When I drink coffee, 99% of the time, it's decaf. My understanding is that caffeine from coffee hinders the ability of the immune system to function.
UPDATE: Based on speno's question below, I'll include a few sources. My apologies. I should have done this initially.
Basically, in layman's terms, I need both rest and magnesium (among other nutrients) for my body to function right (ie. process food and deal with stress and other stressors such as exercise and environmental attacks). The caffeine from coffee causes my body to have less of both.
From the article, Enhancing Your Immune System the Natural Way, comes this quote:
Caffeine is a diuretic that
contributes to the body's loss of
important nutrients, such as calcium,
magnesium, and potassium. Caffeine
places stress on the adrenal glands
(already stressed out from our hectic
lifestyles) and adversely affects the
nervous system, resulting in anxiety,
hyperactivity, and insomnia.
Healing occurs when the body is
relaxed... Regular consumption
of caffeine deprives the body of this
relaxed state. The acid in coffee eats
away the villi of the small intestine,
reducing their effectiveness in
supporting nutrient assimilation. Thus
the acids in coffee may cause as much
problem as the caffeine.
This article, listed on paleodiet.com gives a little more insight into the calcium-leeching:
Coffee reduces inositol levels in the blood. Inositol is a regulating
factor in calcium metabolism. It inhibits formation of the cells
(osteoclasts) that draw calcium from the bones into the bloodstream (14).
It also exerts a direct influence on transport of calcium into cells
Teeccino, on their website, list a number of articles that cite the relationship between caffiene comsumption and stress and the negative effect of their combination on the body's health.
From the article, Caffeine: The Socially Accepted Killer, comes this quote:
Excessive caffeine intake overworks
the glandular system and can quickly
deplete the body of vitamins B, C,
magnesium, and several micro
nutrients, according to nutritional
psychologist Marc David MA (David,
There are lots of references at the bottom of this one.
From the article, Caffeine and the Adrenal Glands, comes this quote:
...caffeine keeps the cortisol levels
high preventing the body from resting
when it should and preventing the
adrenal glands from operating as they
Basically, if I want my body operating optimally, I will try to throw as few stressors at it as I can.
That said, perhaps my body is more sensitive to caffeine than others' bodies.
ANOTHER UPDATE (05/06/11): "That Paleo Guy" just did a blog post that essentially says that coffee can contribute to insulin resistance and glucose tolerance. It also increases cortisol levels. Check it here.