That = "how much that food is valued?"

Context:

"Our basic conclusion is that under these conditions, having to work harder to get a certain food changes how much that food is valued, and it does that by changing how good that food tastes," Johnson said. "This suggests that, down the road, obese individuals might be able to alter their eating habits so as to prefer healthier, low calorie food by manipulating the amount of work required to obtain the food. Of course, our study didn't delve into that aspect. But the implications certainly are there."