So nutrition is really what people are eating; and that overlaps with the concept of diet, which is when people are making specific choices about what to eat, how do they partition their nutrients.

营养学研究的是人们吃的东西,及其与饮食概念的交叉学科,也就是人们决定吃什么之后,如何区分其中的营养素

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选

Our third example for using the "That's what" pattern is when you explain a specific point about a larger issue.

使用 "That's what" 的第三种情况，是解释一个大问题中的特定方面。

That's what课堂 - SpeakingMax英语口语达人