Nutrition While Training for Marathon

For you, does it change much during training? Do you adjust your diet much during your training to take in more calories and energy?

First, I am asking this with this in mind: this year I increased my triathlon training a lot, competing in more and longer races. I lost about 13 pounds since a year ago- at this point I don't think it would be too healthy to drop any more (5'7, 112 lbs).

I am training for a marathon on Oct 10th now and have been finding myself tired more often than usual, feeling very spent, nauseous at times and just an overall a lack of energy. I get 7-8 hours of sleep a night, so I suspect it could be nutrition-related. I've been told I don't take in enough calories throughout the day, but I feel full very easily and feel very uncomfortable when full- so it becomes a bit of a problem trying to eat more than usual. On a typical day, I basically eat:

Small bowl of irish oatmeal, one walnut, 1 tsp flax, 1/3 banana and 1 strawberry

1 banana

1/2 wheat pita with about 3 slices turkey, lettuce, tomato

apple or some other fruit

4-6 oz chicken breast with a vegetable and/or rice/quinoa

1 yogurt with 1/3 banana, kashi cereal, berries of some sort


I've also been making sure I get in some natural peanut butter- I probably have it every other or every day now that I'm getting in more miles- for healthy fats and protein.

Anyhow- any thoughts or similar experiences? Thanks for your input.