Percent carbohydrate by kcal or by weight?

[Adrian and Chino]

When looking for LC, MC, and HC options on charts, do you go by the % of calories that come from carbs or the % of carbohydrate by weight? It seems like many lists are sorted by % calories, though that's the one that changes.

I know these aren't realistic numbers for cat food macronutrients, but for example:

Protein, Fat, Carbs

40g, 20g, 40g... 160 kcal, 180 kcal, 160 kcal...
32% kcal from carbs (40% carb by weight)

30, 30, 40... 120, 270, 160...
29% kcal from carbs (40% carb by weight)

20, 40, 40... 80, 360, 160...
26.6% kcal from carbs (40% carb by weight)

All 3 cans weigh ~3.5 oz and all contain 40g carbs (which amounts to 160 kcal), but the % of calories from carbs changes depending on the ratio of fat and protein.

 

[Larry and Kitties]

Yes. There are different calories per weight (e.g. gram) for protein and carbs compared with fat:
Protein and carbohydrates both contain 4 calories per gram, while fat provides 9 calories per gram.
http://healthyeating.sfgate.com/gram-protein-carbohydrates-contains-many-kilocalories-5978.html

 

[Adrian and Chino]

Right, so what would be the rationale behind sorting a list by % calories from carbs instead of % carbohydrate by weight? I want to make sure I'm not missing something.

 

[Larry and Kitties]

Because a cat's nutritional requirement is calories and you want to minimize the the % calories from carbs.

 

[Adrian and Chino]

Because a cat's nutritional requirement is calories and you want to minimize the the % calories from carbs.

But 2 different cans of cat food could have the same amount of carbohydrate (the same # of calories from carbs), yet different % of total calories coming from carbs, depending on how much protein vs. fat is also in the food. It seems like % carb by weight is more reliable...

Though it would be so much easier if cat food came labeled the way human food is labeled (with grams of carbs, protein, and fat).

 

[Elizabeth and Bertie]

There are different ways to calculate carb values of foods.

But it's been found that diabetic cats happen do better on diets that have less than 10% calories from carbohydrates.
And many cats may do best on foods that don't exceed 4 - 6% calories from carbohydrates. (These cats may have higher rate of remission. I had an article but can't find the link at the moment...)
So, many folks (though not all) tend to use 'percentage calories from carbs' as the means for determining the suitability - or otherwise - of foods for their diabetic kitties.

If protein, fat, and carbs all had the same calorie values then it may not be necessary to calculate percentage of calories from carbs; the percentage of carbs by weight alone may be sufficient information. But because fat has much higher calorie value than protein and carbs, the amount of fat can have quite an effect on the 'percentage of calories from carbs'.

For some reason calorie values are calculated differently in pet foods. Protein and carbs are multiplied by 3.5, and fat is usually multiplied by 8.5.

Edited to add:
There's an old thread here that discusses the calculation in more detail. Just adding the link here in case anyone is interested. Calculating food Carb %

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[Adrian and Chino]

Thanks. That makes sense.