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“These are startling results. It shows that high-fat feeding even over short periods of time can markedly affect...metabolism and physical performance. By optimizing diets appropriately we should be able to increase athletes’ endurance and help patients with metabolic abnormalities improve their ability to exercise and do more.” Professor Kieran Clarke, Department of Physiology, Anatomy, and Genetics, Oxford University, UK
High-Fat Diet Reduces Endurance 50%-In 9 Days
Mental Abilities Also Affected
Fat is a less efficient fuel then glucose (sugar). That’s what “hitting the wall” at about 20 miles in the marathon is about. Runners use up the sugar stored in their body, and are forced to rely mainly on fat stores for the last 6 miles. Fat is a more concentrated form of energy, but sugar is faster burning. A study at Oxford University in England, published online August 10, 2009, in the FASEB Journal, sought to determine whether a high-fat diet would be good or bad for endurance capacity.
The report is highly technical, but here’s how the issue is framed in a press release: “Physical endurance—how long we can keep exercising—depends on how much oxygen can be supplied to our muscles and how efficiently our muscles release energy by burning up the fuel we get from the food we eat. In particular, using fat as a fuel is less efficient than using glucose from carbohydrate, but the metabolic changes induced by different diets are complex and it has been controversial whether high-fat feeding for a short time would increase or decrease physical performance.
Some have argued that a high fat diet improves fat burn during exercise, which would spare sugar stored in the muscles (glycogen). Theoretically, that would put off “bonking” until later in the race.
In addition, earlier studies have shown that high-fat eating over the long term impairs cognitive function in rats and humans. That stands to reason, because the brain works on glucose; fat is unable to cross the blood-brain barrier.
The Oxford team, led by Dr. Andrew Murray, set out to investigate whether rats fed a high-fat diet for just a few days showed any change in physical and mental function.
The study design is logical and easy to understand.
All 42 rats were initially fed standard lab chow for up to 2 months. (Carbohydrate 75%, protein 17.5%, and fat 7.5%) During this period, endurance was measured based on how long the rats could run on a treadmill. Cognitive function was measured using a maze test of working memory. Half of the rats were then switched to a diet containing 55% fat (29% protein and 16% carbs); the other half continued eating the low-fat lab chow. Starting on day 5 and continuing through day 9, endurance and cognitive function were again measured for all rats. This, of course, allowed the researchers to compare performance on the low- and high-fat diets.
Dr. Murray described the standard lab chow as “a pretty low-fat diet, much like humans eating nothing but muesli.” The diet with 55% fat, he continued, “sounds high but [is] actually not extraordinarily high by human standards. A junk food diet would come close to that.”
“Some high-fat, low-carb diets for weight loss,” Murray added, “can even have fat contents as high as 60 per cent.” He cautioned, however: “It’s not clear how many direct conclusions can be drawn from our work for these diets, as the high-fat diet we used was not particularly low in carbs.”
As noted, carbs were 16%. He must mean it wasn’t extremely low carb. It was certainly low in carbs.
The results are eye-popping.
The five day period allowed the rats to adjust to the diet change. On the fifth day, the first day back on the treadmill, the rats on the high-fat diet were already 30% behind the rats that continued to eat the standard lab chow. By day 9, the last day of the experiment, the rats on the high-fat diet were running only half as far as those on the low-fat diet.
The rats on the high fat diet were also making more mistakes on the maze test. The contrast wasn’t as stark, however. The high-fat-feed rats took 25% longer to complete the maze than at baseline, when they were eating the low-fat lab chow.
The researchers also analyzed muscle and heart cells to see what metabolic changes the high-fat diet was inducing in the rats. (This would be problematic with humans.) They found increasing levels of a specific protein called the “uncoupling protein” in the cells of rats on the high-fat diet. This uncouples the process of burning food stuffs for energy in the cells, reducing the efficiency of the heart and muscles. “This could, at least in part, underlie the impaired endurance performance of rats fed a high-fat diet,” the report stated.
The rats on the high-fat diet also had significantly bigger hearts after nine days, suggesting that the heart had to increase in size to pump more blood around the body and get more oxygen to the muscles. Remember that the body has to work harder to produce energy from fat than from carbohydrate.
“In little more than a week, a change in diet appears to have made the rat’s hearts much less efficient,” commented Professor Jeremy Pearson, Associate Medical Director of the British Heart Foundation, which funded the research.
Message for Humans
“Taken together, these findings argue strongly against the use of high-fat diets as an ergonomic aid during endurance training,” the researchers concluded.
While this study was done with rats, Dr. Murray and his colleagues are now carrying out similar studies in humans.
The press release included this forward-looking and optimistic comment: “The results will be important not only in informing athletes…, but also in developing ideal diets for patients with metabolic disorders such as diabetes, insulin resistance or obesity. People with such conditions can have high levels of fat in the blood and show poor exercise tolerance, some cognitive decline, and can even develop dementia over time.”
Here’s my take.
I’m not ready for a muesli diet, although I do like Familia™ Swiss Muesli for an occasional snack. For sure, I’m not switching to a high-fat, low-carb diet. A balanced diet of whole foods, however, is sounding better all the time. See “Simple Diet Patterns for Good Health” http://www.cbass.com/SimpleDiet.htm .
I am encouraged by a new study led Heidi Wengreen, RD, PhD, Assistant Professor of Nutrition at Utah State University, which found that a healthy balanced diet may protect against cognitive decline. “Our results suggest that including whole grains, vegetables, low-fat dairy foods, and nuts in one’s diet may offer benefits for cognition in late life,” Wengreen reported.
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