The most important diet advice is extremely simple:

• no sugar
• no grains
• no seed oils

It matters more what you get rid of than what you add. "Superfoods" are more about marketing than health. As long as you stop eating the foods and "food-like-substances" that destroy health, you're moving in the right direction.

Sugar (principally fructose) is linked to a range of significant health problems for people: fatty liver disease, metabolic syndrome, diabetes, heart disease, kidney disease. If you do literally nothing but eliminate the consumption of added sugar, you're taking a major step toward better health. And likely toward weight loss as well.

Grains are highly problematic for human health because they are full of anti-nutrients and deliver a massive dose of unnecessary carbohydrates. As predators and carnivores, we should eat meat, not grass. Remember, the total requirement for carbohydrates in the human diet is zero. Our liver manufactures all the glucose that the body needs (gluconeogenesis). Highly-processed grains (aka "white flour") are especially bad because they have their few nutrients stripped out in processing. Flour is finely ground so that it digests rapidly and spikes blood sugar faster. The fiber that might help slow down the blood sugar spike was removed in processing. However, this doesn't mean that whole grains are much better. They are just "less bad". Better to avoid grains entirely. People eat grain-based products out of habit. They are convenient, cheap, and versatile; they aren't the best foods to eat for optimal health.

Seed oils are implicated in a number of major health problems like heart disease and metabolic syndrome. They are also completely unnecessary because better fats are available. They only exist as products because they are very cheap and profitable for the industries that manufacture them. The industry has branded them "vegetable oils" because everybody knows that vegetables are good for you. However, they are all highly-processed seed oils. The irony is that they were originally promoted as the "heart-healthy" alternative to saturated fats like beef tallow or coconut oil. That experiment has not worked out very well.

These ingredients - refined sugar, processed grains, and seed oils - tend to come together in the form of highly-processed, convenience foods. These products are a perfect storm of toxic, food-like substances. They provide a quick spike of energy and almost nothing else.

We should avoid these foods as much as we can. But what should we actually eat instead?

• Meat. Eat mostly ruminants (beef, goat, bison, etc). You can add fish, poultry, pork, and other meats for variety, but most of your meat should come from ruminants. Preferably grass-fed and pastured in all cases. Eat "nose-to-tail" if you can.
• Eggs. Get your eggs from free-roaming chickens if possible.
• Non-starchy vegetables. Make sure you cook them with plenty of natural fat and salt to make them taste good. Generally, if a vegetable grows above ground, it may be a good idea to eat it; if it grows below ground, avoid it.
• Fermented dairy products. Natural cheese, yogurt, and butter are good. Full-fat yogurt is especially healthy because of its beneficial bacteria. However, avoid plain milk because of the high natural sugar (lactose) content
• Lower-carbohydrate fruits. Examples include avocado, berries, tomato, olive, etc.

A canonical healthy meal is "steak and eggs". If you like it, add some beef liver - it is perhaps the most nutrient-dense food in existence. In general, eat as much as you need to stop feeling hungry (i.e. to the point of satiety).

If you did nothing else other than follow these steps, you would likely avoid most of the "diseases of civilization" that plague the wealthiest countries of the world.