The medical profession and national health authorities often do not have sufficient information about the effects of various treatments on people's health, because new diseases arise, formal research on long-term effects takes a long time to plan and conduct, and studies that explore different research questions often appear to yield contradictory findings.
Some of the controversy is due to inferring causality from associative links between an illness and a factor of interest. Another factor is the dependence of journalists upon unbiased interpretation from experts, which is not always available.
Consequently the public believes it has no reliable source of advice on these topics. When a particular health topic is popular, public attitude and behaviour is swayed violently by each new possibly relevant study that is reported, reinforcing and debunking fads overnight.
Jogging has been a long-term craze, but may or may not be beneficial. A diet containing a maximum of 30% fat is recommended, but recent findings do not show that this mitigates heart disease. Obesity is linked to diabetes and hypertension. Some studies show premature death from these causes in obese people, but others show elderly obese people surviving at rates similar to their thinner counterparts. Low salt diets have been recommended for years, but there is no evidence that they benefit people with normal blood pressure, and very little evidence that they help reduce high blood pressure.
In the case of obesity, the controversial results may be due to medicalizing disapproved behaviour. Thus, a socially or politically unpopular condition is given possibly unwarranted medical status as a medical disorder, and the results from studies of it are interpreted in light of the bias of the researchers.