Answer by Steven Fowkes:
Myth has many meanings. "Belief in truth without conclusive evidence" is the meaning that I will use here. This is quite different from "belief in something false." Myths can be highly empowering, so myth is not all bad.
This list might grow.
1. Polyunsaturated fatty acids are essential. This myth is widely believed despite the fact that (1) the spectrum of symptoms displayed by EFA-deficient animals were conspicuously the same as those of B6 deficiency, and (2) the researchers never tested whether addition of B6 would eliminate the supposed EFA-deficiency symptoms.
2. Metabolic rate (BMR) is predicted by TSH levels. This is true in young, healthy men. It is not true in many, many situations; I would say most situations where symptoms of low basal metabolic rate are present (low body temperature, cold hands and feet, edema, constipation, digestive complaints, sleeping problems, depression, brain fog, low pulse rate, low blood pressure (before becoming high blood pressure), slow wound healing, etc. This also applies to T4, free T4, and T3 levels, in increasingly less non-predictability. The highest correlation is the T3:rT3 ratio, and even that does not predict BMR in all cases. Thyroid hormone resistance sabotages all thyroid tests regarding correlation with metabolic rate.
3. Masturbation leads to insanity. I mention this here to break the monotony, introduce a note of humor, and point out that myth can be true. This is a past myth that turns out to have a basis in truth: adolescent males masturbating excessively lose significant amounts of selenium and zinc in their seminal fluid, and zinc deficiency can be a cause of mental illness. This is most likely to manifest in teen boys because of their high libido and high zinc-requiring growth rates. Generally, girls do not grow as fast and do not ejaculate when they masturbate.
4. Carbohydrate is needed to burn fat. This myth is fact and fiction at the same time. In absolute terms, fat can be burned quite efficiently in the absence of any dietary carbohydrate, for example, while fasting or starving. But there is loss of lean body mass because gluconeogenesis from the glycerol (glycerine) from hydrolyzed triglyceride is not sufficient to meet body/brain requirements. So carb is needed to most efficiently burn fat.
5. Insulin resistance in prediabetics and diabetics is best managed by complex carbohydrates in the diet. This is long-standing advice from the diabetes NGOs and it is false to fact. Beta-oxidation of fat and ketosis are far more effective. Maybe this is more a diet myth than a metabolic myth?
6. Trace mineral deficiencies can be treated by foods and dietary supplements. Sure, some can. But type-II copper and zinc deficiencies are not reliably treatable with copper and zinc supplements, or eating shellfish. Such deficiencies are caused by liver-sequestration of copper and zinc and supplements of copper and zinc absorbed in the gut go through the liver before going the the rest of the body.
7. Anemia should be treated with iron supplements. Not only is iron similarly sequestered by the liver in response to allergy, infection and inflammation, the utilization of iron in the manufacture of hemoglobin and red blood cells is dependent on many non-iron factors. And since iron sequestration can become deadly when it reaches the limits of ferritin storage, anemia in the presence of high iron stores should not be treated with iron supplements. If iron leaks from supersaturated ferritin, it acts as a growth factor for more species of bacteria than not. This is potentially life threatening.
8. Drinking water is sufficient to hydrate you. This myth is alive and well. But what most people (and advice givers) do not know is that metabolic imbalances alter hydration dynamics at the cellular membrane. Those people who have edema (water retention in the tissues) also have cellular dehydration. This interstitial fluid is often seen in brain scans of people with dementia, which is interpreted as a sign of brain atrophy rather than brain dehydration. Hyper-metabolic individuals will appear to have dry tissues, but underneath can have hyper-hydrated cells.
I'm sure that some of my other posts could be considered answers for this question. I hope this has been a useful (and interesting) set of answers.