Intake of certain dietary fats decreases toenail arsenic levels. However, the presence of long chain n-3 fatty acids in the diet corresponds with higher arsenic concentration in toenails.

Arsenic strongly binds to a protein present in nails and hair called keratin. Hence, toenail arsenic levels are considered as strong indicators of dietary arsenic. The current study investigated the relationship between toenail arsenic and dietary factors like folic acid, vitamin B12, protein and fats.
The study population, comprising of 920 individuals, consisted of bladder and skin cancer patients, aged 25 to 74 years, from New Hampshire. The household usage of well groundwater in this population was analyzed to assess their exposure to arsenic through water consumption. Data for intake of each food item was recorded by means of a food survey.
The data analysis revealed two significant findings. One, toenail arsenic levels were inversely proportional to dietary vitamin B12, animal and vegetable fats and animal protein. Secondly, high toenail arsenic levels accompanied higher intake of long chain n-3 fatty acids. These fatty acids are usually found in fish; it is noteworthy that fish is also rich in arsenic content. Toenail arsenic levels were also found to be associated with alcohol intake.
The researchers concluded that some dietary lipids and other factors decrease toenail arsenic concentration, while long chain n-3 fatty acids intake is directly proportional to increase in toenail arsenic concentration.
Reference: Associations between toenail arsenic concentration and dietary factors in a New Hampshire population Joann et al; Nutrition Journal 2012