Most commonly, when you see diet plans laid out, the intake of the various macronutrients is presented in terms of percentages of total caloric intake. So you might see a diet which was 60% carbohydrates, 30% protein and 10%fat or some other set of percentages. Or you'll see recommendations that '...athletes only need 15% of their calories from protein.' or 'don't eat morethan 30% of your total calories from fat', that sort of thing.
In this chapter, I just want to run readers through the basic calcluations, to show them how they can be used in various ways. In the next two chapters, I'll talk about problems with the use of percentages.
A quick recap on calories
In a previous chapter I gave you the caloric content of the various macronutrients. To save you needless paging, I'll review them here.
Protein: 4 calories/gram
Carbohydrate: 4 calories/gram
Fat: 9 calories/gram
Alcohol: 7 calories per gram
With the above values in hand, and using some basic math, we can do several different operations in terms of diet and food analysis. Let's look at each one in turn. I'll give examples but don't read too much into the numbers. They are only examples.
Setting up diets based on percentages
Probably the most common use of methods is to do actual diet set up, to determine how many grams of each nutrient someone will be consuming. Let's say we have a 170 pound male with a maintenance calorie level of roughly 2700 calories per day and let's say we wanted to put him on a diet that was 60% carbohydrate, 20% protein and 20% fat (again, don't read too much into these values, I'm using them for example only). We want to find out how many grams of each nutrient he will be consuming per day.
Step 1: Calculate total calories of each macronutrient
The first thing we'd do is multiply his total caloric intake (2700 cal/day) by the percentages of each macronutrient as this will tell us how many calories will be coming from each nutrient. To convert percentages, just divide by 100 so 20% becomes 0.20, 60% becomes 0.60, etc. The calculations appear below.
Carbohyrate: 2700 * 0.60 = 1620 calories from carbohydrate
Protein: 2700 * 0.20 = 540 calories from protein
Fat: 2700 * 0.20 = 540 calories from fat
Note: It should be obvious that the percentages need to total 100% (or 1.0).
Step 2: Determine total grams from each macronutrient
Now we simply divide the total calories from each macronutrient by the caloric content of each macronutrient. This tells us how many grams of each food our guy will be eating each day.
Carbohydrate: 1620 calories / 4 cal/gram =405 grams carbohydrate
Protein: 540 cal / 4 cal/gram = 135 grams protein
Fat = 540 calories / 9 cal/gram = 60 grams fat per day
So this particular diet, with 2700 calories and 60% carbs, 20% protein and 20% fat yields a diet of 405 grams of carbohydate, 135 grams of protein and 60 grams of fat per day. For the remainder of the diet setup, you'd divide that up across some number of meals including pre- and post-workout, all that jazz.
Working backwards part 1: Determining diet composition
You can just as easily work the math backwards, to determine what percentage of each nutrient a given diet is. Let's say someone was eating 150 grams of protein, 200 grams of carbohyrdate, and 50 grams of fat and we want to find out how many total calories they are eating and what the percentages of the diet are.
Step 1: Determine caloric intake
First you simply mutiply the total grams of each nutrient by the caloric content of that nutrient. That tells you how many calories they are eating each day.
Protein: 150 grams * 4 cal/gram = 600 calories from protein
Carbs: 200 grams * 4 cal/gram = 800 calories from carbs
Fat: 50 grams * 9 cal/gram = 450 calories from fat
From those values, you can calculate total daily caloric intake by simply adding up the numbers.
Total calories = 600 + 800 + 450 = 1850 calories per day.
Step 2: Determine percentage from each nutrient
Now simply divide the calories from each nutrient by the total number of calories being consumed to determine the percentage each nutrient is providing. Multiply the decimal amount by 100 to get the percentage.
Protein: 600 calories/1850 calories = 0.32 * 100 = 32%
Carbs: 800/1850 = 0.43 * 100 = 43%
Fat: 450/1850 = 0.24 * 100 = 24%.
So our example person is consuming 1850 calories per day with 32% from protein, 43% from carbs and 24% from fat.
Working backwards part 2: determining food or meal composition
You can use the identical math above to determine the composition of a given food (based on the food label) or a given meal.
So say you wanted to determine the macronutrient percentages on a food or a meal that contained 10 grams of protein, 20 grams of carbohydrates, and 9 grams of fat.
Step 1: Determine calories from each nutrient
First you'd simply multiply the total grams of each nutrient by the caloric content of that nutrient.
Protein: 10 grams * 4 cal/gram = 40 calories
Carbohydrate: 20 grams * 4 cal/gram = 80 calories
Fat: 9 grams * 9 cal/gram = 81 cal
Although most food labels list the total caloric content, even if they don't, you can easy figure it out by adding up the totals above. This food/meal would contain 201 calories (40 cal + 80 cal + 81 cal).
Step 2: Determine percentages from each nutrient
Now you simply divide the total calories from each nutrient by the total calories in the food.
Protein: 40 calories/201 calories = 0.2 * 100 = 20% calories from protein
Carbohydrate: 80 calories/201 calories = 0.4 * 100 = 40% calories from carbohydrate
Fat: 81 calories/201 calories = 0.4 * 100 = 40% calories from fat
So this food or meal would contain 201 calories, with 20% protein, 40% carbs and 40% fat. Whether those percentages mean anything is the topic of the next chapter.
A note on food labels
Many people become perplexed when they do the math above on food labels and find that the caloric content listed isn't the same as what they calculate. So you might see a food that was listed as containing 212 calories with 10 grams protein, 20 grams of carbs and 9 grams of fat (which, as above, only yields 201 calories). There are a couple of reasons that this happens.
The first is that determining the caloric content of a given food isn't doesn't give perfect values, there is always a little bit of slop. As well, the 4, 9 and 4 cal/g values are rounded values in the first place. Finally, food labels almost always round off the values for protein, carbs and fat grams (for example, a food containing less than 0.5 g of fat can list it as 0 grams of fat). If the food listed above actually contained 10.5 grams of protein (44 calories), 20.5 grams of carbs (84 calories) and 9.5 grams of fat (85 calories), that would make up for the difference in values.
Ultimately, these types of tiny differences are no big deal. Even under the best circumstances, caloric estimates are only estimates and there's always going to be a little bit of slop either direction. We're not doing clinical nutrition here and, as long as it's not excessive, small discrepancies in calore values are nothing worth worrying about.