It's possible that nutrition is to blame. However, you could also have other things going on in your body that are affecting you. I'm 43 and two years ago, when I was training my hardest and riding my fastest, my period came and the next club ride I did, I started the first half strong, then limped home . . . like you. After that I really couldn't ride worth beans for about a month. Had a lot of fatigue and some dizziness. Thought it was low iron. Upped my iron intake and then started feeling better and actually did my fastest century ever (shouldn't have done it at all in retrospect). The next few months I didn't ride as much, but when 2011 hit, I got serious about training again. However, like you, when I'd do a long distance ride, I'd "bonk" at around 40 miles or so and limp home. In March I somehow managed to do a really strong half century (after giving up the notion of doing the full century). Then my period came and, once again, on the next club ride, I rode the first half strong, then limped home. And that was my last "real" ride for a very, very long time. Again, I had a lot of fatigue and when I stood, I'd get the black out sensation (though never actually passed out). I took a lot of naps and if I rode my bike at all, it was 5 miles, flat, at around 10 mph (or less).

The first thing I discovered I had was adrenal fatigue. Eventually I learned that "all" my hormones were low. My naturopathic doctor said if she didn't know I was still having periods, she'd assume I was in menopause by my test results. I also have subclinical hypothyroidism and candida. All of the things I listed are often interchangeable, meaning when you have one, you may have one or more of the others. I was also low in vitamin D, and just found out I have high levels of mercury and lead poisoning. (Mercury due at least in part to amalgam (aka:silver) fillings, which I'm getting replaced soon). Every one of the things listed can cause fatigue and mercury poisoning can potentially be the culprit of them all (along with other symptoms I have had for years . . . and the periodontitis that took all my upper teach several years ago. I've had so many dentists shake their heads trying to figure out how I got periodontitis. Well mercury poisoning can cause it). I've been doing a natural method of mercury detox for four months, even before actually being tested, cause I knew it was a likely culprit.

I have never felt 100% healthy. Since my 20's I have felt like I should have a lot more energy than I do. I think some of my issues have been underlying conditions that didn't completely rear their ugly heads until I got serious about cycling. Pushing my body as much as I did, which was completely normal exertion for a "healthy" person, pushed my body over the edge until I HAD to stop riding and figure out what was wrong.

I'm just now starting to see some progress. Some of my hormones are in the normal range now. My vitamin D is way up, but still not optimal. My TSH (thyroid test) is better (not optimal, but better than it's been). I'm feeling less fatigued and less dizzy. I've been slowly amping up my rides and exertion level.

So . . . like I said, nutrition may be your only issue. I don't know. But what you describe in your rides sounds so much like what I was experiencing and when you said you were 42, it threw up another red flag. You may want to have your hormones tested, vitamin D levels, thyroid, etc.

I don't want to start a debate with anyone (been there on this board with this topic), but I personally, avoid conventional doctors and stick with natural medicine as much as possible, but that's me. Most conventional doctors don't recognize adrenal fatigue as an actual condition because there are no drugs to cure it and the drug industry funds medical schools. Conventional doctors also often read thyroid results differently than natural ones, meaning, one doctor may think your thyroid is fine, while the other realizes it may at the least need some "help". I am also not a fan of taking drugs, so I've been on herbal tinctures to help balance hormones and other natural supplements.

Anyway, I can go on all day really, so I"ll stop right here. Feel free to PM me if you want to ask me anything. I do hope you only issue is nutrition, but wanted you to be aware of these other possibilities.

Take care!