Weight loss question: mechanisms of adaptation

[jbank]

I certainly don’t weight things I don’t sweat the accuracy too much. I only use the spreadsheet
when I’m actively trying to cut weight, not while I’m just maintaining weight. I’m usually shooting for
a daily deficit of >500 calories, so a couple 100 one way or the other isn’t a huge problem, I’m still
going in the right direction.

I found velocity to be a pretty good rough metric for cycling. I mostly used it for outdoor training
in the past, but my KK trainer has a pretty reliable speed/power curve. Tools like trainer road manage
to use speed as a reasonable proxy for power with some trainers. I certainly agree that I wouldn’t use HR.
Distance seems like a pretty bad proxy for the bike. I just looked at an example in my logs and found
an easy recover ride used 34 kcal/mile vs. a hard ride that used 43 kcal/mile. I guess if you always go
at a similar effort, the error might not be so bad.

[bradl016]

Been trying to cut some weight for races this year and have managed fairly
well with a simple spreadsheet tracking calories in and calories out. I weigh
myself daily and plot a 5 day moving average and try to validate that the
actual weight loss matches the theoretical weight loss based on total
net calories/calories per lb. I’ve seen various threads here where people
debate “its just the calories stupid” vs. “your body adapts”, but I haven’t seen
a discussion of the actual mechanisms for how your body adapts discussed.
Is there a good understanding of what actually happens? Here are some
possibilities off the top of my head:

1.) Your body modifies how efficiently it extracts calories from the food you
eat. In the extremes, you could imagine it either just lets food pass through
entirely (0 calories), or uses 100% of the available calories.
2.) Your body regulates something (temperature?, something else?) to cause
more/less energy to be used per unit time. If this is the case, what is actually
being regulated?

On a quasi related note, I also wonder about what seems like a counter intuitive
aspect of measuring calories from exercise. Being fit seems to allow me to burn
more calories per unit time. For example, after some off-season, I can go all out
on my bike for an hour and average say 200 watts, and my power meter tells me
I produce ~700 kj (just ball parking here). After a few months training, I go all out
for the same hour at 250 watts and produce 1000 kj. The usual thing I’ve seen is
that due to efficiency being ~1/4 and roughly 4 kj/kcal, you can just use kj as a good
proxy for kcals. Did I really use more kcals/time when I was more fit, or did my

Hey, I’m sorry if some of my thoughts are redundant but I couldn’t keep up with all the responses you received:

On your first question - I’ve never heard of the digestive system adapting to respond to calories consumed. We rely on bacteria to digest a large proportion of our food which could mean that a change in the microbiome may affect calories available. On a related subject, the pancreas will adjust levels of enzymes to match what you typically eat. So, if you (previous to your diet) ate mostly fatty foods, your pancreas will produce more enzymes that digest lipids. This doesn’t really apply, since you are talking about cutting foods out, not adding. It IS possible that (because of your starvation), the body may be more sensitive to what carbs it gets and the insulin response will be more dramatic. There are far more opportunities, however, for the body to regulate its calorie consumption. Down-regulating things like metabolism (which includes cellular division, body temperature, heart rate and digestion) is a natural starvation response. So, in terms of weight, while (a calorie not consumed) = (a calorie saved) in Week 1 of the diet, (a calorie not consumed) probably = (.95 calories saved) in Week 6. Does that make sense? I feel like I can explain it really well to myself. I’m not sure that it comes across clear. So, just to reiterate, if you plan to lose 1 lbs a week, you would have to progressively cut more calories from your diet.

Secondly, it is my understanding that as you get more training, your muscles become more efficient at . So, the calories it takes to power them is less because your muscles have: a)stored more glycogen, which means that your muscle relies less on body fat and more on its own energy stores b) you recruit fewer muscles to perform the movement, which leads to greater efficiency.

Over all, you may be burning more calories but fewer of those calories would come from stored fat reserves. So while your body may be more efficient at metabolizing fat after a season of cycling, it will be less reliant on the body fat.

In each instance you would see a diminishing return on dieting.

To finish up with a personal interjection: I have been trying to diet in this ‘base/build’ period since January. Using an online tracker (very handy) and IGNORING my calories from workouts, I’ve got from ~166 to 157 lbs (yesterday morning). It seems like your spreadsheet is working but I thought I’d add the endorsement for MyFitnessPal and ignoring workout calories.’
’
Either way, best of luck with your races this season

[jbank]

To finish up with a personal interjection: I have been trying to diet in this ‘base/build’ period since January. Using an online tracker (very handy) and IGNORING my calories from workouts, I’ve got from ~166 to 157 lbs (yesterday morning). It seems like your spreadsheet is working but I thought I’d add the endorsement for MyFitnessPal and ignoring workout calories.’
’
Either way, best of luck with your races this season

Thanks for the interesting response. My spreadsheet seems to be working fine, so I’m not
really looking for a new technique, just a deeper understanding of what else is going on.

I’m not that surprised that you get results ignoring calories; for me much of the benefit of
any tracking is eating thoughtfully. The process of recording what I eat makes me take a
bit of extra time to evaluate what and how much I am eating. It does seem a bit odd to
me to ignore workout calories completely if you do large workouts. I’m still in my build
phase and there are days where I burn >2000 calories. As I continue to ramp up, I expect
that will turn into >3000 on some days. That is a lot of calories to not take into account.
If I didn’t eat a lot extra on those days, I think my training would suffer dramatically.

[kyle_s]

I suspect I didn’t ask my question at the end very clearly. I’m wondering
which of the two models is correct:

-) 1 hour at my max effort always is X kcals burned, regardless of how fit I am.
-) When I am more fit, 1 hour at my max effort is more kcals burned

If model 1 is correct, the difference must be in efficiency, since the kj’s
produced is more. If model 2 is correct, it sucks to be out of shape

(Edited for slightly improved clarity)

I think Ill be repeating some people but Ill still throw my .02 in. My understanding is that you can in fact burn more calories at a higher “fitness level” because calorie expenditure is directly related to oxygen consumption. So if your VO2max goes up, you should also have the ability to burn more calories (at a max effort anyway…at a given submaximal VO2 nothing will change).

The substrate burned will change with intensity. Lower intensity = more ketone bodies (fat), less glycogen (carbs). Higher intensity is the opposite. There is a crossover point, something many people argue does not exist, where the two switch over. The theory of “metabolic efficiency” would suggest that you can, through diet and training, alter that crossover point to allow yourself to burn fat at a higher intensity, preserving glycogen stores and allowing for longer duration of exercise before bonking or simply the need to fuel less. Since fat contains roughly 9 calories/gram and carbs have about 4 calories/gram, you could theoreticlly burn more calories per hour if you tap into the fat more. And everyone (except the SUPER lean 5%BF human anomolies) had plenty of fat to tap into.

When talking max effort, you are talking glycolytic pathways and very little fat burning so it will become a function of your muscle capillary density, fiber type make-up (fast/slow twitch) and the ability for you to over-ride your central governor (brain) to endure the effort without imploding.

Also, keep in mind, the algorithms used for Kcal burned are all different for every device manufacturer and accuracy is always suspect. Compare your time over distance by doing repeatable marker distance rides to see improvement in efficiency.

To the person discussing fat and carb utilization…you should basically be burning the same number of calories per distance (run, lets say) regardless of the relative percentages of carb and fat being used. You would not be burning more calories by tapping into fat more, especiallly since you would have to operating at a lower intensity (and consuming less oxygen) to do so. And for what its worth, even someone with 5% body fat should be fine for an ironman at least, even if they are burning mostly fat.

The crossover point isnt so much where they “switch over” in the sense that a switch is flipped and it switches from fats to carbs. Each is a gradient relative to intensity (fat decreases with intensity and carb increasing) and the crossover point is simply the term applied to intensity at which one is burning 50/50 carb and fat.