A few questions about exercise and weight loss.

Factual Questions
1

I’ve been doing a fair bit of bike riding this summer; just for general health reasons and to lose a little weight. I’ve been keeping a spreadsheet to track my progress; mileage, average speed, weight, etc. More than 1,000 miles so far, 600 in just the last 30 days, which gives me a lot of time to ponder the mechanisms of exactly what’s happening. There are a few questions I don’t have any answers for.

  1. Is it really accurate to correlate a change in weight in terms of net energy input? Is my body such an efficient processor of the food I eat that I extract and store the energy from it and the weight of the food is irrelevant? I’ve heard that 3,500 calories burned is equal to one pound lost. Is there some calorie-dense food such that I can eat 3,500 calories with less than a pound of bulk? What happens when my body tries to process that, does it run out of building blocks to store that much energy?

2a. How long is the delay between burning calories and the corresponding change in weight?

2b. By what mechanism does the weight leave my body? Not to be too graphic, but I haven’t noticed myself excreting any more than usual. I sweat while I’m riding, but I usually drink about two full water bottles, so weight-wise, that seems like a wash. I suppose if I’m breathing in O[sub]2[/sub] (amongst other gasses) and breathing out CO[sub]2[/sub], that would account for some of it, but 15 pounds seems like a whole lot of exhaling to me.

  1. How much variation in weight is to be expected? A few days ago, I was down to 236.8, then popped back up to 239.something. I know my bathroom scale isn’t exactly lab-certified, but the results seem to be somewhat consistent with what I’d expect; up a pound or two after dinner, that sort of thing.
2

These are like physics style, perfectly efficient sort of measures. But yeah, the weight of the food is irrelevant. Some foods may carry a lot of, say, water, which runs right through you. Similarly, if you chow down on 5 pounds of ball bearings you won’t gain any weight once your body is done with them. Some foods may weigh less than a pound but contain more than 3,500 calories. Those calories are simply in a lighter form than the body fat you would store them in. Also remember, that if you are exercising, you are also likely building muscle while losing fat.

Okay. Good question. I got this one nailed though. I may gain as many as 5 pounds the day after I’ve finished a 50 miler. I’ve eaten a lot to do it, and consequently, there’s a lot of mass in my gut. My body is also filled with the waste products of exercise, lactic acid, etc, and the water that carries them. One week after a 50 miler though, I’ve found myself about 12 lighter. So, it can take a little while.

There’s too many variables. Have you weighed your poops before and after beginning your program?

Probably not. But you excrete sweat, feces, urine, and by breathing. I can lose two pounds overnight just by breathing out water vapor and sweating if it is hot and dry.

I’m 180 and I eat and exercise a lot. In normal times, I fluctuate about 5 pounds. So, I would say about a 6 or seven pound fluctuation would be normal for you.
While weight is a useful measure, don’t think in terms of weight as your goals. Really. Muscle weighs more than fat by volume, so you could gain weight and still shrink in size as you exercise. Your bone density may increase as well. These are both good things. They may cause you to weigh more, but decrease your overrall body fat.

There is good weight to carry.

3

In terms of intake, yes, it is the calories of the food that matter almost exclusively. The caveat is that some foods are absorbed less well and some take more energy to process. Nut consumption, for example, is associated with increased loss of fat in the stool, and possibly increased calories burned in processing the food (thermogenesis) such that a diet modestly higher in calories secondary to increased nut consumption may be associated with weight loss and beneficial effects on the lipid profile.

Olive oil actually has almost 4000 calories/pound (248 calories/oz), more than the 3500 that is 1 pound of body fat (as body fat also includes non-fat components), so yes, if you absorbed it all, you could gain more than a pound by consuming a pound of olive oil.

The calculations become messy because few remember (or sometimes even realize) that the output side is very dynamic - and not just as a result of the exercise burning X calories: the body responds to weight loss by slowing down it’s metabolism. Also hormonal levels vary over the course of the day and the year, effecting metabolism and water balance.

The rate of weight loss is extremely variable. Some of that is because of what Scylla already referenced, and some is because water weight responds differently than does fat and muscle loss or gain. Some “diets” cause a fairly rapid weight loss, but most of it is water loss. And again, the body can fight against fat loss by changing its baseline metabolism, only to have a sudden reset with persistence - plateaus are real.

The waste products of catabolism - that is the end products left over after tissue (be it fat or muscle) is used for energy production is excreted - in exhaled gases (such as CO2) and in the urine. The water is a wash. Your body regulates that balance pretty tightly and short term shifts caused by diet changes are not the stuff of lasting weight loss.

I have been unable to find any definitive studies on how much weight varies over the 24 hours of a day, but my personal experience is that 2 to 3 pounds is not unusual, heaviest at night, lightest in the mornings. Probably more fluid shifts than anything else (including exhaled vapors).

Agreed with Scylla - weight is just a crude proxy for fat free mass - the beneficial health effects are less achieved by the weight loss (which can be muscle loss) but by the healthy diet and exercise itself, even if no weight is lost at all.

4

Thanks for the answers so far. A couple of follow-ups:

I’m curious, but I’m not that curious.

But in general terms, do the waste products of exercise tend to show up in one place more than another, or do all excretory channels ramp up by the same percentage, keeping their relative proportions the same?

How would that work, though? It seems to me that as I digest the olive oil, it would be broken down and rebuilt into whatever molecules my body uses to store energy. But the olive oil weighs less than a pound. I’m going to run out of atoms before I run out of energy. Are there just always enough spare molecules circulating through my gut to make up the difference?

I’m not thinking of variation within one day, but from one day to the next. I get back from my ride, have something to drink and cool down, then I weigh myself before I get in the shower. It’s not the exact same time every day, but it’s under pretty similar conditions from one day to the next. Still, it goes up and down by a few pounds.

By the way, I mistyped up above. Force of habit, I suppose. I’m actually in the 220s now, not the 230s.

I’m not obsessing about it, just curious. While I’m doing this, I might as well learn what I can. And having something like that to keep my mind engaged is one of the things that keeps me at it.

I’m 6’1", so no one’s going to accuse me of anorexia for a while.

5

Depends on what is being broken down. Fat and glycogen/glucose break down into CO2 so its waste is exhaled. Protein breakdown produces CO2 and also nitrogen compounds that are mainly excreted in the urine. Any exercise breaks down some protein.

In short, yes. There is never a shortage of the basic building blocks floating around the body. In the longer version, don’t think so much unit for unit, think balance sheets at the end of the day.

6

The primary waste product of fat metabolism is carbon dioxide, which you exhale. Yes, your selfish desire for a healthy weight contributes to global warming! If your eating habits have not changed significantly, you won’t notice any particular difference in your poop.

Fats are built from carbon, oxygen and hydrogen. Much of the oxygen comes from what you breathe. Much of the hydrogen comes from water.

As others have said, there’s simply too much noise from day to day to get any meaningful data. There’s really no point in weighing yourself more frequently than once a week or so. As long as the long-term trend is downwards toward your goal weight, you’re good to go.

7

Scylla’s answer of five pounds sounds about right to me. Personally, I’m at about 165, my weight can fluctuate as much as 9 pounds in a 24-hour period. When I exercise, if I don’t take in any fluids, I lose 4-5 pounds in water weight. If I weigh myself after taking in a big meal with a lot of fluids and then the next day weigh myself after exercising, it can be (and sometimes is) a 9-pound difference. I am, however, a prodigious sweater.

So, definitely weigh yourself at the same time in the day (first thing in the morning is good), and look at long term trends, not daily weight fluctuations. If you’re retaining a lot of water weight for whatever reason (high salt intake, for example), the scale can be deceiving.

8

My suggestion is to get a digital scale if you are not using one already.

As already posted, there is a delay between intake and actual weight gain or loss. Your body can really change it’s metabolic rate too as you diet. Sometimes the weight can hang at a certain level even though you are eating far fewer calories than supposedly needed to support that weight. These plateaus are hard to understand. They say the body is conserving and going into starvation mode. The secret to breaking out of starvation mode is to “eat more and weigh less.” It’s unbelievable, yet so true. You can eat more and lose weight. The important thing is to eat the right foods and no junk.

PS. Keep a journal of weight as you are doing and then average it over four days to see more of the true direction and avoid those wild daily fluctuations.

9

I have a question along these lines open to anyone. How does a body know where fat is to be burned? In other words, how does it know there is fat on someones arms, gut, butt, legs, chin, etc–and how does it target those particular areas?

10

Former wrestler (who cut 1/3 of his body weight down for competition via starvation and exercise, which is not recommended). The key thing to keep in mind is that the law of conservation of mass can be (almost) discarded here, while the law of conservation of energy cannot.

As mentioned above, some foods contain high caloric energy and will allow you to metabolize more per pound (or store more), while others will not. This is the reason for “carb-loading” and similar exercise diets.

IME, for gradual weight loss, maintaining high levels of cardio exercise such as yours whilst maintaining fairly high caloric intake will not be much of a factor (as long as they’re “good calories”); your metabolism will accelerate with the workout and demand more calories (making you hungry), and then level off. Hence, the athlete’s lamented “plateau” period.

11

It doesn’t. When your body needs energy, certain hormones are secreted into your blood which signal fat cells to release stored fat. Other complex hormonal processes then break the fat down and use it to produce energy. In general, fat burning is a last-in-first-out situation. There is no such thing as targeted fat loss. Anybody who says there is is selling snake oil.

12

The reason for the discrepancy is that the human body also stores water in it’s fat tissues. So by the time your body reaches equilibrium, it will be retaining some extra fluids in addition to the fat from the calories you just ate.

FWIW, I track my weight with an Excel spreadsheet, plotted on a graph with a 7-day moving average. I consider that average to be my current weight because, as others have said, there’s a lot of noise in the measurement. I’ve noticed I can drop 2 lbs. overnight as I sleep, just from sweat and exhaled water vapour alone.

13

You breathe, sweat, and urinate it out: more details at Mayo Clinic’s explanation.

14

Actually, I find that kind of inspiring. Not only have I rid myself of 15 pounds of ballast, I’ve made it disappear into thin air.

Are we sure it’s noise, though? Maybe those fluctuations are the result of a sequence of metabolic and hormonal affects that operates on a time scale of a few days.

I’ll be sure to mention you all in the acceptance speech for my Nobel.

I am.

Is there any way to tell which side of the curve I’m on? If I eat a big, healthy dinner tonight, will those be excess calories that go straight to my thighs, or will it kickstart my metabolism and burn the fat away?

Update: 31 days, 620 miles.

15

It is really hard to tell which side of the curve you are on. This is why the moving four day average is so helpful. It overcomes the day to day frustration and anticipation of the morning weigh in. One important factor is have a very good idea of what you are taking in as calories. If you are convinced that you are taking in less than you SHOULD be burning, it’s easier to watch those plateaus. I recently went through such a plateau where the weight was hanging at 130# constantly even though I knew I was only taking in about 1300 calories and exercising all day long. Hell, I must have been burning 1300 just in exercise alone. This was very frustrating. I boosted the calories up to 2,000 and withing two days I was dropping weight by half pound to a pound per day. Something kicked in. I am convinced of this “eat more, weigh less” philosophy withing reason. So if you have been losing and think you are on a plateau, then be willing to throw caution to the wind and say, “I’m going to try this and what’s the worst that can happen?” Add 500 or so calories and be willing to gain some weight before proving yourself wrong. Test the theory. If it doesn’t work for you, go back to the fewer calories.
This is not merely my theory. Google and you will find it all over. I have known people who dieted so much they could barely eat anything because their systems became so efficient. One woman was counting grapes for gads sake. She would allow herself to eat only thirteen grapes! My goal is to be able to eat as much as possible because I am a glutton at heart. That’s just what I do now as I have boosted my calorie intake well above that 2,000 calorie level and still losing!

16

Well, I treated myself to a great steak last night, and I’ve got a lunch date at a Mexican restaurant, so that’s a start.