Calorie Slashing and Overexertion Can
Stress the Immune System
Week after week of little sleep and extreme overexertion can put even a
young, physically fit body at higher risk of infection when that body isn't
getting enough calories to replace those it burns each day.
That's what ARS' Tim R. Kramer found when the U.S. Army asked him to look
into why men in a Ranger training class at Fort Benning, Georgia, were
developing infections uncharacteristic of healthy, young adults.
"The men are deliberately sleep deprived and food deprived, to see who
can set aside personal needs to complete a mission," explains Major Karl
E. Friedl, an Army physiologist at Fort Detrick, Maryland. "But medical
researchers were called in after a class was decimated by strep
pneumonia," he explains.
So Kramer tested blood samples of men in two Ranger training classes and one
class of aspirants to the Special Forces as well as a fourth group of new
inductees--all womenduring basic training. He was looking for differences
in immune response under different exercise and diet regimes.
All three groups of men burned about 4,000 calories daily during their
rigorous training, but their intakes differed markedly. So did the ability of
their T-cells to divide and conquer invading pathogens, Kramer says. T-cells
are the player coaches of the immune system: they disable pathogens themselves
and signal for help when it's needed.
Men in the first class of Rangers tested had consumed 1,300 fewer calories
than they burned each day, and they had a 50- to 60-percent drop in T-cell
response, says Kramer. So the next class was given more food but was still 900
calories shy of what they burned.
That helped somewhat. This group had a 30-percent drop in T-cell response.
Special Forces aspirants in training at Fort Bragg, North Carolina, got even
more to eatjust 250 calories less than they burnedand had even less
of a drop in T-cell response20 percent.
"The reduction in T-cell function due to hard physical stress seems to
be worsened with calorie deficiency," says Kramer. "The bigger the
deficit, the more it compromised immune response."
On the other hand, T-cell response improved 150 to 200 percent in the women
recruits during basic training at Fort Jackson, South Carolina. They had gotten
all the calories they needed, along with the military regimen. A little out of
shape to begin with, he says, the recruits got daily exercise, regular meals,
and no cigarettes.
Kramer measured an indicator of physical stressinterleukin 6
(IL-6)in all four groups. He says the women began with typical levels;
the men began with very high levels indicative of their high level of physical
activity. "It increased somewhat in the men and then started downhill.
Their bodies were saying. 'We can't keep up with the physical demands.' Such
demands would also tend to reduce their defense against disease."
By the end of 8 weeks, says Friedl, the Rangers were down to 4 or 5 percent
body fat. And they had lost an average 7 percent of their lean massmostly
A month later, he continues, "everything was back to normal, except
they had put on excess body fata typical reaction to extreme calorie
deficits. And that was back to normal after 6 months. There is no evidence of
any lasting health problems."
So, as the old axiom says, too much of a good thingin this case,
cutting calories and increasing exercisecan hurt you. At least,
By Judy McBride, ARS.
Human Nutrition Research Center, Building 307-C, Room 117, 10300 Baltimore
Ave., Beltsville, MD 20705-2350; phone (301) 504-8157, fax (301) 504-9381.
"Calorie Slashing and Overexertion Can Stress
the Immune System" was published in the
January 1995 issue
of Agricultural Research magazine.