Answer to Question 1
Thiamin
Answer to Question 2
It is wise to consider taking a supplement:
When food availability and/or variety is limited.
If you decide to avoid certain food groups, e.g. dairy products.
During periods of rapid growth, development, illness, or injury.
As you age, if consuming adequate amounts of a nutrient becomes difficult (e.g., calcium, B12).
If you must struggle to buy the food you like or need.
When you eat a low-calorie weight-loss diet.
When you have a health problem that increases nutrient requirements.