There is a window of time in which sufficient physical activity can help mediate some of the damage caused by eating an unhealthy meal.
I’ve previously discussed studies that show a single meal high in saturated fat can impair artery function in men, as measured in the arm, but what is more concerning is blood flow to the wall of the heart. Researchers randomized men to eat either a high-fat meal that was more than 60% fat, half of it saturated, with more than an egg’s worth of cholesterol, or a low-fat meal that was mostly carbs, less than 10% fat, and had 50 times less cholesterol.
Below and at 0:47 in my video Exercising to Protect Your Arteries from Fast Food, you can see a Doppler recording of the left anterior descending coronary artery, known as the widow-maker, before the high-fat meal (top). Its nice strong signal was squeezed down within hours after eating; the image (bottom) was taken five hours after the high-fat meal.
The coronary flow reserve decreased after a single high-fat meal, but not after a low-fat meal with the same number of calories.
What does “coronary reserve” mean? When part of a coronary artery is blocked for any reason, the surrounding vessels expand. That extra expansion capacity is called the coronary flow reserve, and it’s clamped down within hours of eating a fatty meal, undermining the heart’s ability to compensate for clogged arteries. That’s how a high-fat meal affects blood flow to the heart.
In extreme cases, you can even witness it in the back of someone’s eye. Below are before-and-after images of a retina, which you can also see at 1:34 in my video.
The first image shows milky-colored blood vessels, and the second shows what happens after a low-fat diet and drugs help clear the fat from the bloodstream. Can you see the difference? In the first photo, the blood looked like a milkshake.
What happens if you exercise right after eating that high-fat meal? Post-meal inflammation following the extended elevation of fat in the blood after high-fat meals is a likely explanation for increased cardiovascular disease risk, but substantial evidence suggests that acute exercise is an effective way to clear out some of that fat after a meal. However, the beneficial effects of acute exercise on postprandial lipemia (after-the-meal fatty blood) appear to be relatively short-lived. Going a few days without exercising may completely negate any benefit, no matter how fit you are. The time window appears to be between 18 hours before the meal and around 90 minutes after the meal. And how much exercise do we need? About an hour of moderate-intensity exercise should do it.
In another study, it only took 20 minutes of stair climbing, broken up into five-minute intervals every hour for four hours after a McDonald’s breakfast of hash browns, eggs, pancakes, an English muffin, sausage, and a milkshake. Following such a meal, artery function significantly decreased when the subjects just sat around after eating, but not when they did the hourly stair-climbing exercises. So, hourly exercise may attenuate the negative effects not only of prolonged sitting but also of eating a high-fat meal, “suggesting that stair climbing should be incorporated as an easily accessible lifestyle strategy to protect vascular [artery] function.” Of course, it goes without saying that the other way you can protect artery function is to not to eat breakfast at McDonald’s in the first place.
Such a meal would also have more than 2,000 mg of sodium. That’s more than the 1,500 mg the American Heart Association recommends we stay under for an entire day. Give someone a meal with less salt, even a third less, and that alone can still impair artery function within an hour of consumption, even independent of the increase in blood pressure.
When it comes to blood pressure, some people are “salt-sensitive,” meaning they suffer a large bump in blood pressure when they eat salt, but others are said to be “salt-resistant.” Their blood pressure doesn’t really depend much on their salt intake. So, is salt okay for these people? No. High dietary sodium intake reduces artery function regardless of whether your blood pressure is salt-sensitive or salt-resistant. Your artery function is impaired either way, going from a low-salt diet to a high-salt diet, which you can see below and at 3:53 in my video.
There is an influence of dietary salt beyond blood pressure. Despite the “seemingly unanimous consensus,” some researchers (too often funded by the salt industry) claim that it’s actually not good to cut down on salt, but the evidence is against these dissenters. Like the saturated fat in meat, dairy, and junk, the science indicates that sodium—not sodium reduction—is “the real villain.”
Doctor’s Note
This is the second in a three-part series on saturated fat and artery health. The first was How a Single Meal Can Cripple Your Arteries and Lungs. Up next is Protect Your Arteries from Saturated Fat with These Foods.
Still not sold on the dangers of salt? Check out The Evidence That Salt Raises Blood Pressure.
