Table 1.

Summary of the main clinical studies that have directly or indirectly investigated the relationship between vitamin A and Parkinson’s disease

Design	Factors quantified	sample size	Method	SNc changes*	Main outcome(s)	Study
Cased-control study	Dietary factors in PD	103 sporadic PD, 103 control cases	Food frequency questionnaire		No association between consumption of vitamin A-containing foods and PD.	[58]
Cased-control study	Serum levels of retinoids in a PD case-control study	61 sporadic PD, 61 control cases	Whole blood collected		No association between carotenoids / retinoids serum levels and PD.	[62]
Cased-control study	Dietary factors in PD	144 sporadic PD, 432 control cases	Food frequency questionnaire		No association between consumption of vitamin A-containing foods and PD.	[63]
Cased-control study	Serum levels of retinoids in a PD case-control study	104 sporadic PD, 52 control cases	Venous blood collected		Serum level of β-carotene (but not retinol) is lower for PD patients compared to control, and is also lower in early PD group than in advanced PD group.	[67]
Cased-control study	Reduction of ALDH1A1 expression in DA neurons	13 sporadic PD, 14 control cases	Brain SNc sections	ALDH1A1 ↘	ALDH1 mRNA is specifically and markedly down-regulated in DA neurons of SNc in PD.	[71]
Cased-control study	ALDH1A1 as a peripheral biomarker for diagnosing sporadic PD	22 sporadic PD, 33 control cases	Whole blood collected		ALD1A1 is a biomarker with high sensitivity and specificity to identify the risk of developping PD.	[72]
Cased-control study	Reduction of ALDH1A1 expression in DA neurons	10 sporadic PD, 9 control cases	Brain SNc sections	ALDH1A1 ↘ DA neurons ↘	The clustering of ALDH1A1-positive and -negative DA neurons in human SNc is different between PD and control cases. A reduction of ALDH1A1 may render DA neurons more prone to degenerate.	[73]
Cased-control study	Serum levels of retinoids in a PD case-control study	41 sporadic PD, 41 control cases	Whole blood collected		No significant changes of retinoids and carotenoids serum levels in PD patients.	[207]
Prospective study	Risk factors for PD	13,979 people at start; 395 incident PD and 2,320 control cases, 17 years later	Food frequency questionnaire		No significant association between vitamin A intake and PD.	[64]
Prospective study	Link between dietary antioxidant vitamins and risk of PD	371 incident PD in 124,221 population cohort	Food frequency questionnaire		High intakes of carotenoids do not appear to reduce the risk of PD.	[66]
Prospective study	Link between dietary antioxidant vitamins and risk of PD	1,329 incident PD in 84,774 population cohort	Food frequency questionnaire		Dietary intake of β-carotene is associated with a lower risk of PD.	[68]

^*compared to control cases.