- Flow-mediated dilation (FMD) is the ability of the artery to dilate beyond it's normal size when blood flow increases. Dilation of 7% above the normal size is considered healthy, with higher being better and lower being worse.
- A low FMD is a significant predictor of atherosclerosis and decreases in FMD over time indicate the progression of heart disease.
- Dilation is important because it reduces the stress experienced by arterial walls when blood flow increases.
Diet, Part 2
In the previous post on FMD and diet, we looked at a study that showed a high fat meal suppresses FMD, whereas a high carb/fat-free meal had no effect . However, after a deeper look, the results weren't so black and white. While saturated fat is a target of mainstream health today, the majority of the fat calories were unsaturated. Specifically, they came from various vegetable oils. These oils contain large amounts of omega-6 fatty acids (FA). Omega-6 FA, when taken in much higher quantities than omega-3 FA, lead to cardiovascular inflammation and increased risk for heart disease. Not only did the group eating a high fat meal consume a lot of omega-6-dense vegetable oils, but many of those oils were hydrogenated. Hydrogenated oils are known to increase many of the risk factors for heart disease (lowering HDLs, increasing triglycerides, etc.).
I also mentioned vitamin C is known to improve FMD , yet it did nothing for the high carb/no fat group in that study. I believe this happened because vitamin C was only able to attenuate for the damage caused by the high carb meal, which brings us to today's question: does a high carb meal affect artery function?
A group of Japanese researchers may have found the answer to this question. They put three groups of people through an oral glucose tolerance test, basically giving each subject 75 grams of glucose . The three groups were split into individuals who had healthy blood sugar levels and insulin responses (NGT - normal glucose tolerance), people with slightly abnormal insulin responses and blood sugar (IGT - impaired glucose tolerance - these people will likely develop diabetes), and those who currently have diabetes (DM). Each subject fasted for 12 to 14 hours before the test, and measurements were taken one and two hours after receiving the glucose.
Blood glucose levels skyrocketed after the glucose meal. The NGT group went from a baseline of 91 to 154 and 114 mg/dl in one and two hours following. IGT started at 105 and jumped to 220 and 173. DM started at 128 and jumped to 240 and 242. The healthy blood glucose range is 70 to 99 mg/dl.
The FMD results are charted below:
All three groups significantly digressed one and two hours after taking the glucose. The NGT group started at a desirable 7.53% but plummeted to 4.24% and 6.35% during the following two hours (recall that 7% is the lowest "healthy" FMD). The IGT group started at 6.5%, then fell to 1.4 and 4.0% in the two hours after. The DM's baseline FMD was 4.77% (ouch!) and fell to 1.35 and 1.29 in hours one and two after the test.
As you see, a large dose of glucose can drop a healthy individual's FMD into an unhealthy range. In addition, the groups with high baseline blood sugar were slower to recover from the meal. Both the IGT and DM groups fell to FMDs of less than 2% following the meal. This means their arteries could dilate only about one-forth of the minimal healthy amount and could hardly dilate overall!!! The authors of the study speculated that the hyperglycemia increases the concentration of free radicals, which inactivates nitric oxide. Nitric oxide is the catalyst for arterial dilation.
In getting back to the question about high carb meals, it's clear that a high sugar meal raises blood sugar and is detrimental to arterial function. Fat and fiber both slow the metabolism of sugar, and in turn, blunt the spike of blood glucose. Therefore, this study indicates that having a meal dense in carbohydrates without any accompanying fat or fiber is very damaging to arterial health. This also supports my previous suggestion: the high carb, fat-free meal presented in the high fat meal study  was likely damaging but that damage was controlled by vitamin C or other nutrients in the meal.
Concluding the Discussion on FMD
For those of you who may have day-dreamed your way through my last few posts, I'm going to make this simple for you: here are my concluding points on how diet and exercise affect flow-mediated dilation, and therefore, how they affect your cardiovascular health:
- Cardio in the traditional steady-state format (one speed for long periods of time) improves FMD, but high intensity interval training (alternating short bouts of 90-100% effort with longer bouts of light effort) can provide the same benefits in a fraction of the time.
- Strength training is beneficial for arterial health in just about any unhealthy population. However, for healthy populations, the key may be how much time is spent between sets. Hustling between exercises could be the key for strength training to improve FMD.
- A meal high in vegetable oils, including hydrogenated vegetable oils, can cause hours of arterial dysfunction. Vegetable oils are commonly found in fast food and processed foods.
- A high amount of sugar causes a state of high blood glucose which also leads to hours of arterial dysfunction. Based on the study discussed in this post, I recommend avoiding meals rich in carbohydrates, especially in the absence of fat and fiber.
- Vitamin C improves arterial function, but I don't recommend relying on it to save you from meals featuring high amounts of vegetable oils or sugar.
You state that normal blood glucose is 60-99, however medical standards consider normal 80-110. Anything less than 70 is considered hypoglycemia.
How does one measure FMD?
These results are not shocking, we know that diabetics are at increased risk of CVD due to hyperglycemia :)
Thanks for reading. The 60 was an error - thank you for pointing it out! Recent ACSM guidelines consider 100 (measured on two separate occasions) to be prediabetic. I've seen 110 as well but stick to ACSM for risk factor definitions.
I agree that the results aren't shocking, especially with the diabetic group. What I did find interesting is the magnitude of impact that hyperglycemia has on FMD in healthy individuals. A 3% digression in just one hour is extreme - it demonstrates the power of a single bout of hyperglycemia.
FMD is measured differently in various studies, but the most common method I have seen is to put a blood pressure cuff on the upper forearm for five minutes at 200 mmHg. An ultrasound machine is used to measure brachial artery thickness prior to the administering the cuff, and then shortly after releasing the pressure. The percentage is found by subtracting the premeasure from the maximal dilation size, then dividing the difference by the premeasure (hopefully that's not too confusing!).
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