One of the most popular posts on SmarterFitter is my list of 100 most protein rich vegetarian foods. The table is based on values in the USDA National Nutrient Database and lists protein amounts in grams and percentages, with leafy cowpea tips having the highest percentage of protein at a whopping 85%.

Recently, one of you wrote to ask how I calculated these percentages. Something didn’t quite add up:

1 gm of protein or carbs contains 4 calories, and 1 gm of fat contains 9 calories. Using these measurements, the percentages of protein listed next to each food are unrealistically high. I used a calculator to double-check my estimates, calculating both % protein by weight and % protein by calories.

Take the cowpeas for example. According to the USDA database, cowpeas have about 4.6g protein, 0.1g fat, 2.8g carbs and 22 Calories. But if you calculate calories based on the 4-9-4 rule, you get

4.6*4 + 0.1*9 + 2.8*4 = 30.1 Calories

This is much higher than the 22 Calories listed in the USDA database (the value I used to calculate the percentages in the table, thus giving higher percentages than my reader expected).

So why the discrepancy? The USDA website has a helpful FAQ that explains part of the story:

Calorie values are based on the Atwater system for determining energy values…The Atwater system uses specific energy factors which have been determined for basic food commodities. These specific factors take into account the physiological availability of the energy from these foods. The more general factors of 4-9-4 were developed from the specific calorie factors determined by Professor Atwater and associates. For multi-ingredient foods which are listed by brand name, calorie values generally reflect industry practices of calculating calories from 4-9-4 kcal/g for protein, fat, and carbohydrate, respectively, or from 4-9-4 minus insoluble fiber. The latter method is frequently used for high-fiber foods because insoluble fiber is considered to provide no physiological energy.

So the Calories that come from insoluble fiber aren’t included in the Calorie value listed in the USDA database. It still doesn’t tell me what values I should be using to calculate percentages.

To get a definitive answer, I sent an email to the USDA. To my amazement, they actually got back to me within 12-hours of sending the email. Here’s how it went down:

My email to the USDA:

Hi there,

I would like to calculate the percentage of protein in food based on the data in the USDA database. I know that the basic formula is (calories from protein)/(total calories), however, I’m not sure what value I should use for “total calories”. Do I use the value listed for “Energy” or do I calculate total calories from the values listed for protein, carbs and lipids? I assume I use the value listed for “Energy”, as the latter calculation would include calories from insoluble fiber, but I’m really not sure. Please help!

Many thanks,
Monica

Response from the USDA:

Dear Monica,

Thank you for contacting Nutrition.gov.

The correct value that you would use to calculate “total calories” would be the value listed as “energy”. To calculate the percentage of protein in a food, you need to know the grams of protein (which is usually how the food label provides the information) and the number of calories per serving.

For example, you have a food that has 5 grams of protein and 200 calories. You will multiply the 5 grams of protein by 4 because there are 4 kcal/gm of protein. Then you will take that number (20) and divide that by 200 calories. This will provide you with the percentage of protein per serving of a food.

The Nutrient Data Laboratory also has a section on frequently asked questions with background information that you may find helpful. The question, “I multiplied protein, fat and carbohydrate values by 4-9-4, but my energy value is different from USDA’s. Why?” may be found here: http://www.ars.usda.gov/Main/docs.htm?docid=6233#4-9-4

I hope you find this information helpful. Please feel free to contact us if you have any further questions.

Sincerely,

Laryessa Worthington, MS, RD
Nutrition Information Specialist
Food and Nutrition Information Center
National Agricultural Library, USDA
10301 Baltimore Ave., Room 105
Beltsville, MD 20705
Phone: (301) 504-5368

In a nutshell, the percentages listed in my table of 100 most protein rich vegetariain foods ARE correct. And the way to calculate the percentage of protein in food based on the USDA database is

Percentage of Calories from Protein = (4 * Grams of Protein) / (Energy)

Phew!