I'm about to say something that is easy to dismiss as bullshit, so feel free to, but you're right, people were healthier in terms of weight in the 60s, and right around that is when we changed dietary guidance to be carb-focused, low fat, etc. Since then, obesity has skyrocketed. The book Obesity Code covers this.

As sibling comments have pointed out, 3800 is availability, not consumption, so it's not an indicator of how much people do eat, just how much they could eat.