicken breast, 6 almonds, stuff like that. Then I go over to their blog, where they describe the "WHOOPS" moment of eating an entire (large) bag of Dorito Chips. They do not seem to realize that although they suffered through 6 days of low cals, they blew their entire deficit for the week with that bag of Doritos. This is Leigh Peele's whole thing, and she's right. When you make people record everything, the slow rate of fat loss becomes clear.