I am not a nutritionist. What I am is a retired philosophy professor who is reasonably good at doing research and evaluating the preponderance of evidence for or against certain claims. Regarding scientific claims, the best we can do is follow Bertrand Russell’s advice,
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• When Experts Agree: If all experts agree on a topic, a non-expert should generally accept this consensus as the most likely truth, rather than holding a contrary opinion, which cannot be deemed certain.
• When Experts Disagree: If experts are not in agreement, no opinion on the subject can be regarded as certain by a layperson.
• Suspension of Judgment: When experts agree that there is insufficient evidence to form a positive opinion, the ordinary person should follow suit and suspend their own judgment.

However, it’s rare for there to be 100% agreement among experts. For example, while multiple studies confirm that the vast majority of actively publishing climate scientists—often cited as 97% or more—agree that human activity is the primary driver of recent global warming. (This consensus is supported by reviews of thousands of scientific papers and endorsements from major scientific academies worldwide.) In such a case, I would accept this claim as true.

Now, scientific claims differ regarding their certainty. The natural sciences, like physics, chemistry, and biology, are very precise, and hence we can be very certain of their foundational claims. Quantum, relativity, atomic, and evolutionary theory are true beyond any reasonable doubt. The claims of the social sciences, such as psychology and sociology, are much, much less certain, and thus we are more justified in doubting their claims. I’d place nutrition science much closer to the natural sciences, but it is not as precise as, say, physics.

Now, there is broad agreement among experts in the field that fruits, vegetables, whole grains, legumes, mushrooms, seeds, and nuts are healthy, and that refined sugar, processed foods, and those high in saturated fat are unhealthy. There is disagreement along the margins, but contrary to what YouTube influencers and others, either ignorant of the facts or with financial reasons to deceive you, argue, there is a consensus among the experts concerning the statement above.

Now, when someone asks me if food x is good for them, I always ask, “Compared to what?” So, is a candy bar healthy? Compared to a banana, no, compared to a pile of poop, yes. Is a hamburger healthy? Is a banana healthy? Compared to a candy bar, yes, compared to strawberries, no. Is a all-beef hamburger healthy? Compared to a burger made of soy, no, compared to potato chips, yes. Is butter healthy? Compared to margarine made of trans fat, yes, compared to fake butter without trans fats, no. Well you get the idea.

Now, is olive oil healthy? Compared to lard, yes; compared to nuts, no. But is it healthy in the abstract? Well, unsaturated fats (those that remain liquid at room temperature) do improve cholesterol levels, lower inflammation, and support heart and brain function. On the other hand, they are calorie-rich and nutrient-poor. One tablespoon of olive oil contains 14 grams of fat and 2 grams of saturated fat. That’s a lot of calories in a tbsp. of something with little nutritional value, so you want to limit your intake. That’s the same reason why you would limit your intake of nuts—which are an even better source of healthy fats as they contain omega-3s, which are generally lacking in the American diet.

Now, as I have written previously, I follow a whole-food plant-based diet. I do this because those I trust—based on their expertise in the field, their lack of conflicting financial interests, the quality of their documentation of real scientific studies, etc.—those who recommend it. There is much to say about all this, but in an effort to save time, a simple way to begin understanding all this is to watch the entertaining documentary “Forks Over Knives.” It introduces you to The China Study: The Most Comprehensive Study of Nutrition Ever Conducted and the Startling Implications for Diet, Weight Loss and Long-term Health. (A summary can be found here.) And you can follow that up with “The Game Changers,” a documentary that destroys the myth that you need animal rather than plant protein. (Where do animals get their protein? From plants. So vegans are just cutting out the middle cow.)

But perhaps the best way to really understand the current research is to read the books or visit the website of Dr. Michael Greger. (The proceeds from all his books are donated to charity.)  I’d especially recommend:

How Not To Die or How Not To Age.

Warning, though, How Not To Die is just over 600 pages without footnotes. (More than 13,000 footnotes are available online; otherwise, the book would have been over 2,000 pages!) So he isn’t some YouTube influencer telling you that bacon, eggs, steak, and pork sausage are good for you—they are not, no matter how much you want to believe this. But if you’re interested in a summary of the totality of the evidence from decades of scientific research based on a meta-analysis of virtually every scientific study about plant-based rather than animal-based diets, then Dr. Greger and his team have assembled it.

Also, if interested, here is the quick AI overview of a whole food plant-based diet,

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