It seems to me there are two different ways of looking at diet.
In the first model, which is the most common, foods are appraised on their strengths and weaknesses. Candy is bad because it’s high in sugar. Vegetables are good because they’re high in fiber. Partially hydrogenated vegetables oils are bad because they’re high in trans fats. Tuna is good because it’s high in omega-3 fatty acids.
Under this model, it’s common to hear foods talked about in absolutes, like “whole grains are a good source of fiber” or “potatoes are a good source of vitamins B, C, E and K”.
But this kind of language has always bothered me. What does a “good source” actually mean? How can we quantify what counts as a good source of vitamin C, as opposed to a mediocre source or a poor source?
The intuitive answer is “based on how much vitamin C the food has”. If it has lots, it’s a good source. If it has a small amount, it’s a poor source.
But how do you measure how much vitamin C a food has?
100 grams of orange juice has about 50mg of vitamin C1. 100 grams of spinach has about 28mg. Can we say orange juice is a good source of vitamin C, and spinach is a mediocre source?
100 grams of orange juice contains 45 calories, whereas 100 grams of spinach contains only about 23 calories. A person on a calorie-controlled diet can eat about twice as much spinach than they can orange juice. Using calories as a measure, 100 calories of spinach provides 11mg more vitamin C than 100 calories of orange juice. Can we say spinach is a better source of vitamin C than orange juice?
100 grams of orange juice costs, on average, $0.162, whereas 100 grams of fresh spinach costs on average $0.86. A person with a strict food budget can buy about 5 times as much orange juice as spinach. Using price as a measure, a dollar’s worth of orange juice provides 280mg more vitamin C than a dollar’s worth of spinach. Can we say orange juice is a better source of vitamin C than spinach?
100 grams of orange juice provides, on average, 0.2 grams of fiber. 100 grams of spinach provides, on average, 2.2 grams of fiber. A person trying to increase their fiber intake can get 11 times more fiber from spinach than orange juice for the same mass of food. If we need to add 100 mg of vitamin C to our diet, spinach containing 100mg of vitamin C provides about 7.4 grams more dietary fiber than orange juice containing 100mg of vitamin C. Can we say spinach is a better source of vitamin C than spinach for people who want to eat more fiber?
Comparing so many variables simultaneously becomes extremely complex. In the real world, most people care about the mass of food they eat and the calorie content of the food they eat and the cost of the food they eat and the amount of fiber in the food they eat, in addition to dozens of other factors.
The second model of diet attempts to resolve this problem by measuring not just the strengths and weaknesses of a food, but also the opportunity cost of eating that food.
Opportunity cost is a concept from the field of economics. It measures the loss of a potential gain when one opportunity is chosen at the expense of another. For example, suppose you’re invited to a party and poetry reading on the same night, and that you can go to either but not both. Choosing one option precludes choosing the other option, so the opportunity cost of going to the party is not being able to go to the poetry reading, and vice versa.
So what’s the opportunity cost of eating candy?
At the most basic level, it’s losing the opportunity to eat something healthier. But in order to quantify how much of the healthier option could have been eaten, we need to agree on a basic unit of scarcity for eating.
For most people in the world, this unit is money. For them, the opportunity cost of portion of candy is the money used to pay for it, which could have been spent on a different kind of food.
But in many developed countries, people can now afford to buy more raw food energy than they could ever hope to eat. Viewing the opportunity cost of candy in monetary terms for these people makes less sense, because most of the time they could happily afford both the candy and another food. If the price of the candy is not the primary factor influencing their decision to buy, it makes little sense to assume that money is the source of scarcity in their diet decisions. For most people, the decision to eat or not eat candy is based on their desire to eat healthily and maintain a given weight.
For people in developed countries, then, we could say that the basic unit of scarcity driving food choices is the calorie itself. In order to maintain a reasonably healthy diet, there is a hard limit on the number of calories one can consume.
Using this metric, the opportunity cost of 100 calories of candy is 100 calories of broccoli, or grapefruit, or strawberries, or peas, or tuna that we could have eaten instead. In fact, we could say that the opportunity cost was 100 calories of the mixture of foods that yielded maximum nutritional value in terms of macronutrients and micronutrients. The exact composition of this hypothetical 100 calorie mixture would differ for each person based on their existing diet and their health needs, but for the average person it would likely include, for example, lots of green vegetables.
It’s strange to think about food in this way, because it’s very hard to imagine someone choosing between candy and cabbage. Intuitively, it seems like no-one actually behaves this way.
It reminds me a little of when there’s an adjustment to the base rate of interest in the USA or UK, and financial analysts talk about the various effects it will have on the economy. I find it hard to imagine someone would delay getting a mortgage because the interest rate was a tenth of a percent higher, or would spend more of their savings because the interest rate was a tenth of a percent lower. But in fact, very small motivational effects, aggregated over a lot of people and a long period of time, can result in very large changes.
In the same way, the effects of eating candy on appetite and calorie budget add up to have a real effect on consumption of other types of food.
And the opportunity cost of candy is extremely high, because it’s incredibly energy dense. A large bag of skittles weights about 100 grams (3.5oz) and contains 402 calories. The opportunity cost of eating 100 grams of skittles is 980 grams (2.1 pounds) of carrots, 1.18 kilograms (2.6 pounds) of broccoli or 2.4 kilograms (5.2 pounds) of zuchini.
I agree it’s doubtful a particular person would choose to eat 5 pounds of zuchini instead of a small bag of candy. But I find this general way of thinking about food useful because it demonstrates the vast quantity of micronutrients and other general goodness we miss out on when we eat very energy dense and nutritionally sparse foods.
It provides an argument against eating foods like candy quite apart from the concrete negative effects of eating sugar.
- All nutrition data in this post is taken from the National Nutrient Database for Standard Reference Release 25 [↩]
- All food price data taken from the USDA ERS [↩]