How are percentages of macronutrients worked out based on the weight of each nutrient?

[ritaknowles]

How are percentages of macronutrients worked out based on the weight of each nutrient? Is there a constant number that is used to work them out?

[PatinNC]

It is based on their calorie contribution.  Carbs and proteins are 4 cals per gram fat is 9 cals per gram.

[Dominique]

There is no universally "optimal" ratio of macronutrients. The minimum requirements for protein and fats are very low, and as long as you meet them, you can arrange your mix of macronutrients according to your needs, personal preferences etc. In other words, a wide range of macronutrient breakdowns can work just fine.

You roughly need a minimum of 0.3 gram of protein per pound of body weight and 0.3 gram of fats per pound of body weight from your diet for survival. Carbohydrates are not required for survival: however, they’re the preferred fuel for physical activity and the brain, so it can be a good idea to include them, and to adjust their amount based on your activity level (as opposed to body weight, like with the other two macronutrients).

Total calories are the constraint for the upper limit of each macronutrient. For instance, someone who weighs 150 lbs and needs to consume 2000 calories to maintain their weight could split them as follows:  75 grams of protein (would be 0.5g per lb of body weight, i.e. meet the minimum requirement), which is 75 x 4 = 300 calories, 45 grams of fat (would be 0.3g per lb of body weight, i.e. also meet the minimum requirement), which is 45 x 9 = 405 calories and fill in the rest with carbs. Carbs would thus be 2000 - 300 - 405 = 1295 calories (324 grams). Such a diet would qualify as LFHC (P: 15%, F: 20%, C: 65%). Another approach would be to consume the same amount of protein (75g/300 calories), but 167g of fat (1500 calories) and only 50g of carbs (200 calories): this would be a typical LCHF diet (P: 15%, F: 75%, C: 10%). Now if the same person were more active and could consume 2500 calories instead of 2000, the low-fat/high-carb diet would put them as P: 300 calories, F: 405 calories, C: 1795 calories and the ratio would become  P: 12%, F: 16%, C: 72%). Which shows that absolute amounts give a better picture than a percentage breakdown (which is influenced by activity level).

Of course, these are just a couple of examples. Someone interesting in gaining muscle could consume a much higher amount of protein, for instance. All things being equal (same calories constraint), this would leave less space for fats and carbs.

[ritaknowles]

Thank you. That is what I thought but I am trying to put a formula in my spreadsheet using these figures but the result does not compare at all on what fitbit is showing as my percentages. I thought that Fitbit might be using different formulas.

[Dominique]

What kind of discrepancies are you seeing between your calculations and Fitbit’s own?