Hey there
I've recently started Low-carbing again (I was doing well for a few months last winter, but fell off the wagon as finals approached and stress loomed), and I do a pretty strict regimen at the gym about 6 days a week. Normally it consists of weight training (3 seperate workouts - Chest/biceps, shoulders/triceps and legs, essentially. I know there are other muscles to work, I do them too).

However, I then go for an hour on the elliptical trainer about 4 times a week, depending on how I feel after the weighttraining. I typically burn 800 calories. However, how much of this is bodyfat? When I burn 800 calories after the induction phase, is it 800 calories of fat leaving my body? Is it perhaps the fat that I ate at lunch, some 4 hours ago? I'm just wondering how I should count the progress I make - I know there are the visual results, but I'm in engineering and I love to do the numbers.
Thanks for any input!

Logic says it's mostly fat.. after all: your glycogen is mostly gone.
However: the less carbs you eat, the more carbs are coming from protein. Day by day about 100grams of carbs are derived from protein, even when you eat zilch, nada carbs.

If you don't eat the protein, it'll be derived from muscle. That's what happened to me when I reeked after ammonia during exercise this summer.

Also: 800 kcals/hr doesn't sound very likely.. 500 kcals seem more likely. at least that's what I burn per hour when I do interval cardio for 25 mins on a Johnson elliptical. Other brands go way higher, while resistance feel lower.. go figure.

So... it would not be around 800 but more like 500-600 .
and about 10-20% would be coming from carbs I guess.
More like :500-50= 450 kcals = 50 grams of fat?

fietster- or anyone- what do you mean "carbs are derived from protein"?????? Carbs and protein are different;...Am I missing something? or do you have your wording wrong? As far as everything I've read and know says, carbs cannot be derived from protein....
Do you mean energy? That the body is using protein for energy, and if you don't eat enough protein, you'll consume your own muscle? That is what happens, and makes sense.

Well if I lose 100g of protein a day to carbs (since both are 4 cal), then there is no way that I am eating enough protein (I maybe get 80-90g a day now). Is there something that I am missing here? I'm hoping that there isn't a linear conversion between protein and carbs based on caloric content.

Also, I have been doing 800 cal in an hour at constant resistance on about 3 different brand machines (from cheap to expensive). I keep the resistance at halfway and work constant. My heartrate is generally about 180 to 190, but I have always exercised at high heartrates.

Also, I recall that the body was less efficient in burning fat than carbs. Does that mean it takes 9cal of exercise to burn 1g of fat (assuming that all the energy obtained came from the fat, ideally).

Thanks again!

Ever heard of gluconeogenesis???

It means: creation of glucose out of nothing.. err that is: protein.
The N-part is taken away and then the remaining Carb-part is easily changed into glucose. We easily do so.. have done so for millions of years.. any living organism does so.. whether we eat carbs or not, we always need glucose and will make it out of protein (is faster) or fat (slower , is done in aerobic phase).

So, a minimum protein intake would be 2x body weight starting at 150 grams a day if you are lighter than 75 kg. (2,2 x kg = lbs).

Past summer I dieted too much and ate too little. While I ate enough protein, it was a bit alarming to find out that the awful ammonia smell came from protein being broken down. YOu never really know whether it's food-protein or muscle, but from my bodyfat measurements it appears to have been muscle as well.. I lost fat alright but muscle went faster.
By eating approx. 75-100 grams of carbs/day I have enough glucose for the brain and more of the protein I eat.. will be used for muscle building or maintenance.

Normally nearly all of the glucose goes to the brain.. even though it works well on ketones we will always need glucose as well for the brain.
For exercise this only applies for anaerobic exercise.. if you go anaerobic, your body needs ketones (and steals it from the brain) and glucose.. which is also stolen from the brain and derived from either muscle or food protein. Or if you're clever enough from the small amt of carbs you eat during or before exercise..

I don't believe in ketosis as a means for optimal weight loss.
Ketosis is a means to
- diminish hunger in the early phases of your diet
but above all:
- to kick the carb-addiction-habit

It looks like I may invest in a protein supplement. I'm not a huge guy, although I can only manage to eat so much protein without packing my stomach. At my current eating habits, that would mean about 50g more protein intake. Well thanks for the helpful advice.

IMO, Dr. Ellis would be the perfect person to answer this question, however, I'll try and shed a tiny bit of light on this for you.

I believe that it has more to do with your diet composition over the past several weeks. In the absence of carbs, your body becomes more efficient at burning fat for energy (fat adapted) over time. The range of carbs should be somewhere below 25% of your daily caloric intake. The main thing is that you burn the calories. If at the end of the day your body needs more calories to feed your lean body mass, I believe it pulls most of it from stored fat and to an even lesser degree (thankfully), muscle tissue. I don't know the exact numbers here (or science), but you can minimize the loss of muscle mass by providing optimum stimulus, protein and fat.