Polyunsaturated fats (found in most vegetable oils) causes obesity in rats. Lachowicz et al. (2009) has tested groups of rats, feeding them each with a different type of fat. One fat was a polyunsaturated fat, soybean oil, a monounsaturated fat, and a saturated fat, palm oil.
The study has found that rats have different body weights and metabolic rate. The rats which were fed a polyunsaturated fat (soybean oil) weighed the heaviest and had the slowest metabolic rate. In contrast, rats which were fed a saturated fat (palm oil) weighted the lightest and had the highest rate of metabolism.
In another study by Ailhaud et al. (2006):
Just 10% of the diet as corn oil (roughly 20% of calories), with no added omega-3, on top of an otherwise poor laboratory diet, caused:
Obesity Osteoporosis The replacement of bone marrow with fat cells Diabetes Insulin resistance Generalized inflammationElevated liver weight (possibly indicating fatty liver)
Stephan Guyenet has summarized that:
I found a well-controlled study in which investigators put rats on three different isocaloric, high-fat diets. Each one contained an identical amount of total fat, protein, carbohydrate, omega-3 alpha-linolenic acid (from flax oil), and variable amounts of omega-6 linoleic acid:
All rats gained weight on the high-fat diet, but their body fat composition differed. Fat tissue in the tallow group was 10.3% linoleic acid, 15.2% in the olive oil group and 54.5% in the safflower group. Relative to the tallow group, rats in the olive oil group saw a 7.5% increase in total body weight, and the safflower group saw a 12.3% increase. The latter is the equivalent of an average American gaining 21 lbs (9.5 kg). All rats were eating the exact same number of calories. The most straightforward explanation is that the rats' metabolism was slowed in direct proportion to the linoleic acid content of the diet.
- 59% of calories from beef tallow, for a total n-6 of 4.4% of calories.
- 59% of calories from olive oil, for a total n-6 of 7.7% of calories.
- 59% of calories from safflower oil, for a total n-6 of 36.6% of calories.
- Lachowicz, K., Koszela-Piotrowska, I., & Rosołowska-Huszcz, D. (2009). Dietary fat type and level affect thyroid hormone plasma concentrations in rats. Journal of Animal and Feed Sciences, 18(3), 541-550.
- Ailhaud, G., Massiera, F., Weill, P., Legrand, P., Alessandri, J. M., & Guesnet, P. (2006). Temporal changes in dietary fats: Role of n−6 polyunsaturated fatty acids in excessive adipose tissue development and relationship to obesity. Progress in lipid research, 45(3), 203-236.
- Vegetable Oil and Weight Gain