Early, small, conflicting, or animal-only. Not actionable alone.
Finnish men who took 4–7 saunas a week had a 63% lower adjusted risk of sudden cardiac death, a figure resting on 201 men and 10 deaths. Then 20 randomised trials of heat, most of them not Finnish sauna, found almost nothing.
A month-long trial put box breathing against two other breathing techniques and meditation. All three breathing groups got the same small lift after each session, and smaller studies find the square tying with the alternatives more often than beating them.
Cortisol falls in the pooled trials, most of them paid for or supplied by the extract makers. Whether people feel less stressed is contested. Sleep is the best-supported claim, testosterone rises in men but not women, and liver injury is very rare but can be severe.
The trial behind the 2024 headlines was retracted in September 2025, four days after a meta-analysis pooled it, and a year on that meta-analysis still carries no correction. What survives: a real but modest effect on blood sugar after meals, and about 2 lb of weight across twelve short trials, most of it back within a month of stopping in the one trial that checked.
Pooled across 72 measurements, instant oatmeal scored 75 on the glycaemic index and large-flake oats 53. Regulators have signed off on oats for cholesterol, at 3 g of beta-glucan a day, not for weight. On weight, pooled trials of beta-glucan from oats and barley found about 2 lb at best, and the newest, in overweight adults, found none.
The 2023 study linking erythritol to heart attacks came with a real mechanism — more reactive platelets, more clotting in animals. But the cohorts measured erythritol in blood, not diet, and your body makes its own: plasma levels track metabolic disease and fall after weight loss. Marker or cause is genuinely unresolved.
Swapping sugary drinks for diet ones took off 2.3 lb across 17 randomised trials, and swapping for water did nothing. But the Framingham study found daily cola drinkers had 3.7% lower hip bone density — and diet cola looked the same, so switching does not buy you out of that one.
The WHO guideline covers 'all synthetic and naturally occurring' sweeteners and names stevia in the same list as aspartame — natural is written out as an exemption on purpose. And stevia has about a third the randomised trial evidence of aspartame, so the confidence people feel about it is not coming from a deeper evidence base.
The WHO advises against non-sugar sweeteners — as a conditional recommendation, on evidence it grades low certainty, from a harm signal that is observational, in a document that names reverse causation itself. Meanwhile 17 randomised trials found swapping sugary drinks for sweetened ones took off 2.3 lb, while swapping for water did nothing.
The famous 30% metabolism boost works out to about 24 calories, and two later labs could not reproduce it at all. What two randomised trials do support is duller and more useful: a glass before meals, for a couple of pounds over twelve weeks.