I'm not a food or nutrition expert having never studied it. However, I have had to recognise that food plays an important part of most peoples triathlon story. Afterall logically, if you don't feed the body properly its not going to perform very well.
Women's hormones fluctuate a lot, sometimes during the month, you just want to curl up with a book, a blanket and sit by the fire and be grumpy, fortunately as I am much older (some might say ancient) I'm well past the whole time of the month thing, although the menopause wasn't a Joy either. Some days I could eat for England, other days food holds no appeal at all. We are all different and being too rigid can be just as bad for motivation as not doing anything. Find your happy balance.
Generally the info tells you to eat pulses, fruit, vegetables, fish, meat, plant based stuff followed by a huge list of what not to eat. It tells you to prepare everything from scratch, don't eat refined sugar and so the list goes on. Speaking personally, time is limited so I do cut corners, when you work all day and in many peoples cases have families and other demands on their time, following all of the good advice is not possible. Please don't get me wrong, I'm not saying ignore the advice, if you have time, the money and the energy then go with it.
From a personal perspective I stick to what my Nan used to tell me. That everything in moderation is good for you and every in excess is bad! it seemed reasonable when she said it, I know times have changed, but her general principal is a good one so I try and stick to it. I do prepare food from scratch most days, eating my 5 portions of fruit and vegetables, I eat lentils, nuts and seeds. Just once in a while though, its nice to go out with the Girls or my OH have a Steak or a curry, too much wine and a good giggle.
I'm not sure where I read it, but the advice of "you cannot outrun a bad diet" is true we operate the same as data entry on computers, if you put rubbish in, you will get rubbish out. If you eat rubbish food, high in unhealthy fats and carbs, generally your performance will reflect this in some way.
So my advice would be eat wisely and if you are training for a big event, cut back on the booze.