Trials comparing named diets against each other repeatedly find smaller differences between them than between individuals following the same one. The explanation lies in what the approaches share rather than in what distinguishes them.
Every approach restricts something
Whatever the stated rationale, each popular diet rules out a portion of what a person previously ate, whether by food group, by timing or by an explicit limit.
Removing options reduces variety, and reduced variety reliably reduces the amount eaten, because habituation to available foods occurs faster.
The mechanism is therefore shared even where the reasoning differs, which is why approaches built on opposing theories can produce comparable early results.
Early change is dominated by water
Carbohydrate is stored in muscle and liver bound to a substantial quantity of water. Reducing carbohydrate depletes those stores and the associated water leaves with them.
This produces a rapid fall on the scale in the first week that has little to do with fat tissue.
Approaches that cut carbohydrate sharply therefore appear dramatically more effective at the start, and the difference narrows considerably once stores stabilise.
Attention changes behaviour independently
Starting any structured plan involves recording, planning and paying attention to food in a way most people were not doing before.
Self-monitoring alters intake on its own, which is why control groups asked simply to keep a food diary often show changes.
Because this effect accompanies every approach equally, it inflates all of them in the early weeks and fades at a similar rate.
Divergence appears later and reflects adherence
Studies with longer follow-up find that results within any given diet spread widely, with some participants maintaining change and others returning to baseline.
What separates those groups is whether the pattern remained compatible with their circumstances, preferences and household, rather than which theory it was built on.
This is the basis for the finding that the best-performing approach for an individual is generally the one they can sustain.
Why comparative trials are hard to interpret
Participants in dietary trials drift towards each other over time, with intervention groups relaxing and control groups tightening.
By the end of a long study the groups may differ far less than assigned, which compresses any genuine difference between the diets themselves.
Anyone with a medical condition affecting how they should eat needs individual advice from a clinician or dietitian rather than a general comparison.