Sat, Dec-31-16, 07:59
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Senior Member
Posts: 15,075
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Plan: mostly milkfat
Stats: 190/152.4/154
BF:
Progress: 104%
Location: Ontario
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A diet that induces obesity causes inactivity first
https://www.sciencedaily.com/releas...61229141901.htm
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Inactivity in obese mice linked to a decreased motivation to move
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Starting a regular program at the gym is a common New Year's resolution, but it's one that most people are unable to stick with for very long. Now a study done in mice is providing clues about one of the reasons why it may be hard for so many people to stick with an exercise program. The investigators found that in obese mice, physical inactivity results from altered dopamine receptors rather than excess body weight. The report appears in Cell Metabolism on December 29.
"We know that physical activity is linked to overall good health, but not much is known about why people or animals with obesity are less active," says the study's senior author Alexxai V. Kravitz, an investigator in the Diabetes, Endocrinology, and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases -- part of the National Institutes of Health. "There's a common belief that obese animals don't move as much because carrying extra body weight is physically disabling. But our findings suggest that assumption doesn't explain the whole story."
Kravitz has a background in studying Parkinson's disease, and when he began conducting obesity research a few years ago, he was struck by similarities in behavior between obese mice and Parkinsonian mice. Based on that observation, he hypothesized that the reason the mice were inactive was due to dysfunction in their dopamine systems.
"Other studies have connected dopamine signaling defects to obesity, but most of them have looked at reward processing -- how animals feel when they eat different foods," Kravitz says. "We looked at something simpler: dopamine is critical for movement, and obesity is associated with a lack of movement. Can problems with dopamine signaling alone explain the inactivity?"
In the study, mice were fed either a standard or a high-fat diet for 18 weeks. Beginning in the second week, the mice on the unhealthy diet had higher body weight. By the fourth week, these mice spent less time moving and got around much more slowly when they did move. Surprisingly, the mice on high-fat diet moved less before they gained the majority of the weight, suggesting that the excess weight alone was not responsible for the reduced movements.
The investigators looked at six different components in the dopamine signaling pathway and found that the obese, inactive mice had deficits in the D2 dopamine receptor. "There are probably other factors involved as well, but the deficit in D2 is sufficient to explain the lack of activity," says Danielle Friend, first author and former NIDDK postdoctoral fellow.
The team also studied the connection between inactivity and weight gain, to determine if it was causative. By studying lean mice that were engineered to have the same defect in the D2 receptor, they found that those mice did not gain weight more readily on a high-fat diet, despite their lack of inactivity, suggesting that weight gain was compounded once the mice start moving less.
"In many cases, willpower is invoked as a way to modify behavior," Kravitz says. "But if we don't understand the underlying physical basis for that behavior, it's difficult to say that willpower alone can solve it."
He adds that if we begin to decipher the physiological causes for why people with obesity are less active, it may also help reduce some of the stigma that they face. Future research will focus on how unhealthy eating affects dopamine signaling. The researchers also plan to look at how quickly the mice recover to normal activity levels once they begin eating a healthy diet and losing weight.
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http://www.sciencedirect.com/scienc...550413116305964
The study itself is full access.
I think some of this applies to lean "lazy" people as well. A healthy person isn't on a forced march, all gritted tooth and determination--moving is supposed to feel good, I don't think it's all a matter of character (unless we admit that much of what we call "character" is just how an underlying physiology manifests in a person's behaviour). And, not to rule out a little bit of self-imposed forced marching, but I think it will be more likely to be effective when the exercise itself promotes the sort of changes in brain physiology that promote activity--providing diet induced obesity animals with running wheels does help, perhaps by providing an activity that's a bit more fun than just scurrying around a cage. Similar to the thing with food, perhaps, where an obese animal will eat less of a low reward food like low fat chow. Obese mice have a higher threshold for sweet and fatty flavours--subtle foods become less enjoyable if they can't taste the light sweetness, something like an oreo is so blatantly sweet that an animal that's less sensitive to sweetness can still enjoy it.
http://www.sciencedirect.com/scienc...550413111003093
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Laboratory animals usually become relatively insulin resistant and obese. In this issue of Cell Metabolism, Cao et al. (2011) find that mice living in a complex environment are resistant to diet-induced obesity because they produce energy-dissipating brown fat cells within white fat depots, a process orchestrated by brain-derived neurotrophic factor.
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