Friday, January 3, 2020

Measuring Nutrient Density: Calories vs Weight

The Nutrient Density Cheat Sheet considers the nutritional yield of foods per standard serving size. Serving size is a food measurement based on the weight of a given food, and is meant to reflect the food portion sizes that people typically eat. Recently the Nutrient Density Cheat Sheet was criticized by a somewhat fanatical vegan. She claimed that by considering nutrition per serving, I was unfairly biasing all of my scores toward animal foods. She doubled down and further claimed that if my scores were recalculated using calories instead of weight, leafy green vegetables would certainly be revealed as the most nutrient dense foods on the list.

We went back and forth about the correct methodology for some time. It took quite a while for her to coherently distill her objections down into something I could actually work with. Essentially, she asserts that water and fibre confound the weight and volume measurements in unacceptable ways, and that calories are a more accurate way to measure nutrient density. She suggested that I completely remove weight as a variable, and do my calculations strictly with nutrients per calorie. So, I decided to humour her and I did precisely what she asked me to do.

I spent a couple hours going back and recalculating all of the nutrient data by calories instead of serving weight. The results are still adjusted for bioavailability, nutrient absorption capacity, and metabolic conversion inefficiencies (my methods for each adjustment are detailed in an earlier blog post here). There are absolutely no weight measurements considered in the nutrition per calorie scoring calculations. Here are the results:

Animal foods seem to still come out on top. However, in order to insulate myself against criticisms regarding my nutrient yield adjustments, I also produced two other unadjusted scores. They use the same methodology— one is nutrient density per serving, and the other is nutrient density per calorie. Neither is adjusted for nutrient bioavailability, nutrient absorption capacity, or metabolic conversion inefficiencies. It's literally just each nutrient divided by its respective DRI and divided by calories. Results are summed across all nutrients per food, and the results across all foods are sorted and ranked. Here are those results:

As you can see some leafy green vegetables do get a boost, but ultimately animal foods are still dominating the top of the list. But, why is this? Leafy greens are low calorie and animal foods are higher calorie. So, it seems intuitive that leafy greens would be some of the lowest calorie foods, so why does measuring nutrient density per calorie actually produce these counter-intuitive results? It's because dividing nutrition by calories just gives you a silly little ratio. That's it. The results don't actually have to favour low calorie foods at all. The results just favour foods that have a similar ratio of nutrition to calories, which can include both high and low calorie foods.

Highly nutritious, high calorie foods (like Atlantic salmon) get similar scores as poorly nutritious, low calorie foods (like spinach). Think about it. If you divide 1000 by 100, you get 10. If you divide 10 by 1, you get 10. It's just a ratio. It tells you nothing about realistic portion sizes or caloric density. Which is why dividing nutrition by calories is a foolish and uninformative way of quantifying nutrient density. For example, black coffee can be found in both the adjusted and unadjusted scores when calculating nutrition per calorie. Which could leave one with the false impression that black coffee is a great source of nutrients, or is at least comparable to oysters or mussels. However, one would have to consume approximately 1.7 litres in order to exceed the RDA of a single essential nutrient found in coffee (riboflavin in this case). Whereas eating just 10g of either mussels or oysters yields more than the RDA of vitamin B12. 10g is barely the size of the tip of your finger. Whereas 1.7 litres is an insane amount of coffee to drink to get the RDA of riboflavin.

One of this vegan's primary arguments in support of nutrition per calorie measurements was that humans have a limited calorie budget (approximately 2000 kcal/day), so assessing nutrition per calorie is best. While it's true we all eat within a similar calorie budget, it's not true that measuring nutrition per calorie actually gives you much meaningful insight into the calorie yield of a food. It's just a ratio. The foods on the top of the list need not be low calorie foods at all.

Ultimately, my position is that calories are a subjective value-judgement. Calories are something you assess completely independently of nutrient density, and you increase or decrease calories according to your goals. On this basis alone, I suggest that factoring calories into the nutrient density score necessarily injects subjective bias into the results. Nutrition divided by calories has a number of unacceptable drawbacks. 

Firstly, considering nutrition per calorie assumes that calories are always a disadvantage. Secondly, it punishes foods for having essential nutrition. Both essential amino acids and essential fatty acids contain calories, so they actually lower the nutrient density score. Which clearly doesn't make any sense. Lastly, it just doesn't actually give you any meaningful information about calories. So, why even bother? Nutrition per serving also has significant interpretive challenges, but they are far less severe, and far less limiting, than measuring nutrition per calorie. Serving size simply gives you a better approximation of how humans interact with food, and that's what matters.

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