I had a conversation with a client today that started with answering her question on how many grams of carbohydrates she was supposed to have in a day. It evolved into a discussion about how to figure out nutritional numbers (calories, grams, ratio of carbohydrates, protein, fat). The mission of Guiding Stars is, of course, to help you avoid the need to work out these numbers on your own, but if you're curious about taking your nutrition tracking to the next level of complexity, this post is for you.

Let's start with how many calories you should be eating each day. There's a standard formula called the Harris-Benedict Equation that predicts what your daily caloric need (DCN) should be to maintain your body weight. This takes into account your age, gender, height, weight, and activity level. A Harris-Benedict calculator can be found on WebMD. For weight loss, you then adjust this number by reducing it by 500 calories or 15%, which ever value is less. Reducing calories too drastically can actually slow down weight loss. If your DCN is 2000 calories, for example, reducing your intake by 500 calories would mean 1500. Reducing your calories by 15% would mean eating 300 less calories, or 1700 a day. Believe it or not, 1700 calories would be more effective for weight loss.

Okay, say you've figured out that you should be taking in 1700 calories per day to lose weight. Where should those calories come from: carbohydrates, proteins, or fats? According to the U.S. Department of Agriculture U.S. & Department of Health and Human Services Dietary Guidelines for Americans 2010, we should consume somewhere between 10 – 35% protein, 45 – 65% carbohydrates, and 20 – 35% fat.

portion size
Portion Size (of meat) / Robert Couse-Baker / CC BY 2.0

To calculate this, figure out how many calories make up the percentage range. (Hint: cross-multiply and divide!) Divide the amount of calories by the calories per gram figure for each category to get the number of grams you'll need each day. Here's the math for your 1700 calories:

  • Protein: 10 – 35% = 170 – 595 calories. At 4 calories/gram, you need 43 – 149 grams each day.
  • Carbohydrates: 45 – 65% = 765 – 1105 calories. At 4 calories/gram, you need 191 – 276 grams each day.
  • Fat: 20 – 35% = 340 – 595 calories. At 9 calories/gram, you need 38 – 66 grams each day.

With that information we are all set to balance our diets, right? Well, there's obviously a little more to it than that, but it at least gives you an understanding of some of the numbers that diets are based on. As always, if the math here is just more work than you want to do when you're planning your weekly menu, you can use FoodFinder to search products for their Guiding Stars rating. FoodFinder will give you the numbers you need to do the more difficult food calculations I've outlined above as well as lighting up an easy path to good nutrition. Anytime you're pinched for time (or a calculator!), just follow the stars and you can rest assured that you're making an easy choice that is healthy.

Mark Nutting, CSCS*D, NSCA-CPT-AR*D, National Strength and Conditioning Association's 2009 Personal Trainer of the Year, holds 12 certifications in the field and has 30 years experience in personal training. A national presenter and an educator of Personal Trainers, Mark's areas of expertise include weight loss, post-rehab conditioning, and brain fitness. Mark contributes regularly to the Guiding Stars Blog.