These three prominent conspiracy theories have changed the way that we think about health.

1. Vaccines cause autism.

Vaccine conspiracy theories have been around nearly as long as vaccines. But the purported link between vaccinations and autism first appeared in the 1990s, when a British doctor named Andrew Wakefield published a study that assessed the cases of 12 children who had received the measles-mumps-rubella (MMR) vaccine.

The study suggested evidence of a link between the MMR vaccine and autism symptoms. However, not only did it fail to prove a causal relationship — that is, there was no evidence to rule out other potential causes of autism-related symptoms — it was followed by numerous studies which failed to find any link between MMR and autism. The study’s co-authors retracted the paper, as did the publication. Wakefield and his co-authors were later exposed for fraud.

Today, a number of prominent medical organizations, public health officials, and scientists have spoken out against the dangers of the anti-vaccination movement. But the anti-vaxxers just refuse to give it up.

2. Fat is bad for you.

We've been waging war against fat since the middle of the 20th century when heart disease rates were rising and researchers were struggling to get to the bottom the epidemic. However, when a physiologist by the name of John Yudkin pointed out that sugar might be the cause, his hypothesis was ridiculed by those the food industry, along with several prominent nutritionists. Meanwhile, the sugar industry started to take an unusual interest in the studies they were funding, even going so far as to contact researchers about the results.

For decades, food products and health guidelines suggested that dietary fat was responsible for conditions such as obesity, stroke, high cholesterol, coronary heart disease, and a number of other chronic illnesses. Low-fat and fat-free labels graced everything from salad dressing to crackers, while saturated fats were in the doghouse and trans fats were outlawed altogether.

Today, public scrutiny has shifted towards sugar. After years of being ignored, sugar is the latest culprit in the fight against heart disease and its related conditions. And while sugar isn’t all bad, there’s compelling evidence to suggest that eating too much added sugar can significantly increase your risk of dying from heart disease.

3. Can exercise actually help you lose weight?

For a long time, diet plus exercise was the accepted solution to losing weight. While fad diets come and go, most health professionals recognized the importance of a balanced diet and regular exercise. The idea that exercise helps trigger weight loss is so accepted that few would think to question it.

But a trio of British doctors recently did, reopening the debate around the root causes of the obesity epidemic. In an article published in the British Medical Journal, the authors suggested that the link between physical activity and weight loss has been overemphasized in the past three decades.

According to the authors, the truth of the matter is that while exercise is an excellent way to reduce your risk of developing a number of chronic health conditions — including heart disease and dementia — it doesn’t “promote” weight loss. In other words: “You cannot outrun a bad diet,” the authors write.

A number of other prominent health researchers were quick to point out the flaws in the authors’ argument. For one, exercise has proven benefits. Most experts would agree that there’s no question that staying active is part of a healthy lifestyle. Does it contribute to weight loss? It looks like the jury is still out on that one.

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