David Friedman Shatters A Myth

Many years ago, I heard someone on television talk about a charity that went into the homes of poor people and showed them how to cook properly. The idea was that the poor suffer from bad nutrition because the cheapest food is also the least expensive. Thus, the poor rely on food from, say McDonald's, rather than taking the time and money required to produce home-cooked meals.

As a poor college student, I myself relied heavily on inexpensive fast food to get me through the years. So, based on my own personal experience, the claim seemed highly credible. Poor people are obese because they are forced to rely on the comparatively poor nutrition offered by the least expensive foods.

All was well with the universe until the excellent David Friedman took time to question the prevailing assumption:
Flour, the main ingredient in home made bread, costs about $.50/lb and has about 2000 calories/lb, so about $.25/1000 calories.
Wonder Bread, the classic example of supermarket bread, has 1100 calories/loaf and cost $1.99/loaf on sale at Walgreens, normally more. So about $1.80/1000 calories, or seven times as expensive.
Comparing lentils to fast food, 1 kilo of lentils has about 3500 calories and costs a little over $2. So about $.60/1000 calories
A McDonalds Quarter Pounder with cheese has 520 calories and costs $3.10.  So about $6.00/1000 calories. For that comparison, fast food costs about ten times as much per calorie.
Not only are the claims wrong, both are wrong by close to an order of magnitude.
To question the claims even further, I would add that - because the obese poor are actually obese - that means they are not engaging in cost minimization. They are eating more than they "need" to (but probably less than they would like to eat - I leave that assumption uncontested).

So if the claims are mathematically nonsensical and logically inconsistent with observed reality, we may have to toss this one out as a myth. But I am open to being persuaded otherwise.