Saturday, October 09, 2010

The Fate Of Ingested Fat Vs. Carbohydrate

I was going through some old papers and saw this:1
"It is well known that when dietary carbohydrate is converted to fat by de novo synthesis the obligatory cost is 23% of original calories from carbohydrate, whereas the cost of carbohydrate storage as glycogen is only 7%. The cost of deposition in adipose tissue of dietary fatty acids as triglyceride, in contrast, is only 3%."
Okay, so...

If we eat 50 calories of sugar (about 3.3 teaspoons):
  • We spend about 11.5 calories storing it as fat, or
  • We spend about 3.5 calories storing it as glycogen (the storage form of glucose in humans).
If we eat 50 calories of oil (about 1.25 teaspoons):
  • We spend about 1.5 calories storing it as fat.
  • Eating 50 calories of carb leaves us with only 38.5 calories of stored fat. (Eating 100 calories of carb leaves us with only 77.)
  • Eating 50 calories of fat leaves us with fully 48.5 calories of stored fat, almost the entire amount. (Eating 100 calories of fat leaves us with 97.)
Not only does it take more volume of food to ingest the same calories, when we eat carbs vs. fat, but we likely fritter away more of those carb calories storing their energy as fat.

Of course, it's not this simple. Everyone's physiology is different. We all digest and absorb in different ways and at different rates. How and how much of a nutrient we absorb is also dependant upon the form in which it's presented* and what other nutrients and items are simultaneously present. How, and whether, and into what form those nutrients are deposited also varies by individual. It's not straightforward.

Nonetheless, I do wonder if any of this will affect Chris Voight's post-lab results. (Voight is eating 20 potatoes a day for 60 days. Here are his pre-potato lab results.)

*For example, we absorb fewer calories from a given sample of starch if it is in the form of resistant starch. Starch can, in fact, be made more resistant to our digestion by heating it and allowing it to cool before ingesting, e.g. we absorb fewer calories from cold pasta than we do from that same pasta if it was hot.
1 Expenditure And Storage Of Energy In Man, The Journal of Clinical Investigation, 1987.
Photo: Bix.


Perovskia said...

What does that mean, "de novo synthesis"? I've never heard that before.

Bix said...

I think of "de novo" as "directly." The first thing it does.

Phoebe said...

You need to keep in mind in your estimation of ingested sugar that sucrose is composed of one glucose and one fructose molecule, and the body does not deal with them in the same manner. Additionally, every cell in the body can break down glucose, while only the liver can metabolize fructose.

Angela and Melinda said...

I'll certainly be interested to see Chris's results!