I've changed my mind so often on these health issues that nobody should take much notice of my opinion. For what it's worth here it is.
Firstly here are some facts which seem reasonably likely to be correct:
- People lose weight on a low carb diet, and reduce their blood sugar levels.
- People lose weight on a low fat diet, at least if it is low in sugars and other refined carbs.
- Statistics used to show that being overweight was bad for your health and reduced life expectancy.
- Statistics now show that being overweight is protective and increases life span.
- Gluten allows leakage from the gut into the blood circulation. In celiac (and maybe other gluten sensitive folk) this gets to be significant.
- Emulsifiers may do the same.
- When obese people have a gastric bypass operation, so the food bypasses the top 1/7 of the gut, then they almost immediately stop having high blood sugars, long before they lose any weight.
- If you are fit and active you will live longer if you stay overweight.
- If you are unfit and inactive you will live longer if you are overweight.
- A very high protein diet (particularly if from mammals) is bad for your health. So you've got to eat carbs or fats or both.
- Processed food, particularly meats, are generally bad for health.
- A healthy diet, particularly one low in refined carbs, makes people enjoy being active more.
- UV (direct sunlight) modifies molecules in your blood, and in some cases (at least one = bilirubin) turns them into something your liver can deal with better.
So why did statistics previously favour not being overweight? The reason is that most fit active people don't stay overweight. Previously more than half the population were fit and active (and got out in the sun) and these tended to normal weight. Being overweight was highly correlated with inactivity which was the actual cause of the statistical bad outcomes. Now things have changed. Nearly everyone is inactive, and now the health advantages of higher weight show up directly.
We used to think that being overweight caused bad health. So it was assumed that things which lead to lower weight must improve health. All these interventions that address risk factors rather than outcomes are highly suspect, particularly medications.
Kids are rarely overweight until they become inactive. But that doesn't mean that anything that reduces their weight is good. Rather it is a wake-up call on activity levels.
We can see that some significant part of metabolic syndrome is caused by problems in the top 1/7 of the gut. So it makes sense that healthy diets are ones which can't be easily processed and require processing lower down in the gut. I suspect that this includes nearly all fats and proteins, so that is why a low carb diet addresses metabolic syndrome in the same way as a gastric bypass.
However I think it is hard to have a healthy diet unless it is mostly plant-based. Sugars and refined carbs are immediately satisfying because they get processed in that top 1/7 of the gut. So grains, if any, need to be minimally processed. I suspect that people would have a lot less trouble if all gluten, and many similar molecules, were not available for processing at the top of the gut.
TL;DR: Mostly eat vegetables/fruit/nuts/grains. For protein mostly fish and birds (and plant protein, particularly pulses). Mammal meat every 2 weeks. Get active. 15 hours sitting plus 1 hour of activity doesn't do it. Get your heart rate up a few times a day. Get a standing desk and keep moving. Get in the sun but don't get burnt.
I don't recommend my home-made standing desk, but it is cheap (card table plus small angle beams held up with nuts and held on with cable ties):
[update: I should have mentioned that there are important nutrients that you need to get. This (http://www.express.co.uk/life-style/health/385884/Supplements-halt-Alzheimer-s-groundbreaking-dementia-prevention-treatment) mentions B6, B12 and Folate. There are others. Selenium is important: brazil nuts often have a lot, depending on where they're grown. Look for brands which specify how much selenium.]