Measuring my metabolism…crudely

 kw: analytical projects, weight, metabolism

For decades we've had a bedroom scale, the kind with a spring. Over the years I found that the weight it showed was a little variable if I leaned one way or another. Naturally, being overweight, I soon learned how to lean so the weight shown was as small as possible, without me falling over. I finally realized I was fooling myself, particularly because my weight in the doctor's office, adjusting for clothing and shoes, was about five pounds greater than my "home weight". That made a big difference to me, emotionally, because it pushed me over a boundary: I am just six feet tall. At home I would get a weight of 215-216 pounds, for a BMI of 29.2-29.3. That's near the top end of "overweight". A couple of years ago the medical scale showed 226 pounds, and with shoes on and my cell phone in a pocket I had five pounds of "accessories", for a naked weight of 221. That's a BMI of 30, which is "obese". Boo-hoo!

Over a period of about a year I did my best to eat more moderately, and my doctor said one day, "Oh, you've lost a little weight." I weighed 224 clothed, or 219, a BMI of 29.7. My "home weight" was 213-214. Better, but not good enough. Then I caught Covid-19, to which I reacted by fasting for several days (low blood sugar reduces the chances of getting pneumonia). That brought my "home weight" below 210 pounds.

I bought a digital scale. Its reported accuracy is 0.2 pounds, which is 3.2 ounces. This scale helped me calibrate the old spring scale, which is actually surprisingly good. If I stand straight on the spring scale I get the same reading, within a pound, as the digital scale. Of course, I am rather enamored of that extra digit, and the digital scale is much easier to read.

I had read enough of recent literature to realize that it is eating sugar and "fast carbs" like potatoes that cause weight gain, not eating fat. I went nearly full-carnivore for a while. When I had eggs for breakfast (I typically fry 3 eggs in olive oil), rather than buttered toast I had breakfast sausage: two of the little links, which each have the same number of calories as a slice of the bread I'd use for toast. I quit having sandwiches; just some lunch meat and cheese, which I'd take to work in a baggie, or prepare on the spot when I ate at home (I work 3 days/week). I began losing about a pound weekly. My present morning weight is 195 pounds.

I've been weighing myself morning and evening for some time, and I took note of a certain regularity. After a morning pee, I weigh a pound less (plus or minus 0.2 pounds) than in the evening before bed. Then for a few days I checked my weight in the morning before visiting the toilet, and found it was either 0.4 or 0.6 pounds less, and another 0.4 to 0.6 pounds would go into the toilet. I thought, "I am losing half a pound overnight just by breathing!"

I realized that I was measuring my metabolism, in a crude way. That 0.4-0.6 pounds (6.4-9.6 ounces, or about 180-270 grams) represents glucose being oxidized and its oxidation products being exhaled.

If you have had any exposure to biochemistry, you'll find this formula familiar:

Glucose, the primary sugar in grapes, is the energy currency of life. Plants use photosynthesis to add carbon dioxide from the atmosphere to water brought up from the roots, producing glucose and releasing oxygen to the atmosphere. Animals consume plants, from which they obtain glucose (and lots of other chemicals); they add oxygen to the glucose to break it down to water and carbon dioxide. This simple chemical formula hides the complexities of the Krebs cycle, and the energy input by ATP to keep it running during respiration, or the production/activation of ATP during photosynthesis.

To see where the weights come and go, here are the atomic masses of these molecules:

• glucose - 180
• oxygen - 32 (a 2-atom molecule)
• water - 18
• carbon dioxide - 44

In chemistry, a mole is the weight in grams times the molecular weight. Thus a mole of hydrogen atoms weighs one gram, and a mole of carbon dioxide, which has a molecular weight of 44, is 44 grams. If we multiply the last three numbers above by six, to correspond to the equation above, we find this:

In respiration, 180 g of glucose combines with 192 g of oxygen to produce 108 g of water and 264 g of carbon dioxide. In photosynthesis, the opposite happens. The two gases outweigh the sugar and water.

These are the gram weights appropriate to the oxidation of 0.4 pounds of glucose. For those of us who are not as familiar with metric, I'll take advantage of the fact that all the weights in the prior paragraph are divisible by 12, and use somewhat greater actual weights, for the following:

In respiration, 7.5 oz of glucose combines with 8 oz of oxygen to produce 4.5 oz of water and 11 oz of carbon dioxide.

7.5 ounces of glucose is 0.47 pounds, so this is similar, and gives me a feel for what is going on.

How much energy does this represent? We are told in nutrition tables that a gram of sugar has four calories (the dietary calorie is a Kcalorie, or the amount of heat that must be used to raise the temperature of a kilogram of water 1°C). I found that the oxidation of a mole of glucose releases 280 calories. One mole of glucose, as above, is 180 grams, almost exactly 0.4 pounds, so at the low end of my nightly weight loss (before urination), my metabolism has produced 280 calories. When I sometimes get a reading of 0.6 pounds, that converts to 420 calories. I suspect that the actual amount is the same each night, but when I have a bedtime weight of 195 pounds, it might really be anywhere between 194.9 and 195.1. Then, if the actual amount of glucose "burned" is 0.5 pounds, my morning weight would read either 194.4 or 194.6. Half a pound of glucose converts to 350 calories.

I sleep 6-7 hours. Let's use an average of 6.5. Dividing 350 by 6.5 yields 53.8 calories per hour. Multiply by 24, and we find 1,292 calories per day. That seems to be my basic (not basal!) metabolism when at total rest. A Basal Metabolic Rate calculator tells me 1,628 calories per day. Further, for an entirely sedentary man my age, daily caloric need is 1,989. Explanatory text points out that "body maintenance" for most people is about 70%. If I understand that correctly, let's see what I get by dividing 1,292 by 1,628: 0.79 or 79%. And then 1,292 / 1,989 = 0.65 or 65%. So it seems to be in the right range.

There is a second factor to consider, which is nitrogen metabolism. Small amounts of protein are discarded daily. They are the source of the nitrogen in urea, the main non-water component of urine, and creatinine. The urine of a healthy and properly hydrated person contains 9.3 g/dL of urea and 0.67 g/dL creatinine. The latter can be ignored for practical purposes. I don't have a general molecular formula for protein, so I'll just do a simple "bonehead" analysis.

Molecular weight of nitrogen: 28
Molecular weight of urea: 60

Thus a half-pound of urine (one cup or about 120 ml, or 0.12 liter) contains about 11g of urea, or 5.2 g of nitrogen. Roughly speaking, protein is 16% nitrogen, so that represents the discard of about 32 g of protein. That is insignificant compared to the amount of glucose metabolized, at least in terms of energy/calories.

Daily urine production for a man my size is about 1.4 liter, and it will apparently contain about 130 g of urea, which contains 61 g of nitrogen, derived from the discard of 380 g (13 oz) of protein. This emphasizes that we need to consume at least that amount of protein daily, because it can't be produced by converting either carbohydrate or fat; they contain no nitrogen.

These numbers are quite at variance with the recommendation that adult men need 0.8 g of protein per kg of body weight. That converts to about 70 g/day for me (195 lbs = 88 kg). I need to do more research, because the discrepancy between 70 g and 380 g is huge. This could take a while…

Bottom line: Someone who weighs about 200 pounds can expect to lose a half pound of glucose overnight, expelled in the breath as water vapor and carbon dioxide. In addition, about an ounce of protein is lost overnight and expelled during the morning pee.