The role that diet plays in overall health, more specifically cardiovascular health, cannot be understated. Indeed, risk factors such as insulin resistance, obesity, the metabolic syndrome, and high blood pressure (to name a few) all contribute to adverse, pathological changes in the function and structure of our body.
One of the more detrimental changes to occur in disease (and “naturally”, with age) is a process called arterial stiffening. Usually elastic and relatively pliable, arteries can modulate pressure in the body by expanding and contracting as needed. Stiffening causes these arteries to lose their elasticity, and thus, more pressure is placed on vital organs, mainly the heart. Higher arterial stiffness is a pretty good predictor of future CVD development and even current risk for cardiovascular complications.
Returning to the discussion on diet. While it isn’t known which particular diet is best (“best” is so vague…) for reducing arterial stiffness, many diets have been shown to improve stiffness (that is, reduce it) — and most of these are accompanied by some kind of weight loss. Weight loss on it’s own can reduce arterial stiffness by a variety of mechanisms. There haven’t been many studies investigating how restricting specifically carbohydrates might influence arterial stiffness. Given the popularity of #LCHF and #Keto diets in the popular culture, along with the compelling evidence that these diets may have some efficacy in metabolically dysregulated individuals, the topic is worthy of study. Conveniently, a recent study from the University of Missouri investigated how a carb-restricted diet might influence vascular health.
Study purpose and design
One of the pathways that has been implicated in the relation between arterial stiffness and metabolic disorders is insulin resistance (the main phenomenon in diabetes). Since insulin resistance might negatively influence stiffness, the authors proposed that improved insulin sensitivity (which occurs in a carbohydrate restricted diet and/or weight loss) might reduce arterial stiffness.
Participants in this study ranged in age from 30–55 years of age, were sedentary to moderately-active and had characteristics of insulin resistance or the metabolic syndrome. Presence of any other disease (diabetes, CVD, etc) excluded participants, which removed any confounding factors and served to “isolate” the variables of interest — metabolic dysfunction.
For the study, participants visited the lab on three occasions. After having their baseline measures taken (discussed below), they received a prepared diet for 2 weeks + education on how to prepare the diet and types of foods they would/could consume. At the end of 4 weeks (entire study duration) the baseline measures were repeated.
What did they measure?
The primary outcome of the study was arterial stiffness. This is measured through something referred to as pulse wave velocity (PWV). Pulse wave velocity is the speed at which the pressure waveform generated when the heart contracts (termed systole) travels through the circulatory system. A wave moving at a faster speed indicates a higher PWV, and signifies greater arterial stiffness. Think of blood flow through arteries like water traveling through a plastic pipe ,where it would be high pressure and higher velocity, vs a stretchy garden hose, where the pressure would be lower, and things would flow slower. Stiffer vessels resemble the pipe.
In addition to arterial stiffness, they measured participants dietary intake and physical activity, weight loss and fat mass (duh), metabolic rate, blood pressures, and a panel of things like sex hormones, insulin and glucose, and markers of inflammation.
What did they eat?
The carbohydrate and dietary restriction in this study was pretty considerable. The table below (from the paper) shows all of the data — but a few key things here.
The groups reduced their overall caloric intake by about 30% — from about 2200 calories/day to around 1500 per day.
Carbohydrates were also drastically reduced. At baseline, participants were consuming about 221 grams of CHO per day (41% of their total caloric intake). By the end of the study, this fell to an average of 54 grams per day (14% of overall intake). A point that I don’t think is emphasized enough in the paper is that, in addition to carbohydrates, the participants sugar in take fell from 88 g/day to just 12! Even in the absence of carb-restriction, I feel like this reduction might lead to improvements in several biomarkers.
Fat and protein intake (grams) stayed the same throughout the study but, as a % of total calories, they went up; total fat from 41 to 58% and protein from 18 to 28%. This pretty much fits the bill for a “low-carb/high-fat diet.”
What were the results
The main outcome of aortic stiffness was reduced when the entire group (n=20) was combined. Even more interesting is that, when separated according to sex, it was found that women experienced a 10% reduction in aortic stiffness — statistically significant — while the men showed no such significant reduction — suggesting that the women benefited even more from the low-carbohydrate diet.
On average, the study group lost around 6 kilograms — women lost 6 kilograms, while the men lost about 7. It is interesting to note that both fat mass and lean mass were reduced in response to the diet. Most of the time, you want to try to maintain lean mass on any diet, but with such a drastic reduction over just 4 weeks, you might expect some weight lost to be lean/muscle mass.
Other interesting changes
All measures of blood pressure — central aortic as well as peripheral (brachial) were reduced in response to the intervention. Blood glucose remained unchanged, while insulin was shown to decrease. Triglycerides (TG) and total cholesterol were similarly reduced in the group after the diet intervention. No signfiicant changes were found in inflammatory markers or sex hormones such as estrogen and testosterone
Reduced pulse wave velocity in response to 4 weeks of a low-carbohydrate diet suggests that this type of intervention might be effective in reducing cardiovascular disease risk — since PWV is “an emerging marker of CVD risk and mortality.” I think it speaks volumes that such a dramatic reduction occurred after just 4 weeks — who knows what a longer intervention of several months might lead to.
The “novel” finding in this study that women benefit more than men (in terms of arterial de-stiffening) in response to a low-carb diet indicates that sex differences may exist in dietary interventions as well as CVD risk factors. This we know — based on some interesting recent literature. It is no longer acceptable to design or interpret interventions in an “all-male” context that has been long commonplace in physiology.
Women who have insulin resistance have been shown to be at a greater risk of developing arterial stiffness than men with insulin resistance. Aging itself affects women differently vs. men in terms of cardiovascular risk factors such as arterial stiffening — and intervention designed to target them specifically are being developed.
“This is the first to suggest that dietary carbohydrate (CHO) intake is a factor that may raise arterial stiffness.”
Low-carb diets are a hot area of research — and a divisive one at that. There is little doubt that a high (refined) carbohydrate diet contributes independently to many diseases and this study provides some evidence that a higher-carbohydrate diet may be detrimental in some populations. Indeed — it was also found that dietary carbohydrate intake (as a % of total calories) correlated positively with PWV — that is, a higher intake of carbohydrates was related to a higher PWV — greater arterial stiffening.
By itself, dietary CHO restriction leads to lower measures of arterial stiffness in just 4 weeks — along with a significant amount of weight loss in both men and women. This highlights the profound impact of diet on CVD risk, and suggests “simple” (if it only were so) changes implemented into one’s lifestyle can impact the trajectory of aging and disease.
Majid M. et al. Effect of carbohydrate restriction-induced weight loss on aortic pulse wave velocity in overweight men and women. Applied Physiology, Nutrition, and Metabolism. 2018