We Mostly Learn Whatever Our Society Promotes.
We begin to learn health lessons before we can talk. Our taste buds are being developed as we transition from milk to solid foods. Our parents pass on to us their best health knowledge, and many of us have carried on those same health traditions. Exercise patterns, diet, smoking, alcohol use, problem solving skills are passed through families, and among friends.
My parents taught me
My parents smoked VERY heavily. I learned that smoking is bad for health.
Roast meat, potatoes and two vegetables were a good meal. My father always encouraged us to eat all the fat on the plate, which we resisted as kids.
To eat porridge, with cream off the top of the milk, and two slices of buttered toast for breakfast.
Or to have bacon and eggs, or sausages and tomatoes, for breakfast.
To have white bread sandwiches for lunch.
The finish a meal with bread and jam or a biscuit, and a cup of tea.
That hard work kept you fit.
My parents took sugar often, and ate puddings, but remained slim.
My parents never fought, at least in my presence, and we were never beaten as children so I didn't learn how to deal with conflict as a child.
Some of the things my parents did were unhealthy but they got a lot of things right too.
Society and Education taught me
That muesli and raw fruit were the best breakfast. (Later we added yoghurt.)
That we should eat five servings of vegetables a day.
We should drink lots of water.
That we should avoid fat in our diet. Especially animal fats.
That vegetable fats were good, especially olive oil.
That we should drink only trim milk and that margarine was better than butter.
That exercise was a good way to lose weight.
The all fats were not the same. Somehow; I learnt that saturated fats were the real enemy, they made me fat and increased the likelihood of heart attack.
That red meat should be avoided, but that chicken was OK.
A high carbohydrate diet was especially healthy. Whole grains were the best energy food.
That we should all be taking a multi-vitamin supplement.
That positive thinking was the key to success, and for good mental health.
That high cholesterol, and fats in the blood, increased the chances of having a heart attack, and that I could avoid that by eating a low fat diet.
Somehow I learn that the prefect diet was probably vegetarian.
Far too much of the above is BAD Information. Misunderstood sometimes, but often there is no basis what-so-ever behind the claims made, except "it seemed a logical idea at the time".
Learning some new health science
I've been generally healthy all my life. I assumed the "good information" listed above had kept me healthy. But I wrong. Much of the information above is certainly WRONG, and can now be shown to be wrong in scientific ways.
Until I was in my mid 50's I was still running marathons and tramping and leading an active life. I thought I knew all I'd ever need to know about how to be healthy.
But when I stopped running so far, I began to get fat. I took me about 10 years to learn that the best way to stop that was to cut the bread out of my diet. However, fluctuating weight remained a problem. I discovered that if I didn't finish my evening meal with a biscuit or cake (No bread at the table.) I didn't need to have supper at 8pm and again at 10pm. That was enough to keep my weight stable, but since 2012, I've been encouraged to find out more, and what I've discovered has significantly changed how I think about health, what I eat, and how I behave.
Medical science is full of stories about the unwillingness of medical doctors to accept new ideas, mostly because they WANT to do the right thing by their patients. Here are three examples. When is was discovered that doctors in hospital spread childbirth fever for patient to patient and that simple hand washing in lime water stopped that transmission, the doctor who made that discovery was persecuted, and it took 50 years for change to happen. More recently, a researcher in the UK, discovered that x-ray treatment for pregnant women was undesirable and likely to harm the fetus. Thirty years later the practice remains common. Thirdly, it's been known for twenty years, and very well understood for 10 years, that for people with diabetes, a low carbohydrate diet which often allows people to stop injecting insulin, is best. Yet the "mixed balanced diet" followed by blood testing and insulin injection is still being recommended. Get informed. Become the expert on what you need to do for your own health.
As I discovered, it's inevitable that if you live in a modern society, much of the health knowledge that you and 
almost everyone around you, including the doctor you rely on, is likely wrong. This is the "information age" but we live in a sea of disinformation. Powerful commercial and institutional forces have created a system that protects their interests, but does not necessarily support the best health outcomes. How can that be?
Firstly, when many of the key ideas now driving low fat and vegetarian diets were first promoted, there was very little science in nutritional studies. Most nutritional experts learnt their skills in Home Economics Departments, or from the text books written by members of the Seventh Day Adventist Church, the were once the foundation of the training of dieticians. A varied "balanced diet" was the objective. Better methods of cooking and preserving food were developed. The idea that low fat diets were good for heart health, was based on some very simple assumptions that seemed so obvious, they were never tested. This simple: Fatness is the result of too many calories in and too few calories out. And, 1 gram of fat contains 9 calories; while 1 gram of carbohydrate or 1 gram of protein only contains 4 calories. A simple but wrong idea, to lose weight you must eat less fat. Simple but wrong idea; Fat makes you Fat: seems so logical, in must be right, but it's nonsense.
In "The Structure of Scientific Revolutions" (1962) Thomas S Kuhn explained that science never progresses smoothly. In any field of science "best practice" is established and sustained for long periods of time. Scientists work with a paradigm, a set of rules that defines "good science". Belonging to the profession, has at it's core, training in and practice of these fundamentals. There are always problems to solve. At some time the progress that is possible within that paradigm seems to come to an and, yet many unresolved problems have been identified. One way forward might be a new scientific instrument, a new way of seeing. Alternatively, a new idea, a new direction, and new theory to explore. All the leading people in the field, are firmly committed to the theories they have spent their lives building. They may be scientists or doctors but they have the same trouble we all have, following a new direction, changing your mind, is never easy.
Usually the roots of any new theory have existed for a long time, but were overlooked and ignored, because it didn't seem relevant. The theory has probably been nurtured for a long time by someone working with no recognition, and little funding. Some younger, up and coming "star" recognizes in this "old" work provides a new approach that makes the "unresolved problems" facing the profession look quite different, perhaps resolvable, and maybe, given this new understanding the old "problems" are irrelevant. If this new approach leads to success, more young people join the "new school" of thinking, and within quite a short time, maybe 10 years, the whole way of doing science in this field can change. Kuhn calls this a paradigm shift.
I might be wrong but I think a paradigm shift in nutritional science is happening now. The Swedish Government has recently recommended a new official dietary plan for Sweden, based on the current science, but in fact it's very much the same diet originally recommended by a doctor in Sweden in the 1920's.
The ketogenic diet, well known in the 1920's particularly as a treatment for epilepsy, but also for diabetes, was neglected after that because more "modern" drug treatments were available. Drug treatments were also seen as scientific, medical, and professionally appropriate ways to offer help to patients. Dietary recommendations and "natural cures" were what quacks and fraudsters offered. In nutrition, carbohydrates, particularly whole grains were highly desirable and red meats and saturated fats were the enemy. Ancel Keys, usually takes the blame for this misdirection of our effort to avoid a heart attack, but the Seventh Day Adventist Church was active 50 or 60 years earlier (pre 1900), training dieticians and openly promoting a vegan diet. (At the time this was entirely a religious justification, there was no science.) Even today, despite a great deal of research, there is no well controlled, randomised study that supports a vegetarian diet. (That's hard to do.) There are 100's of studies over the last 60 years, that show an association between vegetarian diets and improved health. Considering the effort to prove the value, the results are disappointing.
In contrast people seeking to research fats in the diet, and the effect of vegetable oils in the human diet couldn't get funding. "Everybody knows the saturated fats are bad and vegetable oils are good," so there are more important things to spend scarce research money on.
In the USA, several researchers who for twenty years and more have worked is the dark corners of the nutritional world, are suddenly finding that there is an interest in their work. Several books have been written in the few years, One of them called "The Art and Science of Low Carbohydrate Living" (2011) by Dr Stephen Phinney and Dr Jeff Volek; was followed up by "The Art and Science of Low Carbohydrate Performance" (2012). Stephen Phinney had worked for 20 years previously "getting the wrong answers" as he says. His research was successful, but the political climate in nutrition wasn't ready to hear that carbohydrates were driving obesity and inflammation in the body. The writing of the meticulously documented book, A Big Fat Surprise" (2014) by Nina Teicholz and another called "A World Turned Upside Down" (2014) By Prof. Richard Feinman helped to confirm the trend. However if you want to buy one book that's easier to read get a copy of "Death by Food Pyramid" by Denise Minger.
