Nutrition and exercise struggle to be sciences. This is not to say that rationality and experiment cannot, or are not, applied to these fields - but rather that the conclusions so far reached are difficult to help inform a person interested in the “best” way to eat healthy and exercise efficiently. That is to say: sports science has so far been unable to make testable predictions of the sort that are repeatable or informative for the population generally. But this is not to say sports science is not worth pursuing. It is. There always exists an infinite frontier of ignorance before us in all subjects - so more investigation is always called for. But the problem with proclamations in the sciences of nutrition and exercise is that they are always so full of confidence and so devoid of carefully checked content. These fields are complex. Huge numbers -literally incalculable numbers of variables require computation in order for us to approach anything deserving of the word precision. So we must be skeptical of anyone claiming to be in possession of very specific recommendations for what foods you should or should not eat - how you should or should not exercise. Think stuff like: the top 10 list of most healthy foods. Yes, even when it's from a medical website. That link in particular is a catalog of poorly controlled studies. Modulo allergies, more or less: eat what you like.
So first, nutrition. The chemicals we refer to as “nutrients” number in the dozens. And we simply do not know what all the nutrients that a human person requires happen to be let alone how much precisely is required. It is true we have some very general guesses - but that's all they can ever be, because we know that different people will require different amounts and we simply haven't quantified much at all - even with questions apparently as simple as that. For example it happens to be the case that even with the nutrients we are most familiar with - multiple studies disagree about how much we "need". Indeed it seems quite major changes are made periodically when it comes to what amount of (say) sugar we need (if any). What, for example, constitutes a "deficiency" in something at familiar as Vitamin C? This is not a trivial question. Is merely not having scurvy enough to say one has had enough? Apparently not.
Anyone who takes even a passing interest in what is "healthy" versus "unhealthy" food will (or perhaps should) appreciate how misunderstood a nutrient like fat is. Some chemistry is needed to appreciate nutrition - fat, like the other macronutrients is not a single chemical but a word describing an entire class of compounds. Some people might be less interested in the question of what fat actually is to asking: How much do we actually need? What is a "healthy" amount? This question has no answer for different people will have different requirements - one of which is what they want. If the greater part of someone's day is devoted to the sport of sumo wrestling, they might require more because of what they want. And this is what takes nutrition away from being an entirely objective science like chemistry to something with a very personal, subjective, moral element. Desire becomes a factor, nutrition ceases to be a pure science in the same way chemistry is because what is “nutritious” for you is different to what is "nutritious" for me.
Is 100g of fat a day unhealthy? The question itself is meaningless and admits of no rational answer. Is the 100g for me or you or someone else? Is the fat mono-, poly- or un-saturated? If a person is literally starving - and 100g of fat is all that is available - that 100g is more than merely 'healthy' - it could be the difference between life and death. If a person is a marathon runner, they will burn up the equivalent of the energy contained within that fat before the race is over. Is the fat in question saturated or not? Is it eaten with sugar, or not? Does the person already have clogged arteries? Is the person about to die from fatty acid deficiency?
So whether fat is “good for you” or not is not a question like “is cobalt a metal?” and indeed it never well be that sort of question. Because it is a question not simply of science (what is the case) but also of morality - it is about what should be the case.
Of course this analysis does not apply only to fat. It applies to all nutrients - and every food more broadly. What amount is healthy? The answer will always depend upon a multitude of factors. The range of possible dosages of any nutrient will be vast and currently there is no known general purpose answer - no mathematical formula into which you can enter your height, weight, caloric intake per day, metabolic rate, etc, etc - that will be enable you to prescribe for yourself what amount of whatever nutrient you will need to achieve your goal, or what amount - for you - might be harmful. Until that day comes - what amount is "healthy" versus "unhealthy" will reduce to a very rough guess. And those scare quotes around the words "healthy" and "unhealthy" are no accident. There is no good definition that sharpens the meaning of those terms into something that can be useful beyond "not sick" and "sick" (and even there we have circularity). So, it seems a food is "healthy" if it does not make you sick. But, poison and bacteria aside, we do not know which foods, or how much of them, are required to make an otherwise healthy person ill. Or if indeed the same foods make the same people sick. Those with apple allergies can rightly say apples are extremely unhealthy.
Is alcohol a nutrient? Some studies at least seem to suggest it's better to drink heavy than not at all (while moderate daily drinking seems to be best). Yet, perversely in the state of New South Wales in Australia alcohol is taxed more highly than other commodities because of the supposed detriments to health and venues that serve alcohol must suffer severe restrictions on opening and closing times. Bad ideas about nutrition can lead to bad policies and jumping to one conclusion over another only ever leads to irrationality. At every turn it seems someone with not enough knowledge and too little skepticism has more than enough confidence to tell you what to do when it comes to what you should eat or drink. And they might even punish you with fines and prison if you deviate from their ideas about nutrition. This is no small thing.
Next: exercise. There is a mantra that often goes something like this among health gurus: Exercise is more important than diet. Or diet is more important than exercise. Or diet and exercise are equally important.
But important for what? For getting good at exercise? For getting as massive as a sumo wrestler? For the nebulous concept of "health"? Typically, it about weight loss. And often getting "fit".
Let’s say we deal with weight loss because that seems to be the most common reason people become atypically obsessed with how much activity they are doing and what they are consuming. But even when it comes to a well defined goal like "I want to lose 10 kg of my body mass" - no answer can be given as to what is more important - diet or exercise. A person who eats little but high fat hamburgers can still be very thin. Think a person who runs regular marathons. Anyone who runs 3 or 4 marathons (or more) a week will not be fat. There has never been a person who has run regular marathons who is also fat. You simply cannot consume enough kilojoules per day to put on weight if you are expending some tens of thousands per day in exercise. It just will not add up mathematically. This is proof positive that diet is not more important than exercise for keeping fat off your hips. Moreover, we all know of the glutton who seems to be able to eat hamburgers and chocolates and never get fat despite never exercising. From what we know, these people are burning a lot of energy just by resting - and why this is we just do not seem to know. We hand-wave the mystery away by saying "they have a fast metabolism" (in sports science and medicine it's called "Basal Metabolic Rate" or BAL and they have altogether unreliable formulae to help predict what one's energy expenditure might be given certain constrains. But giving a mystery a label, and coming up with an empirical formula doesn't actually provide any answers). Indeed all this simply begs the question: How can two otherwise extremely similar people respond so differently to identical food intake and exercise? This is a good scientific question we do not yet have good answers to. No doubt the answer in large part lurks within the genes - but where exactly, we do not know. These questions are the "dark matter" and "dark energy" of sports science. Merely statements of our collective and scientific ignorance.
One place where people have approached something like a scientific approach to nutrition and exercise is in the sport of bodybuilding. Here we can actually see, before our eyes, the effect of different theories. We can test the theory that eating 1000g of protein per day, 500g of carbohydrates and 200g of fat has upon the body of a 25 year old, 90kg male who follows a very strict, particular exercise regime where, within the best measurements available, burns some number of calories per day.
But even here the error bars are beyond the realms that any other quantitative scientist would regard as acceptable in order to establish trends. Some people will simply put on more muscle and burn more fat, even though they do the same amount of weight lifting and eat the same meals - and so this means general advice is typically not going to apply to - well anyone. Because there is no "general person". When at rest it is difficult to know how many calories John burns compared to Joe. And even if we do get an estimate, we do not know whether 10 minutes later, after the test is done, if John’s caloric burn will increase or decrease due to an almost infinite number of factors. We simply do not know enough about human exercise physiology to make such judgements. And even if we had a perfect experimental specimen - say John who we have studied for some years and in the process minimised those error bars - we know it is a very unreliable thing to extrapolate from this single data point to others - like Joe - let alone someone who is not 25...and a male...and a bodybuilder.
So for all we know about nutrition, using this very general knowledge to make specific predictions (like what should *I* eat to maximise *my* weight loss/muscle gain?) is not yet possible without further experimentation.
Of course what we do know is worth knowing. We know water is essential. We know protein is needed to build muscles. But whether broccoli or blueberries really do contain anti-cancer properties simply is not known. And anyone who asserts they do, is being dishonest. Worse than that - they are actually hurting people by giving false hope. Cancer, for example, we know is not a single disease - but many. And potential treatments, or preventatives, are going to be as broad as the disease itself. If someone tells you "Wheatgrass is going to prevent cancer" ask: which cancer? If they say "All of them" ask the best question of all: "How do you know?" If the best they have to offer is one of the thousands of "natural living" websites you know they don't know. No one does. If something really did prevent cancer, we would know about it. In the same way we all know aspirin works for mild headaches. We all know what works - and we use it. That's a good rule of thumb. Common knowledge when it comes to "health" is often superior to that from some purported expert. This is because "health" is so wooly and nebulous a term that broad general knowledge actually works better than narrow specific stuff.
All this is to say: be extremely skeptical. Almost all the nutrition and exercise advice you will be given is false. Almost all of the posts of facebook, most of the tweets on twitter - most of the stuff on supposed "health" websites - are replete with misconceptions. It would typically take very high level expertise - like degree level knowledge in nutrition, or a PhD in biochemistry - to sift the "discovered through the application of good scientific research carefully checked and repeated by multiple peers" from the "just a rumour" stuff. Most of the time what already passes for general knowledge is pretty robust. It's best to get a bit of protein, fat and carbohydrate each day. Drink when you're thirsty. Have some veggies to keep your vitamins up. It doesn't get much harder than this - and even those very general rules might be too strict.
Most of the time, in a rich country, you can eat most of what you want and do as much or as little exercise as you like. You will be able to tell just by looking in the mirror if you like what you see. In a rich country you would actually have to do more work to suffer some deficiency, or risk cancer from some food you eat, or risk any bad disease by the amount of exercise you do not do. Of course, if you are a fat 200kg and thinking about buying one of those scooters to get you to McDonalds - it should be obvious to you that you're at higher risk of disease. It's common sense. You can see it in the mirror.
We do not know to what extent this or that chemical in food causes diseases like the many cancers that plague the population (remember when I use the word “chemical” I use it in the scientific sense - so I include things like “water” and “oxygen” and “protein” as well as “potassium benzoate” as chemicals). Likely none, on their own really do cause cancer. Almost all "this thing causes cancer" scare articles are complete lies. One of the best lies is the one about artificial sweeteners. The fact is: artificial sweeteners are great. They are not unhealthy - not in the slightest under any definition of "unhealthy". There is nothing wrong with them in moderation.
If it's what the reasonable person would say is "edible" and you're not allergic - and fits the definition of "food" as a starving person might understand the word - you can relax about it. Food is going to be considerably less of a cancer risk than, say, the granite and basalt rock beneath your feet at this very instance that is mildly -and entirely naturally - radioactive. And which it is impossible to escape (and if you think living at higher altitudes away from bedrock might help - think again - there's even more radiation coming from the sky - also entirely naturally). Radiation, like light - just is. Right now, the walls around your house, the sky above your head and the ground beneath your feet contains radioactive materials - that are there naturally. And we live with that. Are they harmful? Not really. Are they harmless? Well no - not really. But they are a fact and we have no means to guard against this background of relatively harmless but nonetheless damaging fact of life. And it doesn't matter in the long run if you live above slightly more granite than your neighbor because they just might get a little more cosmic radiation. So if you are concerned about stuff in your food potentially causing disease - you should be far more concerned about the rocks on which your house is built is the point. The latter we know can, according to our best physical theories, damage the DNA. The former, we are still guessing at the relevant mechanisms.
So eat what you like and just remember you cannot get around the laws of physics. In this case the first law of thermodynamics: energy cannot be created or destroyed - only changed from one form into another.
When applied to nutrition this simply means: if you want to lose weight: the amount of energy you take in must be less than the amount of energy you expend. If you want to gain weight, the amount of energy needs to be more.
So you can eat all the hamburgers and chocolates you like. But if you want to lose weight, you better burn more calories throughout the day so as to not store that stuff as fat. Because - in a healthy person at least - most excess calories will get stored as fat. That really is all there is to it, until - years from now - we have a precise, predictive science of nutrition and exercise. We just aren't there...yet.