Something I think but don't think that you think

I overheard a conversation walking through campus today. I will abbreviate it thusly:

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Person A: I saw on tv how lack of vitamin D causes breast cancer. But I found out today that it is not true. My professor said so.
Person B: But your professor isn't a doctor right?
Person A: No, he's actually a doctor in a hospital and everything.
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So here's what I think. A cursory google search seems to say that Vitamin D may actually be linked to reduced cancer risk. But what is key here is that causal links like these are hardly ever conclusive. Unless results of tests are incredibly clear, given really good statistical analysis, you're going to have academics disagree about results. Way too many people rely on what the professor said. Often the professor is right. More often the professor is half right. Typically, the professor is heaving spoonfuls of subjectivity into each sentence.