I was recently reminded that the world at large perceives that vegetables are healthier than meats, though no one really seems to know why. You often hear the word “superfood” associated with health-food-store favorites like kale, spinach, blueberries, and walnuts. You never think about foods like salami and cheddar cheese, mostly because they are salty, fatty, and generally considered to be deliciously unhealthy.

But on a per-calorie basis (because whether we practice it or not, we can probably all agree that the purpose of eating is not to avoid hunger, but rather to collect nutrients for our bodies), which foods are most nutritious?

Being a graduate from an engineering program, I wanted to look at the numbers. At first glance, as I’ve talked about before, the nutrient contents of many animals foods far outweigh many of the popular, so-called healthful foods. Check out the results. Wow!

• 2.8 times as much vitamin A in Beef Liver than in Spinach!
• 30% more phosphorus in chicken liver than walnuts!
• Infinitely more vitamin D in Salami than any of the plant foods (vitamin D is only found in animal foods).

Sure, walnuts won for magnesium content, and kale won for Vitamin C. Even spinach had way more vitamin K than any of the animal foods (but I have my own theory on that, and it has to do with animals being able to eat grasses and greens—the USDA numbers came from factory farmed animals, no doubt. I suspect that grass-fed animals would have large amounts of vitamin K in their meats, etc). Plant foods offer many micronutrients, flavors, and variety to our diets, but are they really “healthier” than animal foods?

Let’s compare them on a per-calorie basis. We will follow the evidence where it leads.

So then I fanangled my spreadsheet and spinach was the winner on most counts on a per-calorie basis, by a long shot.

Ouch. My theory just went out the window. We all want maximum nutrition, minimal calories, right? But wait a minute. Spinach is a low calorie food, mostly because it’s 90% water. So I can get plenty of nutrients per calorie, but I have to eat LOTS of spinach to get a significant number of calories that are so richly associated with nutrients.

Let’s compare NUTRIENTS versus mass of these two foods, then, and see how much spinach I’d have to eat to get the same overall nutrients as I would from a serving of beef liver.

Though spinach is much more nutrient-dense on a per-calorie basis than beef liver, spinach is not as calorie-dense. So to compare apples to apples, let’s figure out how much spinach it would take to get the equivalent nutrients. First I converted it to weight, but hardly any modern American cooks use a scale in the kitchen—for food, we like volume. So I converted grams to cups, based on the USDA’s estimate that one cup of raw spinach contains 30 grams of this green “superfood.” Now for the comparison.

For some nutrients, like potassium and folate, it only took a few cups of spinach (2.1 and 4.5). That’s easy. That’s like a salad or a slice of spinach quiche. Yum!

But for many of the other nutrients, including phosphorus, zinc, riboflavin, niacin, B6, and Vitamin A, it was more like 9 cups or 33 cups or even 80 cups, or 5 pounds, of spinach! That’s a lot of spinach! A cow could handle that, but not humans with our small mouths, small stomachs, and short digestive tracts! And even then, I’d be missing some vital micronutrients, like niacin. So if I’m stuck eating spinach, maybe I’ll take a multivitamin for the niacin (or just eat some salami). But at least I could keep my calorie intake low, right? Let’s see…

It’s not a totally valid way of looking at it—as they say, if your feet are on ice and your head is on fire, on average you feel pretty good—but I was curious what the overall average amount of spinach was to reach the beef liver nutrient content. Just under 20 cups of spinach! OK, so maybe if I cook the spinach down really well, that’s not so bad.

But back to calories. After all, humans are (or are supposed to be, right?) into low calories, high nutrients. So if I managed to eat 20 cups of spinach to get the same general nutrient content as a 100 gram, or quarter-pound serving of beef liver, I would consume approximately 138 calories. Wait a minute. A quarter-pound serving of beef liver only has 175 calories. We’re talking less than 40 calories difference for (roughly) the same nutrient content!

Now, I am not arguing that you should eradicate spinach from your diet and only ever eat beef liver again. I’m not even agreeing that the calorie theory works (i.e. eat fewer calories, lose more weight), because I don’t think it does. What I am saying, though, is that it is incorrect to assume that plants have more nutrition–per bite, per calorie, per pound, whatever—than animal foods. Overall, they don’t. Plants contain great things for human nutrition and provide lovely textures, flavors, smells, and nutrients to our diets—but according to my research, they shouldn’t be used in place of animal foods—they should be used to complement them!

So next time you see a “superfoods” list come out on the news, consider your capacity for eating, and choose foods that provide the nutrients AND calories that you need to thrive, without you having to spend all day chewing!