(Liquid) Calories, how much is really used by the body?

emsi · November 2017

Hi guys,

I've been thinking a lot about the actual amount of calories absorbed and used by the body.

Now, I know that foods low in calories also sometimes create a negative calorie balance, because they burn more to be processed than they actually give. But apart from this, how much does get lost on the way in digestion?
Do the 77cals of a small apple really get into the system completely?

Thank you!

TexanInNorway · November 2017

Interesting question... I am no expert on this, but will share my thoughts.

Wouldn't the calories burned by your body in order to digest food be included in the "Basal Metabolic Rate"? So for example, if a low-cal food contained 10 kcal but took 20 kcal to digest, it would be a net -10 kcal. CM is taking care of this by balancing your food diary entry against the "burned" calories from activity + BMR.

I've heard the same as you about negative calorie balance foods , so I did a quick search out of curiosity. This article from the BBC came up, which says while theoretically possible, there are in actuality no "negative calorie" foods: http://www.bbc.com/news/magazine-21723312
It gives an example of celery, often touted as a negative calorie food, which is still positive calories. Their math: 10 kcal for celery (minus) only 2 kcal for digestion (equals) positive 8 kcal.

As for absorption efficiency, I have no idea, but it would have to be close to 100% - otherwise all of CM's math would be incorrect! Right? If not, maybe the kcal content quoted for food is what we absorb, rather than the true caloric value?

Susan_RD_101 · November 2017

Hello!

What a great question @emsi . Calories are a unit of energy, so they aren't really absorbed (rather, it's the protein, fat, alcohol, and carbs that are absorbed or burned). When we consider total energy expenditure, or all the ways we burn calories in the day, we look at four things:

Basal metabolic rate - the amount of calories or energy we need to sustain organ function and life (i.e. how much you would need to eat in a coma)
NEAT - non-exercise activity thermogenesis (the amount of calories used moving the body outside of exercie)
Thermic effect of food - the amount of calories used for digestion
Exercise

Digestion contributes ~10% to our total energy output and I believe that Cronometer factors this into their calculation for BMR through the equation they use.

And if you are really curious, protein uses the most calories for digestion, compared to carbs or fat.

Hope this helps!

emsi · November 2017

Ah, great answers! Thanks so much, they are super helpful.

katmando · December 2017

Susan_RD_101,

Maybe you can help me. I am a 63 year old female, unilateral, transfemoral amputee (1 leg, above the knee). In order to walk with the aid of a prosthesis, I expend more energy than someone my age, size and weight (130 lbs). Some studies have estimated 7.2 +/- 1 MJ/d, or oxygen uptake as 24% higher than normal. All of the outputs for energy expenditure here and other places assume that I have two legs, so I don't get "credit" for dragging around a 15 lb prosthesis all day long. How do I get around this? Is there a calculation that can determine the calories expended per day? Can I find an equivalent "exercise" that I can add to each day to make up the difference?

Susan_RD_101 · December 2017

@katmando

Hello,

I wasn't able to find a conclusive answer to your question online so asked in a dietitian group if anyone can share a reference. I will be sure to pass on this information when I receive it.

I think that the most accurate way to determine you energy requirements (at rest) is to having your resting energy expenditure measured. Monitoring for weekly weight stability will also let you know if you are eating to match your kcal expenditure.

Kind regards,

(Liquid) Calories, how much is really used by the body?

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