Table 1.

Key characteristics of the interventions assessed.

Study Design	Was the Dose–Response Gradient Measured?			Sum
	No	Yes	Not Specified
1. Experimental research	52	16		68
1.1. Randomized controlled trial	26	14		40
1.2. Challenge trial	1			1
2. Observational research	3			3
3. Narrative review	3		11	14