I have definitely felt "under the weather" (fatigue, elevated body temp, etc.) after a high carb day (especially if my average intake of carbs has been low).

I always attributed this to the impact of sugar on my immune system. I have also only felt this after eating a lot of sucrose (birthday cake, cookies, etc.) vs eating a lot of starch, even pizza and other refined sources or even fructose (although eating the same quantity of fructose as sucrose is limited as I can cram down a whole lot more cake than apples, bananas, and oranges.)

This seems to be backed up by this article from MarksDailyApple.com...

As to whether sugar directly impacts the immune response, there is evidence that it does play a role: the often-cited 1973 neutrophilic phagocytosis study out of Loma Linda University. Neutrophils are small white blood cells, about 9 or 10 µm in diameter. They’re also the most abundant white blood cell, or leukocyte, in the body. Good thing, too, because they play a crucial role in the defense of the multicellular organism (that’s us). Neutrophilic phagocytosis is the process by which offensive microbes are dispatched by neutrophils.

The Loma Linda study observed the effect of sugar ingestion upon neutrophilic activity. After an overnight fast, subjects were administered oral 100 gram portions of either glucose, fructose, sucrose, honey, or orange juice. Blood was drawn before and after administration of the sugar, then mixed with a shot of staphylococcus epidermidis (a fairly common bacterial strain that can be virulent in compromised immune systems) to determine the neutrophilic phagocytosis response. After ingestion of sugar (but not starch), the phagocytic index (a rough measurement of the neutrophilic response) was significantly decreased, while fasting significantly increased the response. Sugar eating didn’t decrease the number of neutrophils; it simply decreased their responsiveness.

It isn’t exactly clear that sugar and sugar alone exerts a neutrophilic-dampening effect on our immune system. I’m inclined to think that the neutrophils aren’t lying dormant, befuddled and entranced by the fructose. Instead, I’m thinking they’re occupied by the rapid influx of twenty teaspoons of sugar into the body. Let me rephrase that: they’re occupied by the effects of the rapid influx of sugar. To understand what I mean, look at the start of this post. Check out all those negative, inflammatory effects sugar has on our body, and think about how twenty teaspoons of sudden sugar might necessitate an inflammatory response to deal with them all.

(full post here)

It's not flu, more like being in a coma, and having an autoimmune disease, and being bi-polar and being obese. I also have more colds, allergies and sinus problems. Inching up with starchier veggies isn't too much of a problem, except for weight.

Ha no kidding! I actually got a full on flu when I switched to high carb. Spent 5 days at 100 degree temperatures... but then got radically better and made great health improvements. I think my immune system needed the glucose after low carbing/being active for so long.

I'd imagine your going to need to shift your metabolic machinery and there is going to be a lag with a major change in either direction.

My personal experience doesn't count since my high carb days consist of the three P's ...pizza, pasta, and pastries :) ....but I don't really feel "horrible" even then. Just a little less productive for that day.

Yes, for me it's a longstanding issue and the reason I don't eat high carb*. My blood sugar/energy levels go haywire if I go over 200g in a day without tons of exercise (like a full day of rafting or heavy yard work, etc).

I have a carb 'sweet spot' of 100-200g. If I go below that I get hypoglycemia issues, if I go above, reactive hypoglycemia symptoms. 2 years of eating whole foods 'paleo', eating various macro-nutrient proportions, replacing grains with tubers/roots/fruit (and white rice) hasn't changed this basic fact, but wheat products give me reactive hypoglycemia problems like nothing else even in moderation, so eliminating gluten has helped a ton.

*note: I eat a lot of food, so while 200g might be considered 'high-carb' for someone on 1500 calories per day, 800 cals is not even a third of my usual daily intake... and I do not get 200g of carbs every single day.