Issue Date: September 19, 2011
Holding The Line On Weight Loss
A mountaineer climbing Mount Everest can expend upward of 10,000 kcal per day. That’s enough exercise to make anybody lose weight, even if they don’t want to. The problem is exacerbated by the fact that people also lose their appetite at high altitude.
At a Division of Agricultural & Food Chemistry symposium at the American Chemical Society national meeting held last month in Denver, scientists described the nutritional challenges associated with extreme conditions found at high altitudes.
High altitudes are particularly harsh, said E. Wayne Askew, a professor of foods and nutrition at the University of Utah, because they combine severe cold with low oxygen levels. People at high altitudes experience muscle wasting, in which weight loss is primarily due to the loss of lean body mass. Some of this weight loss occurs because the body needs to work harder than it does at low altitudes just to carry out the same level of effort and basic metabolism.
And it turns out that it’s all about oxygen, Askew said. At ambient pressure, the oxygen-carrying protein hemoglobin is nearly saturated with oxygen. That oxygen gets handed off to a cascade that leads ultimately to the mitochondria, the cell’s power plants. But as the altitude increases, the saturation level drops precipitously, Askew said, until it’s only about 50% at the summit of Mount Everest. At such elevations, those power plants have less oxygen to work with.
Low oxygen’s effects on metabolism can also be traced to its regulatory control over several proteins. One is a hypoxia-induced factor, HIF-1α, an O2-sensing protein transcriptional factor that turns on genes that encode proteins that increase O2 delivery. When oxygen is plentiful, HIF-1α doesn’t activate gene transcription, but when oxygen levels fall, HIF-1α accumulates in cells and triggers the synthesis of those proteins.
Another protein affected by low oxygen levels is the signaling protein mTOR, which controls cell metabolism and turns on protein synthesis by phosphorylating other proteins. Low oxygen levels may down-regulate the mTOR signaling pathway and inhibit protein synthesis.
The functioning of both of these proteins is likely affected by the amino acid leucine, Askew said. When mTOR senses sufficient leucine, it activates protein synthesis. Researchers have also shown that leucine is important to the activity of HIF-1α, he said.
With this in mind, Askew and his coworkers Stacie Wing-Gaia and George W. Rodway have been investigating whether supplemental leucine could help combat lean body mass loss at high altitudes. During an expedition on Everest earlier this year, Wing-Gaia gave climbers energy bars containing 12 g protein, including 3.3 g leucine. She gave control subjects similar bars with 12 g protein, but only 1 g leucine. Askew’s team is still analyzing the data to determine whether leucine supplementation indeed slowed down the climbers’ weight loss.
The biggest problem, however, turned out to be the taste of the bars. Although the team managed to mask the bitterness of leucine, at higher elevations the climbers could barely “choke down” bars that had been palatable at base camp, Askew said. In the future, the team plans to put the supplemental leucine in a beverage, because at high altitude the ability to drink seems to remain more intact than the ability—or desire—to eat.
In addition to protein problems, fats are poorly absorbed at high altitudes. John W. Finley, now a professor of food science at Louisiana State University, described an Everest expedition that he accompanied in the 1990s while he was working at Nabisco. He and his coworkers thought that medium-chain triglycerides (MCTs) might be better absorbed than the long-chain fatty acids normally found in food because MCTs are transported in the body as free fatty acids rather than as triglycerides.
They gave the climbers cookies, crackers, and hot chocolate mix containing MCTs. With the fats disguised as treats, Finley and his coworkers had no problems getting the climbers to eat them, he said. The foods helped reduce, but not eliminate, weight loss: The control group lost an average of 15.5 lb during the expedition, whereas the climbers eating the MCT-laced foods lost only 11.8 lb.
At the time, Finley thought that MCTs may help stave off weight loss because of their effect on the hormone ghrelin, whose main job is to stimulate appetite. Later work by others has supported his suspicions, he noted. In particular, researchers have since come to realize that the enzyme ghrelin O-acyl transferase, which activates ghrelin by adding octanoylate to the hormone, has two substrates—ghrelin and MCTs.
Despite the increased interest in protein as a way to protect against unwanted weight loss, there’s still a place for fat-based products, Finley said. “Weight maintenance and energy are critical” for high-altitude climbers, he said. “Fat is the most efficient way to deliver calories.”
- Chemical & Engineering News
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