Adipose tissue expansion and the development of obesity: influence of dietary fat type.

Recent studies indicate that the prevalence of obesity in adults has increased by 30% or more in the past decade, with increases in both genders and in all ethnic and racial populations and age groups. Obesity is associated with many chronic diseases and alterations in physiologic function including cardiovascular disease, hypertension, diabetes mellitus, gallbladder disease and certain types of cancer. Much attention regarding dietary influences on obesity development or prevention has focused on high fat diets. Many studies have confirmed that high fat feeding leads to an expansion of adipose tissue mass through an increase in fat cell size and/or number and to the subsequent development of obesity. However, there is little definitive information on the effect of type of dietary fat, especially palm oil, on adipose tissue cellularity and the development of obesity. These studies were designed to determine whether dietary fat of different sources vary in their ability to produce obesity and to begin to elucidate the mechanism by which such divergence occurs. Male Osborne-Mendel rats were fed either a low fat (15% calories) or one of three high fat diets (65% calories) for 12 weeks. The predominant fat source in the high fat diets was either soybean oil, tallow, or palm-olein (a fraction of palm oil). Final body weight was not influenced by fat level or type; however, percent carcass lipid and fat pad weight were higher in soybean oil and tallow fed rats than in low fat and palm-olein fed rats. Fat pad specific increases in cell size and cell number were observed for tallow and soybean oil fed compared to low fat and palm-olein fed rats. Serum triglycerides were higher in the tallow and palm-olein fed rats compared to low fat fed rats; no significant effects of dietary fat type on serum cholesterol were observed. These results indicate that palm-olein, unlike tallow and soybean oil, were comparable to a low fat diet concerning fat pad weight, body composition and adipose tissue cellularity when fed for twelve weeks as 65% of energy intake. The lower fat storage in the palm-olein fed rats is perhaps associated with a slower rate of triglyceride uptake and/ or a reduced fat cell proliferative capacity. The influence of dietary fat type on the proliferative capacity of the pre-adipocytes and on the production of a local or systemic adipogenic factor is being determined in subsequent studies.