Calories input/output?

In diet discussions, why does no one ever discuss the digestability of the food consumed? Doesn’t a piece of wood have a caloric value since it can be burned? But wouldn’t most of it pass through my body, undigested? All diet discussion seems to assume that all calories consumed are digested. To be even more blunt, everyone seems to assume that the caloric content of our fecal output is nil. Is this true? Even if I start a South Beech Diet comprised entirely of wood? Or if I start chasing all my Big Macs with a dose of laxative?

What do the Teeming Millions think? Or even better, what does Cecil think?

I don’t agree that all diet discussion seems to assume that all calories consumed are digested, or that caloric content of our fecal output is nil. I think most disussions about diet are how many calories on average a particular food contains, which is to say how many calories are used by your body. I think if you had a way to move food through your body faster, you wouldn’t (couldn’t?) consume as many of the calories of that food.

I am basing this on absolutely nothing, by the way. That’s just the way it seems to me.


Because precision is more important than accuracy. The available calories from a gram of oak don’t change between pieces, so the total calories you get is the same.

In the Weight Watcher’s system, which is among the more successful, people get assigned a certain number of “points” per day. Higher calorie foods generally cost more points, but the value is lower if the food contains more fiber or less fat. For example, an egg and a medium apple have approximately the same number of calories, but the egg counts as 2 point while the apple is only one.

When you say “how many calories on average a particular food containts = how many calories are used by your body”, you are proving my point. Your comment implies that the calories contained within the food are completely used by your body.

Your second comment, “if you had a way to move food through your body faster, you wouldn’t consume as many calories…” is my second point, except you substitute the word “consume” for “digest.” If you took a big dose of laxative with your meal, you’d push the food through your body so quickly that your body wouldn’t consume/digest all the food’s available calories.

Why must we always assume that every calorie eaten is a candidate for digestion? Dietary discussions all revolve around burning more calories than we eat, with no distinction made for calories that might just pass through us.

I agree that WW’s point system could have been attempting to take my concern into account (i.e., fibre calories shouldn’t count against us as much as fat calories since they are harder to digest). Reading WW’s explanation of their system, however, suggests that their point system is more about measuring relative healthiness of the foods, not their relative digestability. If their system were attempting to address my question, they should penalize sugars in addition to fats since our body can most easily digest sugar calories.

Thanks for your thoughts, nevertheless.

The OP raises a good question, one which I hadn’t thought about before, but now am wondering about too.

For reference, here’s a link to Cecil on How is the calorie content of foods determined? According to Cecil, the old-fashioned way was to burn it and see how much heat is produced. But, as the OP points out, you could do this with a piece of wood and determine how many calories it has, but this doesn’t mean they’re in a form that the body could use as energy, or store as fat, if you ate the wood.

So, to restate (and maybe clarify) the OP’s questions:

When you read on the food package how many calories a serving of that food contains, are all of those calories the kind that the body can actually use? (And how do they tell the difference?)

And, assuming those calories can be used by the body, will they all be? Could some of them pass right through, without being burned for energy or stored as fat?

Telling the difference is a matter for the biochemists, but there’s some stuff that’s known to be non-bioavailable. btw, “bioavailability” is the magic word here, if anyone wants to Google.

Rate of absorption data just isn’t out there. There is some question as to whether there’s a maximum amount of nutrients the body can absorb. I’m sure there is one, but there’s no indications of what it might be.

The human digestive system is extremely efficient. It’s separated into parts, all of which contribute their piece toward digestion, and food moves through very slowly, ensuring that the body has the greatest possible chance of breaking the food down to its individual absorbable components (amino acids, fatty acids, and simple sugars).

So, yes, the vast majority of the calories that are in the food you eat really are going to be digested and used by the body. And the calories that are listed on food packaging are the calories that the body can digest. Fiber may also be listed, but it is not part of the calorie count because it goes through the body undigested.

There are a variety of reasons, from lactose intolerance to diseases and intestinal damage to rapid transit, that some calories don’t get properly digested in certain people, but if you eat 2000 calories of food during the day, you should assume that as a good first approximation your body has digested 2000 calories. At least you shouldn’t plan any weight loss around the contrary assumption! :slight_smile:

Ah! Thanks, EM :slight_smile: That explanation makes sense to me. The calorie count listed on food packaging only counts digestable components (protein, carb, fat… any others?), so my South Beech Diet would be basically zero calories since wood has little to no digestable components, right?

So what makes some plant matter digestable (e.g., celery, spinach), but others (e.g., wood, grass) not digestable? Is it simply a matter of different levels of carb content?

Not necessarily. Most of the protein bars out there have gelatin in them, which is an indegistible protein, but they get to count it on the label.

No food is 100% digestible, but some are pretty close. Very few foods are completely indegistible–even fiber breaks down to some degree (although much less than other carbohydrates). Why is a matter of chemistry and that’s not my area.

Look up soluble and insoluble fibers.

Wood fiber is insoluble.

Herbivores are much better at digesting fibers than omnivores or carnivores, because of differences in the way their guts work.

It’s true that gelatin counts as a protein, but only a tiny number of foods have sufficient gelatin to make a significant difference to the digestible calories. (I don’t understand the point of eating a “protein” bar with unusable protein but there are quack foods like there are quack medicines.) For most foods, ones that have some resemblance to natural food, the calorie count is still a good first approximation.

I also don’t get the reference to wood in the South Beach Diet. I thought that diet was a sensible one. From

Hardly zero calories. And no wood.

Not South Beach. South Beech.

I found an article that may be of interest here. It gives an overview of how food calorie content is calculated and what the possible sources of error are.

Right. The joke in the OP. :smack:

Good article. Obviously the 4-9-4 rule oversimplifies the complex process of digestion. The official Recommended Dietary Allowances recognizes this, BTW, but doesn’t consider it significant:

[slight hijack, but related to the topic]

Well, I have some raw data handy. I’ve been on a “diet” (really more of a lifestyle change) for the last 120 days (so far!). I’m kind of a control-freak technologically oriented person, so I’ve been able to stay on program/motivate myself by precisely tracking calories in/out.

I’ve been using - a great (free) website that allows you to track calories consumed, and provides a pretty good estimate of calories burned. I also made myself a basic spreadsheet in excel to track my daily/weekly/monthly progress and calories in/out.

For calories consumed, they have a pretty comprehensive food database. You can also add custom foods by manually entering information taken off of food labels, or other nutritional information/databases on the internet.

Most relevant to the discussion at hand - I’m pretty sure that their nutritional values are based on how many kcal your body actually extracts from the food - for example, kcal from carbohydrate is grams of carbohydrate MINUS grams of fiber, multipled by four kcal per gram. This is because fiber is pretty much indigestible.

For calories burned they use a formula which takes your gender, height, weight and general activity level into account. You can also add activites that you do each day, and their intensity/duration, and based on your height/weight it calculates how many kcal the average person your size would burn doing that activity and adds it to your daily total.

Obviously there’s some estimated involved, particularly with the kcal burned equations. And I’ve been very meticulous in recording everything I’ve eaten every day. The results have been pretty accurate.

In my case, I’ve been watching what I eat for 120 days. I started at 270lbs, and am currently at 245.8lbs (as of this morning). My average calorie deficit each day has been about 741kcal (total deficit over 120 days of 88,909 kcal). There are about 3,500 kcal per pound of weight loss (fat). So my calculated weight loss should be 25.4lbs, and it’s really been about 24.2lbs. That’s a pretty darn accurate estimation.

I realize micro-managing calorie input/output isn’t for everyone. But it’s working pretty well for me. And all of the nutritional information that my tracking is based on is either taken directly off of food package nutritional labelling, or the same information online. So obviously, the food labels are fundamentally pretty accurate.

Heh, thanks for clearing that up, Ultra :slight_smile:

Wow! That article is exactly the answer I wanted. The example of hair being zero calories, and the example of measuring feed calories by burning both the feed and the resultant fecal matter are the extreme corner cases that I thought should be happening, but which no one else ever discusses.

As I suspected, the Teeming Millions came through. Thanks again :wink: