1. Go to the doctor.
2. Why are you lighter? Did you intentionally try to lose weight, or did it just happen? If it just happened, I'll repeat point number 1, in bold.

I'm 5'11 and 48 years old, and I lost a whole bunch of weight last year (on purpose) and got down to 146 pounds. I got much faster both on bike and run and felt like I had more energy, so I assumed everything was good, particularly since I was still well in the normal range of BMI. My period stopped coming, but I chalked that up to my age since I've been in peri-menopause for several years. I ran a marathon in early December and started my off season, during which I gained about 7 pounds. Several things happened then: my running paces dropped significantly (e.g., ran a marathon in April a full 30 seconds per mile faster than the one in December in extremely similar conditions, all of my training runs were faster/easier, etc.), my period came back regularly, I felt better and had more energy. So even though I felt like I looked fine and my weight was "normal," apparently 146 was too light for me. Pretty much cruising happily now in the mid 150s. So for me, FWIW, I think I had negative effects from being "too thin."

I'm 47, 5'9 and race at ~140lbs. I've never felt better than I have since getting down to 140-145lbs year round. Just my n=1. Hope you get it figured out.

I wish I had the problem of "being too thin"....I have the problem of "I want that entire pizza in my mouth right now" and then a mental battle on why I wont do it and then a following mental battle on how I have no self control as I am doing it

I'm 60, 5' 8" and currently between 144-147 just like you ... 37 days out from my next IM. That's only about 5 lbs under my off-season target and the only effects is that I tend to get cold at night with the ceiling fan. I'm buried under all the covers and my wife is on top of them complaining about the heat. Other than that, my energy is up and I'm running better.

I would think your weight may be partly responsible for your issue. Race weight isn't supposed to be a weight you stay at year round although I know some folks disagree with that. But it could also be a function of how you got the weight down. E.g., dropping food intake but not getting the right, quality food; or perhaps losing weight from increased exercise volume but not getting enough rest. I would concentrate on eating better, taking a high quality multi-vitamin as insurance, and focus on getting a little more rest to see if that works.

One other thought, I started adding a low dose of iron (pill) to my diet a couple of years ago when I was starting to feel run down. Within a couple of weeks I felt much, much better.

Yep, go see your doctor. Maybe double it up with your annual physical...as a "hey, I've been sick several times this year. Is there something we should look at?" The doc might order an extra test or something (as part of your standard labs), if (s)he thinks there's a legit concern.

I'm 49, male, 5' 11", 152, probably about 13%bf right now. Race weight is 147-149, around 10%bf at that weight. I don't typically have any "issues" like you describe. But, bad luck can sometimes strike.

That said...heaving training can weaken your immune system. So, its not uncommon to be somewhat prone to illness during periods of high TSS. I try and be a little extra sanitary during heavy build periods...washing hands a little more often, maybe a little sanitizer. Paying attention to "sick" people in public. Not brushing my face with my hands. And I'm downright paranoid in the 2 weeks leading up to a race!

There have been periods where I've tried to lose weight too quickly and felt like you're describing. For me, it's actually the result of not taking in enough calories, vs. being too lean. I'm 6'0" 158 lbs and feel great at the moment but I've felt like shit at 165 lbs if my caloric deficit is greater than 500 cal/day. Do you track what you're eating vs. how many calories your burning? It might be worthwhile to do that for a while. Also, see if there are places near you that do DEXA scans so you can get an accurate BF, lean mass and bone density measurement. In the short term, try upping the calories for a few days and see if the lethargy goes away.

It's not about calories, but quality nutrients, from minimally processed protein/fats, which hormones are made from. So focus and getting more nutrients in, rather than what likely can be empty calories

Respectfully, I think it's both. I think eating quality food almost goes without saying, and I wasn't suggesting adding 10 twinkies to the daily diet, but my personal experience is that it is definitely possible to eat quality food and still underfuel to the point that I experience a lot of the symptoms that the OP is describing.

I think it’s largely about calories consumed, your energy levels wont be high if you aren’t replacing what you’re burning.

Guess you did not read my post. You can google " calorie is not a calorie" articles. A 5000 cal diet of crisco and gelatin won't give energy but rather make you fatter, tired and slower as the body cannot metabolize this for structure or energy

That is very true. One needs adequate micronutrients and adequate calories. See a lot of this with female athletes. They think its okay they are getting too thin because they are eating "good food". Most of them are (the too thin ones) but if they would just take in some high density calories.

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They constantly try to escape from the darkness outside and within
Dreaming of systems so perfect that no one will need to be good T.S. Eliot

I'd say blood tests are a good idea to get an idea of overall health and RBC and what not. Also make sure you are keeping an idea on recovery vs over training...

Eric Leader
Ellicott City Training

you only have health problems by being this thin if the cause is malnutrition, you arent even that thin, you are just very thin compared to the average american, i was 6 2 and 145 - 150 for years when i was serious into ironman, half im and marathons, i havent been sick in ten years despite training 12 to 20 hours a week, the reason is that i eat a ton of vegetables, you need to eat green leafy vegetable every day and you dont get sick, they are the real medicine, like kale, collard greens, broccoli, spinach, i would get sick 3 to 5 times a year before i started endurance sports up to the age of 30, then i stared eating right, and age 30 to 41 i literally have not been sick since i started eating a lot of kale, its really that easy

I once had a Dr. Say I should just be happy when I went in saying I could not keep my weight up and was spending an inordinate amount of time and money eating. She actually called me a lucky bitch.

After years of weird symptoms including muscle cramping and having to eat two chipotle burritos as a 5’6” 123 pound female one thing I found that really helped was supplementing magnesium. I agree it’s worth checking for anemia too I take iron on and off if I feel tired since I have tested low in iron before.

If you think something is not right push for answers - even if doctors don’t take your concerns seriously. Treating a magnesium deficiency changed my life for the better.