Diet drinks matched water across two years of weight loss in a Liverpool trial

Every participant was already in a behavioral weight-management program ^[2]. The variable under test was what went in the glass: water for one arm, common diet drinks for the other ^[3]. Because the drink was assigned at random, any gap between the arms could be credited to the drink itself.

The trial was designed around one worry in particular ^[15]. One hypothesis holds that sweetness without calories weakens the body's link between sweet taste and energy. In that account, the body releases insulin by reflex and appetite rises enough to cause overeating ^[9]. SWITCH found no evidence that the diet drinks affected appetite regulation ^[7]. "These latest findings show that concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomised trial," said Joanne Harrold, the study's first author and dean of psychology at the University of Liverpool ^[12].

Using water as the comparison limits what the result can say. According to the authors, earlier randomized trials found diet drinks beneficial for body weight when set against sugar-sweetened drinks and neutral when set against water ^[10]. SWITCH extends the water side to two years ^[2]. With no sugary-drink arm, it cannot estimate how much a regular soda drinker gains by switching.

The Discover account does not report the kilograms lost in each arm, the size of the sugar-intake difference, or whether the trial was designed to test equivalence. That last point decides how strong a claim "similar" is. An equivalence trial fixes in advance how small a difference counts as none. A trial that finds no significant difference has shown only that it did not detect one. With close to 500 people split across two arms ^[2], a modest difference in weight loss could fall below what the study can resolve.

I think the trial is good evidence on the weight-control half of the WHO's advice against non-sugar sweeteners ^[8], for people like those enrolled. The authors say the research pointing to harm from these drinks comes mostly from short-term studies ^[11]. "This study provides the long-term clinical evidence that has been missing from international discussions," said co-author Jason Halford of the universities of Leeds and Liverpool ^[14]. "We hope this evidence informs future WHO guidance and public health policy," Harrold said ^[13].

The other half of that advice concerns lowering the risk of type 2 diabetes and cardiovascular disease ^[8]. Numerous metabolic biomarkers improved in both groups ^[16]. Blood pressure, cholesterol and lab values tracked for two years in about 500 people ^[2] are risk factors, though. Showing that diet drinks prevent or cause diabetes or heart attacks would take a far larger and longer trial.