Exercise is known to higher levels of good cholesterol and is thought to lower levels of bad cholesterol in the blood.
The finding in the study was that LDL cholesterol levels in thin people who did not exercise were almost identical to the levels found in obese people who did not exercise.
Heart disease risk factors in 37 lean exercisers, 46 lean non-exercisers and 28 obese non-exercisers was analysed.
The contrast arose when lean exercisers were fit and demonstrated almost optimal levels of LDL cholesterol and lean non-exercisers were unfit and demonstrated borderline high levels of LDL cholesterol at just 40 years of age.
According to Dr Gary O'Donovan, of Brunel University in London, "Many people, especially slim people, believe that the only benefit that can be achieved from exercising is weight loss. This is not the case. Our study suggests that slim people need to exercise as much as others in order to stay healthy and keep LDL cholesterol in check."
International Journal of Obesity features this study.