Explanation of respiratory quotient?


(BuckRimfire) #1

Can anyone give a reference to a good textbook explanation of respiratory quotient? My wife has to give a lecture on the subject next week and isn’t feeling fully prepared!

Her lectures are not fully keto-preaching, but they’re keto-adjacent or keto-curious! :wink:

(Michael - When reality fails to meet expectations, the problem is not reality.) #2

Start here:

(Bacon for the Win) #3

why is she giving a lecture on a topic that’s not in her wheelhouse? Seriously curious.

(Doug) #4

It’s really just the carbon dioxide one breathes out divided by the oxygen one breathes in and consumes.

CO2 / O2

A person will breathe out a given amount of CO2 over a given period of time. What will change with diet is the bottom number - the O2.

Fats have very little oxygen in them - they’re almost all carbon and hydrogen. In human metabolism, the Respiratory Quotient is ~0.7 if we’re just eating fat.

Protein comes with more oxygen in it, and the RQ will be around 0.8 if we only eat protein - we’re getting more oxygen from the food, and don’t need as much from the air, so the bottom part of the equation gets a little smaller, and the RQ thus ends up a little bigger.

More oxygen yet is in carbohydrates, and the RQ is ~1.0.

For “standard diets” the RQ is usually around 0.8.

(Bacon by any other name would taste just as great.) #5

The one thing I would add is that the body generally tries to avoid metabolising amino acids, preferring to save them for structural purposes. Cleaving the bonds of a protein to separate out the component amino acids is easy, but before they can be metabolised, the individual amino acids must first be deaminated and then converted into glucose and/or fatty acids (depending on the amino acid in question). Then metabolism can start.

Given the amount of energy needed just to produce something that can be metabolised, you see why the body prefers not to use protein for fuel unless it has to.

(Bob M) #6

This is an important point. Here’s my “pet” theory as to why some people like high P:E diets (protein to energy, advocated by Ted Naiman). If you’re not using a lot of energy, higher protein diets can make sense, and this is particularly true if you’re muscular. In the latter instance, I think the musculature help to blunt the insulin response (muscles act like an insulin sink) and therefore any increased hunger you might get due to increased protein would be less.

That’s why people like Dr. Naiman, who does very short intense body weight training (and is muscular) can handle high P:E.

I hypothesize that people who aren’t going to like high P:E will be those with high calorie requirements. If you’re Zach Bitter and running a marathon a day, you will NEED to eat more fat (or some carbs, which is what Mr. Bitter does). Even Shaun Baker, with the amount of lifting and aerobics he does, I doubt he could handle high P:E.

Between these two extremes, it becomes less clear, of course.

I also note it’s not easy to convert protein to energy. I know, everyone will be like, “But what about GNG??” While that’s a factor, I think that converting protein to energy is inefficient, at best.

Unfortunately, not a lot of studies in this area. This once comes close…but compares high protein/no carb with…normal protein/high carb (high for us). Would love to see the same study with high protein/low fat versus low protein/high fat, both zero carb.

Design: Ten healthy men with a mean (±SEM) body mass index (in kg/m2) of 23.0 ± 0.8 and age of 23 ± 1 y received an isoenergetic H diet (H condition; 30%, 0%, and 70% of energy from protein, carbohydrate, and fat, respectively) or a normal-protein diet (N condition; 12%, 55%, and 33% of energy from protein, carbohydrate, and fat, respectively) for 1.5 d according to a randomized crossover design, and EE was measured in a respiration chamber. Endogenous glucose production (EGP) and fractional gluconeogenesis were measured via infusion of [6,6-2H2]glucose and ingestion of 2H2O; absolute gluconeogenesis was calculated by multiplying fractional gluconeogenesis by EGP. Body glycogen stores were lowered at the start of the intervention with an exhaustive glycogen-lowering exercise test.

(BuckRimfire) #7

Because one low-level course covers a lot of ground, and no one’s an expert in everything. Researchers know one or a few fields in excruciating depth, which may not align completely with the content of the course the department needs to find someone to teach. A whole lot of college courses have been taught by a prof who’s one week ahead of the students!