Basic food math

Basic food math

Random Food is Random.

Random Food is Random.

If you live in the US, you’ve most likes seen a food’s “Nutrition Facts” at some point or another. Did you know that some of these numbers actually correlate to each other? Let’s take, for example, this label on the right.  You see here that one serving of this product has 150 calories, but what does that mean?  How do they get this number?  Well, the first thing to explain is what a calorie is.  In short, the calorie is a unit of food energy and is equal to 4.1868 kilojoules.  That’s all it is, it simply is telling you how much “food energy” is contained in whatever it is you are eating.  Let’s look at the rest of the list before we break out how it all relates.

Next in the list we have “Total Fat” with a breakdown of the different types of fat.  Following this we have Cholesterol, Sodium and Potassium, and while they’re important, they have nothing to do with calories, so we’ll ignore them today.  Next up is the Total Carbohydrates and the breakdown of what they are (sort of).  Finally we have Protein.

So, let’s get to the fun part: The math (don’t worry, this won’t be on the test).  First, the easy stuff:  How do you get the “Total Fat” amount?  You simply add up the fats listed and you get the total, right?

Let’s see:  0.5+0+2+5 = 7.5

What in the world happened?  We’re .5 grams off, where are they?  Well, they could be anywhere, or nowhere.  You see, in the US, the FDA allows companies to round on the nutrition facts to the nearest full or half gram (depending on the circumstances).  This means that .5 could really be .6.  That 0 could be 0.4.  That’s right, if a product says 0g Trans Fat per serving it could actually have up to .4g per serving.  Funny, huh?

Rounding aint cool.

Rounding ain't cool.

Moving on down to Carbs you see that they list sugars (2g) and fiber (1g), but that still leaves 13g missing.  These are usually starches and are rarely (if ever) labeled.  Finally we have Protein (nothing fancy with it).

So now that we have all these numbers, how do we relate them?  Well, it’s much easier than it looks.  The first thing you need to do is figure out how much food energy is in each component and luckily this is a standard number (makes it much easier to remember):

–  Fat has 9 calories per gram (you’ll note that the “Calories from Fat” on the label is simply the “Total Fat” multiplied by 9).

–  Alcohol has 7 calories per gram.

–  Carbohydrates have 4 calories per gram (subtract fiber).

–  Protein has 4 calories per gram.

Once you know that, you can just take the following formula and plug in the numbers:

(Total Fat x 9) + ((Total Carbs – Fiber) x 4) + (Protein x 4) = Calories

Ready?  Let’s try it out:

(8 x 9) + ((16 – 1) x 4)+ (2 x 4) = 72 + 60 + 8 = 140

Now, you’ll notice that it is about 10 calories lower than the 150 listed in the label.  What gives?  Well, the truth is that this happens for the following two reasons:

  1. They are allowed to round to the nearest .5 or 1 gram of Fat, Carbs or Protein.
  2. They are allowed to round to the nearest 5 or 10 Calories.

So, because of non-standard rounding, it’s not perfect, but it is very close.  I honestly would love to see the rounding stop, because it really can alter how a food presents itself.  Here’s a great example for those of you wondering what the harm of rounding is.  Let’s say that something has a nutrition label that states 0g of Trans Fat per serving, and there are 100 servings (a 3lb tub of margarine, let’s say), you’d expect that if you ate the entire tub over time, you’d have ingested 0g of Trans Fat (0g x 100 = 0g).  What happens when it actually contains .3g per serving?  Well, now you’ve ingested a bit more (.3g x 100 = 30g) which is totally uncool.  You see, as long as we continue to round, all a company has to do is alter their serving size and they can completely change the apparent nutrition of their food.  Something to always keep in mind.

So, while there is plenty more to learn, I figure I’ll just keep this simple today.  If there’s anything I’ve missed, be sure to let me know!


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