by Craig Weatherby
While the basic physiology of rodents and humans is very similar, the two species can differ significantly when it comes to weight control.
So it's smart to take weight control experiments in rodents with a grain of salt, so to speak.
But the results of a recent rat study seem to dovetail with a very human experience… that is, binging on burgers and ice cream seems to beget more burger-and-ice cream binging.
Scientists from the University of Texas Southwestern Medical Center suggest that the specific saturated fat called palmitic acid causes rats' cells to ignore any appetite-suppressing signals received via the key weight-regulation hormones called leptin and insulin (Benoit SC et al. 2009).
The study was supported by the National Institute of Diabetes and Digestive and Kidney Diseases.
Findings reported by Dr. Deborah Clegg and her colleagues suggest that palmitic acid can change brain chemistry in a very short period, causing appetite-suppressing signals to be ignored.
“Normally, our body is primed to say when we've had enough, but that doesn't always happen when we're eating something good,” said senior author Dr. Clegg, an assistant professor of internal medicine (UTSW 2009).
According to professor Clegg, “What we've shown in this study is that someone's entire brain chemistry can change in a very short period of time. Since you're not being told by the brain to stop eating, you overeat” (UTSW 2009).
She said that the satiety-blocking effects of palmitic acid lasted about three days in rats, potentially explaining why many people who splurge on Friday or Saturday say they're hungrier than normal on Monday.
These findings do not apply to all saturated fats. For example, most of the saturated fat in cocoa is a healthy fat called stearic acid, which does nto raise cholesterol, and is not associated with appetite-suppressing effects.
In fact, cocoa and dark chocolate appear to suppress appetite.
We should note that grass fed beef has about the same amount of saturated fat as standard beef (and presumably a similar palmitic acid content), but more omega-3s, which should help suppress appetite a bit.
Dietary fats affect subsequent eating behavior
Though scientists have known that eating a high-fat diet can cause insulin resistance, little has been known about the mechanism that triggers this resistance or whether specific types of fat are more likely to cause increased insulin resistance.
Dr. Clegg said she suspected the brain might play a role because it incorporates some of the fat we eat into its structure.
These include the polyunsaturated omega-6 and omega-3 fatty acids in plant foods and fish, the saturated fats that predominate in dairy foods and red meats, or the monounsaturated oleic fatty acid in meat, avocadoes, and olive oil... with each kind of fat exerting different brain effects
Based on this suspicion, her team attempted to isolate the effects of fat on the animals' brains.
The animals received the same amount of calories and fat; only the type of fat differed. The types included saturated palmitic acid and monounsaturated oleic acid.
Palmitic acid is a common saturated fatty acid occurring in foods such as butter, cheese, milk and beef. Oleic acid, on the other hand, is the most common unsaturated fatty acid. Olive and macadamia nut oils are the richest sources of oleic acid.
“We found that the palmitic acid specifically reduced the ability of leptin and insulin to activate their intracellular signaling cascades,” Dr. Clegg said. “The oleic fat did not do this. The action was very specific to palmitic acid, which is very high in foods that are rich in saturated-fat” (UTSW 2009).
Dr. Clegg said that even though the findings are in animals, they reinforce the common dietary recommendation that individuals limit their saturated fat intake. “It causes you to eat more,” she said (UTSW 2009).
The other key finding, she said, is that this mechanism is triggered in the brain—long before there might be signs of obesity anywhere else in the body.
The next step, Dr. Clegg said, is to determine how long it takes to reverse completely the effects of short-term exposure to high-fat food.
- Bakker W, Sipkema P, Stehouwer CD, Serne EH, Smulders YM, van Hinsbergh VW, Eringa EC. Protein kinase C theta activation induces insulin-mediated constriction of muscle resistance arteries. Diabetes. 2008 Mar;57(3):706-13. Epub 2007 Dec 17.
- Benoit SC, Kemp CJ, Elias CF, Abplanalp W, Herman JP, Migrenne S, Lefevre AL, Cruciani-Guglielmacci C, Magnan C, Yu F, Niswender K, Irani BG, Holland WL, Clegg DJ. Palmitic acid mediates hypothalamic insulin resistance by altering PKC-theta subcellular localization in rodents. J Clin Invest. 2009 Sep;119(9):2577-89. doi: 10.1172/JCI36714. Epub 2009 Aug 10.
- UT Southwestern (UTSW). Ice cream may target the brain before your hips. Sept. 14, 2009. Accessed at http://www.utsouthwestern.edu/utsw/cda/dept353744/files/548055.html