Why “There’s No Evidence Against It” Isn’t a Good Defence of Fad or Extreme Diets
When it comes to fad and extreme diets, one of the common arguments you'll hear from there advocates is "Well, there’s no scientific evidence to say this diet is harmful." or, "The scientific community hasn’t studied it properly, so we can’t say it’s bad." Or even the more blunt “show me the RCT to prove it’s harmful/doesn’t work.”
It might sound logical, but it’s a misunderstanding of how scientific research is conducted and, more importantly, of how we use scientific evidence to make informed decisions about health.
Why Don’t We Have Good Science on Fad Diets?
It's true that there often isn’t much (or any) high-quality, long-term research specifically evaluating the effects of the latest diet trend. But that absence of evidence isn’t accidental—it’s a result of how science is conducted, what we prioritize, and what we can’t do for ethical reasons.
Fad diets tend to rise and fall quickly. They gain popularity on social media, get picked up by influencers or wellness personalities, and then vanish when the next trend rolls in. Scientific research, on the other hand, moves slowly and deliberately. It takes time to design ethical studies, recruit participants, monitor outcomes, analyze data, and go through peer review. By the time we finished studying one fad diet, public attention would have already moved on.
Time, funding, and human resources are limited. So, researchers have to be selective. We simply can’t study everything. Research priorities go toward questions that are likely to have a meaningful impact on public health. Fad diets often don’t offer us biggest bang for buck when it comes to figuring out which food and nutrition choices best support human health and wellbeing.
Ethical Boundaries in Nutrition Research
Even more importantly: we can’t ethically study diets that we already suspect could cause harm.
If a diet is extremely low in calories, cuts out entire food groups, promotes unproven supplements, or encourages disordered eating behaviours, as researchers can’t in good conscience ask people to follow it. Ethics committees (who oversee all official research) would (rightly) block that kind of study from even starting. Just like we wouldn’t test dangerous medications on people “just to see what happens,” we don’t run nutrition trials that could put participants at serious risk for nutrient deficiencies or excesses, which could cause harm to health in both the long and short terms.
So, by not testing fad and extreme diets on people we (the scientists) are being responsible. Fad diet sales people and extreme diet advocates don’t have the same considerations or oversight. They can make their own choices, and encourage their followers to follow particular diets too, because they don’t have the same checks and balances making sure they don’t do harm.
So How Do We Make Recommendations?
Fortunately, the absence of direct evidence doesn’t leave us completely in the dark. Science isn’t about testing every possibility directly and in isolation—it’s about putting together a body of knowledge from different kinds of research.
We have decades of high-quality evidence on what constitutes a healthy diet. We understand the roles of nutrients, food groups, metabolism, and eating patterns. From this foundation, we can look at a fad diet and say, “This approach goes against what we know works,” or “This cuts out foods we know are protective,” or “This promotes patterns we know are risky.” That is evidence-based reasoning, it just doesn’t come in the neat package of a randomised controlled trial on every new diet that hits TikTok.
Bottom Line: Absence of Evidence ≠ Evidence of Absence
Just because something hasn’t been studied in depth doesn’t mean it’s safe, harmless, or advisable. In science, and especially in health, we don’t wait until harm is proven before we speak up. We look at what we do know, apply it responsibly, and try to protect people from harm, especially when the stakes are as high as they often are with extreme diets.
So, the next time someone argues that you can’t speak up against a fad or extreme diet because “there’s no direct evidence against it,” remember: the lack of direct studies isn’t a loophole. It’s a red flag. And it’s okay (essential, even) for experts to speak out using the full picture of the evidence, as we have it.
