Nineteen studies were included in review: eight assessed as high quality and 11 as medium quality. Nine were described as using case control designs, four as using case cohort designs, four as using cohort designs and two as using intervention designs. Sample sizes appeared to range from 37 participants to around 480,000 participants.
Four studies indicated no statistically significant difference between higher and lower intake groups for gastric cancer risk. Three out of nine studies indicated a statistically significant association between garlic intake and colon/rectal cancer risk. One out of four studies indicated reduced prostate cancer risk with increased garlic intake. Three studies indicated no association between garlic intake and breast cancer risk. Two studies indicated no association between garlic intake and lung cancer risk.
Only one case-control study was used to assess the relationship between garlic consumption and the risk of the the following cancer types: larynx, oesophageal, oral, ovary, renal cell and endometrial. This reported statistically significant positive associations that favoured high garlic consumption for oesophageal, oral, ovarian, larynx and renal cancer risks, but not endometrial cancer.