We seem to be bombarded with information and 'research' on healthy diets, and the narrowly focussed research can be particularly confusing - especially when it is funded by vested interests.
There are some guidelines that experienced nutritionists always advocate, and they are relatively easy to follow.
Fat: The best fats are not saturated, and are naturally occurring. Examples are nuts, oats (much more than other cereals), seeds and their derived oils (sunflower, flax, olive, ...).
There probably are nutritious wonders in hard cheese, but it needs moderation to avoid congested arteries.
Sugar: Refined sugar is too rapidly available to be healthy. It provokes insulin to give the message 'store fat!', which most of us can do without. Sugar founds naturally in fruit and vegetables is digested slowly, and doesn't cause the same problems. We can learn by following the BEST examples. Do world class tennis players eat sweets or bananas between sets? They eat bananas and are clearly very fit.
Even fruit juice is a bit risky because the juicing process breaks down the cell walls and the sugar (fructose, in this case) is available too quickly for our own good. In moderation its okay, but eating an apple or an orange is the best way to get the many other benefits of fruit without flooding your bloodstream with sugar.
Sweeteners may cause problems because the tell the body sugar is present when it isn't. There is no general agreement yet on the impact of artificial sweeteners on our health.
Processed food: I can only think of one case where processed food is recommended. If you have gout - avoid the fibre in wholemeal bread! All nutritionists agree unprocessed food is the rule to follow. Possible exception - carrots and tomatoes gain some benefit from being cooked (vitamin A absorption and the antioxidant lycopene respectively.
Meat: Moderation. We don't have the digestive system of a carnivore, and there are plenty of sources of protein that aren't meat. Studies finding dangers in a high meat diet always seem to find that red meat is the worst offender; lean poultry recommended.
Eggs: The cholesterol in eggs is no longer believed to be a problem, compared to the foods that trigger our own cholesterol-making. The enemy is food containing lots of saturated fats.