March 20, 2012

High-fat, low-carb diets not for obese people at risk of heart attack

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 Low-carb, high-fat diets led to more damaging, more deadly heart attacks and impaired recovery of heart function in study.

High-fat, low-carb foods are a popular choice for dieters, but new laboratory research by University of Alabama at Birmingham cardiologists indicates these may be detrimental to the health of people who have ischemic cardiovascular disease or a predisposition to heart attacks.

UAB cardiologist Steven Lloyd, M.D., Ph.D., presented several studies in four moderated poster sessions during the 2012 American College of Cardiology Scientific Sessions Sunday, March 25, 2012, in Chicago.

atkins_heart_story“Obesity and heart disease are major public health issues and are the leading causes of other disease — including diabetes, stroke and death,” Lloyd says. “Many overweight people turn to a high-fat, low-carb diet because of its effectiveness; but research has yet to adequately assess safety, and there is insufficient evidence to recommend them for people seeking to lose weight.”

Lloyd and colleagues first published in Life Sciences in 2008 that high-fat, low-carb diets affect the types of fuel the heart uses to function, alter insulin-signaling following a heart attack and increase post-heart-attack damage in normal-weight rats.

In this research, the team tested to learn if those findings would hold steadfast among obese rats and rats with heart disease.

“Overall, we found that obese rats fed a high-fat, low-carb diet — comparable to that humans would consume — had larger, more damaging and deadly heart attacks than rats fed the control diet,” Lloyd says. “Our findings also suggest that, at the cellular level, a high-fat, low-carb diet impaired recovery of heart function in obese rats immediately following a heart attack.”

Lloyd says that four studies are not definitive enough to say high-fat, low-carb diets are harmful or beneficial, but they do present enough preliminary evidence of harm to warrant further investigation.

“These findings tell us that for those who are trying to lose weight, if you have coronary artery disease or if for some reason you are at greater risk of having a heart attack, a high-fat, low-carb diet might not be a good thing for you,” Lloyd says.

The four presentations by Lloyd each answer different but connected questions:

  • What is the effect of high-fat, low-carb diets on the size of a heart attack, and how well does the heart recovers after a cardiac event?
  • What is the impact of high-fat, low-carb diets on heart attack size and survival in the immediate aftermath of a heart attack?
  • What is the effect of high-fat, low-carb diets on mitochondrial function and oxidative stress in the heart, and how these affect tissue damage from reduced blood flow to the heart during an attack?
  • How do high-fat, low-carb diets affect the heart’s insulin sensitivity and the fuels it uses to function during both normal function and during a heart attack?

Heart attack size and heart recovery

Lloyd and his colleagues fed obese rats either a control diet — which was a low-fat diet — or a high-fat, low-carb diet. The outcome for the obese rats was the same as described in normal-weight rats the 2008 — that type of diet leads to greater injury to the heart muscle and reduced functional recovery.

“Rat size did not matter,” Lloyd says. “The heart attacks in the obese rats — as in the normal weight rats — were much larger in those that ate a high-fat, low-carb diet and recovery of function was much better in those that ate the control diet than in those that at the high-fat, low-carb diet.”

Heart attack size and survival

Lloyd and his colleagues found that rats fed a high-fat, low-carb diet had greater damage to the heart muscle from a heart attack and were at greater risk of death from pump failure and arrhythmias when compared to the rats fed the control diet.

“The majority of the rats on the high-fat, low-carb diet didn’t survive the initial attack; among those that did survive, the attack size was considerably larger than among rats fed the control diet,” Lloyd says. “And many rats that died during the attack did so because of an arrhythmia. This could be attributed to the size of the attack.”

Oxidative stress

Lloyd says research has shown that oxidative stress during heart attacks plays a role in activating different chemical processes that cause cell death during and after blood flow is restricted to the heart. These chemical processes can last for a few hours or days after normal blood flow returns and be extremely damaging to the heart muscle.

Previous research also has linked high-fat diets to oxidative stress in the brain, causing cognitive impairment, and in the liver, causing fatty liver disease. Lloyd and his colleagues wanted to assess the affect of a high-fat, low-carb diet on mitochondrial function and oxidative stress in the heart and its subsequent affect on tissue during a heart attack.

“Obese rats fed the high-fat, low-carb diet had significantly larger heart attacks than those fed the control diet and had much more difficulty recovering function after blood flow was restored to the heart,” Lloyd says. “This might be partly due to a loss in the number of mitochondria in heart muscle cells, and an increase in formation of proteins that led to high oxidant stress, and a decrease in proteins that reduce oxidant stress. We found that all of these factors occur in rats fed the high-fat, low-carb diet. This could definitely be contributing to the harmful process of a heart attack.”

Insulin sensitivity

The primary fuel for the body is carbohydrates broken down into glucose. Once glucose is created, insulin then causes cells in muscle and other tissues to take up glucose from the blood and use it for energy. When someone eats a high-fat, low-carb diet, the body — including the heart — turns to fat as its next source of fuel. Ketones are molecules generated during fat metabolism. Lloyd says though research has shown the heart muscle can use many sources of fuel for energy, not all of them are as efficient. Glucose, or carbohydrates, tend to be the most efficient fuel when the heart is trying to recover from a damaging event.

“We found that a high-fat, low-carb diet impaired cardiac tissue response to insulin, which is known to be important in protecting the heart immediately after a heart attack, as it helps limit the size of a heart attack and protects muscle tissue,” Lloyd says. “We also found that, while the heart easily uses more ketones to function under normal conditions when the rats were fed a high-fat, low-carb diet, following a heart attack it uses less ketones. While it is unclear why this happens, without utilizing the most efficient fuel — carbohydrates — to the fullest extent, the heart is less efficient at recovering function.”

Lloyd says by no means does this research say no one should be on a high-fat, low-carb diet. In fact, these diets have been shown to improve certain markers in cholesterol profiles and they do help people lose weight. What it does say, he says, is that much more investigation is needed into the benefits and risks of a high-fat, low-carb diet.

“Right now, if I were considering a high-fat, low-carb diet, I would ask myself if the benefits outweigh the heart-attack issues this research has revealed,” he says. “If I had heart disease or I was predisposed to having a heart attack, I would think carefully before starting this type of diet.”

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