Your problem lies right there with the phrasing. In order to understand your problem, you need to rephrase it to "What ARE the secretS to health?" this is a much more mature approach.
I say mature because I see your ego is protecting your view of the problem by reducing it to a simple factor: diet.
Health is a holistic endeavour that requires your healing mechanism (i.e. the soul) to factor in all necessary "components" for everlasting health and, if you pay close enough attention, longevity.
Your ego to me lacks experience and thus protects you from the other major causes of health and sickness. Diet is one of five factors, the others include (in order of precedence):
All of these factor into your physiological homeostasis. One without the others is none without the rest. Your ego is protecting you from the bigger and more important factors due to lack of awareness. This may be intentional because to become aware is to be in pain. To be in pain means to feel the past of trauma. This may start with your mother (usually the case) due to her stressful environment. Thus leading further into your psyche and the ego you may currently have that negates your ability to improve your well being.
Your unconscious mind (which sits in your liver) fires implicit memories to aid your ego's clarification for protection. Your fear may be immature because your ego is immature. Your thought patterns may be immature because your mother's emotional telegraphy was impaired due to high stress.
The first level of change is to expand awareness. To expand awareness is to develop RESILIENCE. Resilience is what helps you to see above the mists of emotions in the landscape of your mind. My biggest recommendation is to take up meditation. Do this first.
I would also advise beyond this to develop social interactions wherever you meet ANYONE. Develop your listening skills for if your ego is immature this is the first place to start, as listening is giving your awareness to someone who needs it. This helps to express their need for you to be in this world, which helps to give you purpose and belonging; a byproduct to a healthy self-esteem.
All this is crucial to your thyroid which is the precursor to your metabolism. Remember, your metabolism is the physical process of your ENTIRE BEING. If you are having trouble metabolizing food, your total constitution is dysfunctional. In order to change this you must expand your awareness.
Your environment is crucial. Not just spatially and physically, but RHYTHMICALLY too. Your unconscious mind puts your conscious mind under hypnosis everyday. What influences your unconscious mind is the environment. It happens in this order:
Environment (Spatial, physical and rhythmical) > unconscious > conscious > behaviour.
The bizarre part is that in order for you to change any dysfunctional pattern you must consciously become AWARE of what it is. The rhythmical environment regarding implicit memory is to do with the past. Your emotions are echos of the past. Ghosts that come back to haunt you until you relinquish them through educating your inner self.
Ray Peat has a great article on metabolism and intelligence. You should check it out: http://raypeat.com/articles/articles/intelligence.shtml
Beyond that, I recommend Gabor Mate's When the Body Says No. There are physical signals to the ghosts that haunt you, you must learn to educate them for their misery to leave.
Misery, another thing to mention. I have known people who have developed a fetish for misery. They enjoy its painful allure and thus thrive on its illusion, but as Gabor Mate says:
"The attempt to escape from pain, is what creates more pain.‚Äù
Happiness in misery is an escape. Pay attention to this. Here is a great article on the misery fetish: http://www.alternet.org/personal-health/14-habits-highly-miserable-people
Next is narrative. Narrative is very powerful and directly ties into the rhythmical environment. It determines whether you hang around toxic people (either at home or at work), your unfulfilling occupation (dissonant to your imagined role your muse keeps speaking to) etc and so forth. I noticed someone mentioned their mother being unloving to them. There is an important factor to notice (especially with women) that until you find out her story and her past, you will never be able to over come why she hurts you. She may do this because of her unconscious behaviour related to her destructive narrative within her past. Always find a narrative to the pain people cause you, they may not know (even if blatantly obvious to you) what they mean to do. It helps to be curious to unravel these painful knots as it helps them become aware too. Narrative is interlinked with everyone. It helps to bring people closer. It helps to forgive. It helps your heart to make closure.
These are the questions worth asking before continuing any further with "diets". A good diet is necessary for good health, but not without the other factors of a healthy well being.
That is the secret to longevity.
"The problem is not that the truth is harsh, but that liberation from ignorance is as painful as being born. Run after truth until you‚Äôre breathless. Accept the pain involved in re-creating yourself afresh. These ideas will take a life to comprehend, a hard one interspersed with drunken moments."
‚Äî Naguib Mahfouz