From a personal experiment standpoint, here were my personal findings.

Background: I am a muscular male with considerable bodyfat remaining (even though it is 150lbs less than what it was, I'm still fat).

As a personal experiment, I decided to follow an IF protocol, paired with fairly heavy caloric restriction. To top it off, I was eating in Paleo compliance. Two meals a day, eating about 700-800 calories per.

On sub-1700 calories /day, while still being relatively low carb (>75gm of carb/day). I did not gain, nor lose weight. I did this for 3 whole weeks where I may have had a single day exceeding 2000/calories. I was physically active (yoga in the morning, weightlifting twice/weekly, daily walks or swimming for 30-min or so). To top it off, I was miserable. My hands/feet were ice cold constantly, I had zero libido, couldn't sleep more than 5 hours, and lost my temper at the drop of a hat (I HATE HATS!).

When I realized that wasn't working, I went back to barebones primal/paleo. As soon as I hit 2300-2400 daily calories, I started losing weight again.

Taking this experiment further, I bumped the calories up to 3400/day. Same weightloss stalemate as the caloric restriction, I didn't gain - nor did I lose.

So, on recovery days I might eat 3000 calories, but otherwise I eat around 2400/cals a day and I'm thriving. Less than that I stall, more than that and I stall, and I gain when I binge on really bad foods.

The point?

The law of thermodynamics is great when you are talking about fuel combustion, but it's a mistake to adapt that law to the human body (in my opinion). The world is still under the impression that the flammability of a food (calories) is the only thing relevant to weightloss. While calories are still relevant, this is a grave simplification of the complex metabolic workings of the human body, considering that we are not furnaces, we have regulatory processes and evolutionary conditioning to thrive a certain way. We don't incinerate food in our guts, we break it down to it's primary constituents and absorb them. If thermodynamics was the only factor in animal metabolism, then the most efficient food we could eat would be crude oil and lumber.

But vitamins, minerals, and toxins are really what we should be looking at.

Eating adequate calories, the right kind of calories, when coupled with attention to foods that cause inflammatory or other physical response, results in a reduction of physical and mental stress. Your body thrives, and it makes itself healthy again. For many people (not all, mind you), inflammation and stress are the most considerable factor in weight gain.

One reason can be that your body thinks it is a time of famine. When that happens, it starts protecting whatever it has, metabolism slows down, and your weight goes up. Even worse, the next time you eat more, your weight goes up even more, because your body thinks "great, food, let's hoard and prepare for the next famine!"

Those are typical starvation diet symptoms. But I honestly have no clue where that starvation mode starts in terms of calories.

Oh, and I know people who are nowhere near starvation mode, yet gain weight until they start eating more fat. Hoping that "It all starts with food" might shed some light on that... currently reading. :)

I've written about this a couple of times on here before:

here http://paleohacks.com/questions/129309/science-behind-4500-cal-diet/129341#129341

and here http://paleohacks.com/questions/41372/calories-combustion-versus-digestion/41533#41533

and probably a couple of other places.

While the laws of thermodynamics are true, the only thing you have control on is what you put in your mouth. You may think you have control on calories out by exercising and such, but you really don't. Your hormones control your metabolism and how much you actually burn. So if you do take in too few calories you'll burn even less if your hormones are telling your body to store energy.

I don't see why this is impossible? People who undereat likely lower their metabolism by doing so. If you got a serious thyroid issue you might not lose weight even if you really restrict calories. And if you do lose weight, it's probably muscle.

I'll put a slightly different spin on the answer.

1. If you coinsistently eat fewer calories than you burn, you will over time lose weight. The rules of physics apply.
2. When you go hypocaloric--eating less than you burn--several things happen including: your basal metabolic rate frops somewhat and your body burns its own mass for energy. This is good usually, if it's fat mass that you are burning.
3. Some people who go hypocaloric do not do so with good quality foods or with macronutrients that encourage fat burning. To prevent muscle wastage, you need to have either 50+grams of carohydrate, or tons of protein.
4. Some people who are hypocaloric binge.

So to sum it up, the trules of physics apply, hypocalric eating will reduce your metabolism, macronutrients matter in hypocaloric eating, and bingieing can do you in.

Hope this helps.

Tony

I think there have been a lot of good responses so far, and I would particularly agree with the comments some have made about metabolism going down when you starve yourself.

Above and beyond that, I'll make another observation: there's a whole lot of biochemistry happening in our intestines that is directly influenced by what we eat, and it varies tremendously from person to person depending on which gut flora dominate. Certain bacterial species (e.g. Bacterioides) are relatively good at converting "indigestible" things (like fiber)into starches and sugars that we can metabolize. This is more likely to occur in situations where we're cutting back on the sugars and such that would normally be keeping e other bugs happy and dominant. Estimates for how much extra energy that can be produced by gut flora vary, but it might easily be a couple hundred kCals. Obviously this can't account for the total lack of response even when you've dropped your dietary caloric intake to seemingly ultralow levels, but it might still be a contributing factor--you could be getting a few more "effective calories" than you think you are, and conversely, under other circumstances the gut flora might be chewing up more calories than they would in a starvation situation.

To prep for "The Machinist", each day Christian Bale had a cup of black coffee and either a can of tuna or an apple.

He didn't gain any weight.

(Wouldn't recommend that approach)

Eat less burn more and you will lose weight.

I've never seen FAT starving people....have you ever seen people in a concentration camp? Not a fat one amongst....

It doesn't lead to weight gain- it leads to diminished body composition.

How? Your body is smart. When you start restricting calories heavily, it triggers a "famine response" in your metabolism. What this means is that your body starts becoming more efficient at operating on fewer calories. It does this by using its most metabolically expensive tissue for energy (muscle) and hording its least metabolically expensive tissue (fat). It does this via ketosis- meaning the inefficient break down of fats for energy, so you don't run through your long term reserves (body fat) so fast and die.

So, certain things become less efficient (the break down of fats for energy), but this is what makes you more efficient at operating under starvation conditions.