Healthy food is much more expensive than junk food, which is one reason why it's so difficult to get the poor to eat a better diet. The
CEDAR centre at Cambridge University did
a study a while ago, which found that the average cost per calorie of healthy foodstuffs with an
FSA Nutrient Profile Score below four was three times that of less heathly food.
This is a plot of cost per 1000kcals vs NPS for the foodstuffs in my diet, you can clearly see that the average trend overall is that more healthy = more expensive.
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This study found that taxing unhealthy food actually increases deaths from cancer and cardiovascular disease, but taxing and using the revenue to subsidise the cost of healthy food saves lives.