During the study, the team led by Dr Guenther Boden took fat biopsies from the upper thighs of six lean and six obese patients and found significant differences at the cellular level.
"The fat cells we found in our obese patients were deficient in several areas," said the researchers.
"They showed significant stress on the endoplasmic reticulum, and the tissue itself was more inflamed than in our lean patients," they added.
Endoplasmic reticulum (ER) is found in every cell and helps synthesize proteins.
According to the researchers, the stress causes ER in fat cells to produce several proteins that eventually lead to insulin resistance, which has been found to play a major role in the development and progression of obesity-related conditions.
The study is published in the September issue of Diabetes.