I'm studying nutrition at a tertiary level for pretty much this exact reason and what it has taught me from speaking to dietitians who teach my course and from doing the course is:

a. Advice for the average person with a typical western diet boils down to "would it kill you to eat a damn vegetable?"

b.  There's a lot of organic chemistry that despite me being 3 years into a 4 year degree hasn't paid off (I am doing the degree that feeds into a masters of dietetics, so I'm sure that's where it was going to pay off. Alas, I'm not going to do that masters any time soon because my government engineering job 8 years in pays more than and end of career dietitian despite the higher level of education, and the masters would require me to work 9-5 for 6 months for free)

But most importantly, and most relevantly for the lesswrong sort of person, is that it is basically impossible to study diets in anything resembling double blind randomised placebo controlled. Diets are strongly linked to culture and personality and strongly influenced by those around you and virtually impossible to double blind. Like, imagine you wanted to study whether eating red meat is good for your health. Imagine getting 10,000 people and randomising them into three groups (high red meat, moderate red meat, no red meat). Would a steer farmer from Texas really stop eating red meat just because he was randomised into the no group? Would I, a vegan for 5 years, start eating 4 serves of red meat because I was randomised into the high group? (no, but I wouldn't sign up for the study for this reason, which is a confounder because people like me then wouldn't sign up). And if the Texan did stop eating red meat, would he keep it up for 20 years? Would someone randomised into the high group who got diagnosed with heart disease and was told by their doctor to cut down on red meat ignore the advice because of the study? 

It's that problem but writ large that makes decent dietary research hard to do. You'll notice a lot of studies are done over 1-3 months, because that's a reasonable amount of time to be able to provide three pre-packaged meals to your participants that you can control exactly (though those participants are probably going to eat other things: who goes to a birthday party without eating cake, for example?). 

So then we have to do animal studies, and our ancestral diet is very different from say a mouse or even a chimp, and is maybe not even what is best for us. 

Also, something that isn't really emphasised in this sort of discussion is the cultural value of food. Sitting and sharing meals with people is good for our mental wellbeing. 

I think some people also stick on "are eggs good or bad for you? is red meat healthy? are tomatoes good?" when this is kinda missing the point. No one thing should be such a big part of your diet that this information is gamebreaking. And I think everyone knows that vegetables are healthy and hamburgers aren't. 

So, where does that leave us? 

Fortunately, pretty much every country in the world has a team of dietitians who come together to make a guide for how to eat healthfully. I'm Australian so the Australian guide to healthy eating ( https://www.eatforhealth.gov.au/guidelines/australian-guide-healthy-eating ) and the associated material on the website is what I'm most familiar with. It boils down to eating a lot of vegetables (50% of your plate!), a moderate amount of grains, a small amount of lean protein, and eating fruit and calcium-rich foods. But the website has a lot of information on it in a very accessible format and I'd recommend it as a good starting point. 

The pedantic Lyle McDonald writes quality material on dietary advice.

• Why Epidemiology is Crap
• A Guide to Basic Nutrition
• How We Get Fat
• Controversies Over Carbohydrate and Fats
• The Reason You’re Not Losing Weight

I find Cochrane reviews to generally be of good quality, even if that means there findings are very often "we have reviewed all available data as of X year and we not able to draw any clear findings". Depending on your technical knowledge it might be useful to point out that they generally include a "plain language summary".

It is important to note that not everyone agrees with their findings (the abstract of https://pubmed.ncbi.nlm.nih.gov/16052203/ is well worth reading, not just as a criticism of Cochrane but as a comment of the field of research in general). I suppose one could reasonably argue that a combining a load of crap observational or small RTCs (with high drop out / low protocol adherence) is not going to teach you very much, yet this is what systematic reviews of the field tend to do.

You do occasionally see nice articles like https://pubmed.ncbi.nlm.nih.gov/32144378/ which follow 142 people for 2 years. It's still not a lot of people and not that long to draw conclusions for many different lifestyles and cultures across the entire human lifespan and across the world.

See https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD002128.pub4/full for an example of a Cochrane review, and for a list of advice see https://nutrition.cochrane.org/evidence

Or see this almost comical example, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7427685/ "12,133 records identified, 30 studies met inclusion criteria ... despite the large number of trials included in the review ... evidence for primary prevention on clinical endpoints is limited to one large trial with methodological issues ... [hence] there is still uncertainty regarding the effects of a Mediterranean-style diet on clinical endpoints and cardiovascular disease (CVD)"

I also second peoples point about examine.com - it's good.

Also, it's not really directly answering the question but I want to rant. Calories-in must equal calories-out but in a way that has so many caveats that it's not really a useful measure of anything.

• Processing: A tree, if put in a calorimeter will tell you it has a lot of calories (350 Kcal per 100g for red oak), but if you eat wood, you will shit out wood, we cannot process lignin so cannot extract the calories. Potatoes are interesting, because uncooked, most people (I think this varies by person) cannot process the calories nearly as well, hence the same potato has a different amount of energy depending on how you cook it. Peanuts are calorie dense, but unless you really chew them into a paste (which most people don't) then you will not be extracting the full calories, hence grinding peanuts into peanut butter increases the effective calories. In short the calories you extract depends on whether you can digest the food, which partly depends on how you prepare it (and partly depends on what species of animal you are and partly on your gut microbiome).
• Set point theory (https://pubmed.ncbi.nlm.nih.gov/30026913/) If you exercise more you will want to rest more and you will want to eat more. So if you burn off 100 calories more than you usually do then you might lower your energy expenditure in other ways to make up for it. Or you might eat more than you usually would. A similar thing is probably true if you change the amount you eat- if you eat more you might expend more energy (I know there are studies on this, I was asked to be part of one where I would have to eat and extra 50% calories per day, but I haven't read the studies). The way these interact is complex and there isn't a whole load of agreement on what is going on.

Nutrition is probably the most corrupt science as of date (or maybe it's just fallible, with all that epistemic bs going on?).

Nutritionists don't have a clue. They are only right about one thing: don't eat sweets and processed foods. Don't eat too much. Drink some water. Etc. But that's still far from healthy. If you wanna learn about nutrition if you to scoop the fringes of the Internet like I have done for years.

The basis is a ketogenic diet with intermittent fasting (IF being the most important part). Whereas currently nutritionists will tell you to eat 6-7 times a day. This is not good for most of us who've wrecked our metabolisms with modern lifestyles. Yet the paradigm is slowly changing.

People like Dr. Eric Berg on YouTube.

Yet keto people don't have it all right. Like mainstream nutritionists, they'll tell you to eat tons of vegetables and seeds. This is completely unnecessary since all the nutrients can be found in animal foods and in much more bio available forms. Vegetables/seeds also have anti-nutrients, whereas animal foods don't. The main concern is oxalates, which is a very direct cause to kidney stones, one of the most painful conditions. The second main concern is that too much fiber is actually detrimental for the colon, since our colons due to evolutionary reasons are not as complex as those of apes. We are apes who've descended from the trees and started eating more animal foods than plants.

But that doesn't make the carnivore-dieters right either. We still need a little fiber to feed the gut bacteria (though a carnivore diet can be healthy if it contains plenty of fermented foods to substitute fiber, like Eskimos do, but some fiber is much more feasible). You can also get more vitamin C from a few plants, more folate, more manganese, and more minerals for the same calories if you choose low calorie plants. Which plants? Low carb fruits, mushrooms, possibly some low oxalate vegetables here and there if you want.

Both of these groups also agree that dairy is good for you. Dairy is probably the most inflammatory food group, for some people even more than processed foods. Adults lose the ability to digest lactose, plus casein is very allergy prone. Plus hormones which adults don't need.

Now the least popular part: salt. We shouldn't eat salt. Any salt. It creates excess water retention, leading to headaches, acne, hypertension, etc. How many other animals eat salt? Ok, deers will have a lick here and there, but I don't think it's several grams a day, and they don't have food abundance like we do. We can get all our minerals from food and high mineral water.

So, the ideal human diet: 1 to 2 meals a day with at least 20h of fasting per day, max 25-50g carbs, plenty of nutrient rich animal foods with some low carb fruits (cucumber, tomato, avocado, berries, etc), mushrooms, maybe a little lettuce, carrots, etc. No (too much) vegetables, no seeds of any kind, no dairy, no spices, no sweets and processed meats/foods (all of these are inflammatory to humans).

I've adopted a two-pronged approach:

1. What did my ancestors eat?
2. What does the science say?

The first approach involves tracing my lineage to figure out what they ate. My family is Russian and Ukrainian, so the idea is to figure out what the most traditional foods are. I assume my genetics is going to be optimized for that environment. For me this means more cabbage and more buckwheat. However, though potatoes are popular in Russian cooking, they're a modern addition.

The second approach is to figure out what vitamins, minerals, etc my body needs. Essentially, you treat the body like an old-school Ford factory where raw materials come in one end and the end product (healthy cells) comes out the other. Another way to think about this is that if I had to put a feeding tube into by blood system, what does modern science say the mix of ingredients should be? Yet another thought experiment: take a healthy person and figure out what elements from the periodic table of elements exist in the body and in what proportion. If a healthy body contains magnesium but it's not in your diet, clearly you'd see some health consequences from this.

Finally, synthesize the two approaches. For example, if you have darker skin, you'll probably need more Vitamin D if you live up North. It may also be useful to pay special attention to "weird" foods of your ancestors since they may contain a beneficial nutritional profile that filled a gap. Buckwheat was one such food for me. It's packed with nutrients, but eating the same foods may not be enough. The same food grown in different geographies will have different nutritional profiles. I know rice can have varying composition depending on where it's from. In theory, you can become nutritionally deficient even if you eat the same foods as your ancestors and it could come down to soil composition.

The hybrid approach may seem complex, but it's simple to start since you can start eating what your ancestors ate and then take a lab test for nutritional deficiency to see if you're lacking in anything. The benefit of the hybrid approach is that they balance out each other's weaknesses. Science can't say anything about a chemical it hasn't studied. Even if it's studied, the science may only make claims about what's been observed or what can be inferred based on known mechanics. Traditional foods tell you what your genetics are likely adapted to. This may include chemical soups that science hasn't gotten to yet. However, just because your ancestors ate a certain way doesn't mean it was optimal for them. Finding the perfect diet is a back and forth between science and tradition.

I recommend the book "The Renaissance Diet 2.0" which does a competent job summarizing the science.

I think a productive way to look at it is to look for absence of evidence, which is evidence of absence.

Much has been said about "the western diet" that is killing tons of people, but in reality, what we really know is that being obese is bad for you, as is having severe nutrient deficiencies. Otherwise not a whole lot much is sure.

Let's take an example. Studies on meat consumption barely find a significant effect on all-cause mortality. But most often they fail to control for things as basic as pre-existing obesity or caloric intake. And if you step back one second, it's rather obvious: people who voluntarily reduce their meat consumption (or abstain completely) tend to be quite health-focused. All the people with a junk diet are on the other side. That ought to tip the scales, but even then, the finding is minuscule.

This absence of conclusive evidence really does tell you something: diet is much less impactful (to your health) than people give it the credit for. Consider, for contrast, that being partnered adds, on average, years to your life (I suspect this finding might also have a control problem, but the magnitude of the finding is in another league).

Something where diet really has an impact is your day-to-day well-being. I don't have a solid proof, but it seems to me that if your diet makes you feel like crap, it might not be that great in the long run, and vice-versa (beware deficiencies though, which take a long time to show up).

Unsolicited diet advice:
- Control your calories (track! I guarantee you will be surprised and learn something)
- Eat enough veggies / supplement to avoid deficiencies. You don't need that much, and you don't need to hit the RDAs necessarily - they're incredibly hard to hit with food only, so I supplement to be on the safe side. Also, animal products do actually have a ton of nutrients, compared to what popular wisdom sometimes seems to imply.
- Eat enough proteins. It's really hard to eat too much proteins, it's incredibly good for a ton of things, and it's generally filling besides.
- Eat enough fibers (the better argument for eating more veggies). Personally, my digestive system is weird and I actually supplement this as well (as psyllium), makes a huge difference, but I expect this is quite personal.
- Don't sweat the rest, and enjoy your food!

Imho, these are the 80-20 (or really 98-2) rules of nutrition. I've never seen any evidence that any of the intricate fluff makes any difference.

... or at least at the population level. It might worth experimenting with your own potential intolerances/needs (e.g. like my need for a ton of extra fibers). But that's not something you'll get as general advice. I do think there must exist books or resources on the subject however.

I get the impression that much of the official advice of the past was actively harmful; the official advice has flip-flopped on many things over the past decades; things which were touted as healthy were later shown to be unhealthy, and vice versa. So presumably some of the current official advice is also actively harmful, and some of it is merely useless. But I don't know which.

This is not, in itself, a reason to stop trusting official advice on nutrition. On the science hierarchy of how certain we are of our understanding, nutrition science is very low and physics is very high. So we can treat all nutrition science recommendations as "we believe this slightly more than we believe the opposite". So it will take a little bit of counter evidence for the field to update its point of view to the opposite. But once that has happened, it will take more counter evidence to flip back. The flip flops will get rarer as evidence accumulates, and at every point in time it could still be that official recommendations are the best that can be made given the data that we currently have. I'm not saying this is the case, but it is consistent with the data of flip flops.

Unfortunately, the official advice lags our current understanding (by decades, maybe). I think that's a much bigger problem than the perceived flip flopping.

I don't have any advice for nutrition, but examine.com is pretty decent for information about supplements and vitamins, and the NIH guide on supplements is even better: https://ods.od.nih.gov/factsheets/list-all/

Have you considered the reason for this is that optimal diet is highly variable, both genetic and environmental.

For example whatever a 16yo school going boy from Jordan and a 56yo recently retired woman from Nebraska ought to eat to maximize health/performance/happiness might be entirely different and, if switched, have disastrous effects.

But these effects could also be present in much more similar individuals and even in the same individual over time. Most humans used to eat entirely different diets depending on the weather.

90% of it comes down to "eat more vegetables" and "cook what you think are balanced meals, from scratch." Also don't smoke and drink less alcohol. And if you are overweight, eat a little less.

But I also appreciate the nutritional advice on places like examine.com.

UpToDate is a good starting point.