While nursing, you should not consume less than 1500-1800 calories per day. Some mothers will require much more than this, but studies show that going below this number may put supply at risk. In personal experience, many mothers hold on to weight while they are breastfeeding.
Peppermint is one of the herbs most commonly associated with a decrease in breast milk. Women even use them to help dry up the milk during weaning. Be careful with peppermint when nursing, but don't worry, if you're not consuming large amounts, you will likely be fine.
While there is no scientific evidence that caffeine directly harms your milk supply, caffeine can dehydrate your body. Dehydration can cause a decrease in milk supply. Proceed with caution, keep an eye on your caffeine consumption and milk supply and stay hydrated!