Fatness boosts the level of inflammatory activity in the body while being physically fit decreases it, indicates research published ahead of print in the British Journal of Sports Medicine.
The findings are based on a representative sample of 452 healthy men, who were taking part in the Aerobics Center Longitudinal Study.
This looked at the levels of different groups of white blood cells during exercise, to see if these bore any relation to measures of body fat and physical fitness.
A high total white blood cell count is a marker of inflammatory activity. And it is recognised as a strong and independent risk factor for illness and death from coronary heart disease.
Total white cell count includes the following groups of cells: neutrophils; lymphocytes; monocytes; basophils; and eosinophils.
None of the study participants had been diagnosed with any serious disease or long term condition, and none was a smoker, factors known to be associated with a higher white blood cell count.
Participants were weighed and measured, and their blood pressure was taken, along with a sample of blood. Their level of cardiorespiratory fitness was then assessed, using a standard treadmill test.
After taking account of age, the results showed that all groups of white blood cells were lowest in the men who were most physically fit.
And the greater proportion of body fat a man had, the higher was his white blood cell count.
But the total white cell count and levels of neutrophils, lymphocytes, and basophils were higher in the men who had a combination of higher body fat and lower levels of physical fitness.
They were also higher among men with lower body weight but lower levels of fitness. And, interestingly, a high degree of physical fitness negated the effect of extra body fat.
White cell counts tend to rise after a bout of vigorous exercise, but regular exercise may condition the body to respond more efficiently to the physical demands made of it, say the authors.