Why Another Weak Diet Study Doesn't Inform

Caroline M. Apovian, MD


March 23, 2023

A recently published study shows that dietary patterns are associated with the prevalence of metabolic syndrome, a condition predictive of cardiometabolic disease (ie, type 2 diabetes, hypertension, stroke, and cardiovascular disease).

Ongoing debate exists in the literature about the optimal macronutrient content of diets. Should carbs or fat intake be high or low?

Caroline M. Apovian, MD

This study tries to establish the association of low carbohydrate consumption stratified by fatty acid classes (saturated, monounsaturated, and polyunsaturated) and metabolic syndrome using the National Health and Nutrition Examination Survey (NHANES 1999-2018) dataset of almost 20,000 US adults.

The study's main conclusion was that metabolic syndrome prevalence was higher among those who had a low carbohydrate intake compared with those who met carbohydrate intake recommendations of 45%-55% of total calories daily, when total fat intake of any class was high.

It showed that a high-fat intake regardless of fat type was associated with an increased risk of having metabolic syndrome in those who ate low carbs when compared with those who ate the recommended serving of carbohydrates.

Those on a low-carbohydrate diet to lower their risk for diabetes and heart disease need to make sure that their fat intake is also low. How to do that? Eat a diet that is high in protein.

Unfortunately, this study has too many unaccounted-for variables, making it difficult to make sense out of the data, although the authors did their best to temper their conclusions.

For instance, a high alcohol consumption shifts an individual from the recommended-carbohydrate category to the below-recommended–carbohydrate category, the authors note.

And the study doesn't account for nondietary variables such as the influence of genetic predisposition on metabolic syndrome and cardiometabolic disease risk.

This nutritional study also doesn't account for body mass index (BMI) or another measure of total body fat, which is highly correlated with metabolic syndrome.

With all these variables, it's no wonder that we're still talking about the association between cardiometabolic risk and diet. It's because it's complicated!

Large cross-sectional association studies such as this one can identify areas of interest, warranting further research. This study leaves more questions asked than it does answered, such as:

  1. What other variables besides dietary pattern can influence the prevalence of cardiometabolic risk?

  2. Does carbohydrate type (simple vs complex) matter in the association between dietary pattern and cardiometabolic risk?

  3. Is alcohol considered a carbohydrate?

  4. How do other variables, such as BMI and physical activity, influence the association between cardiometabolic risk and dietary pattern?

  5. How does genetic predisposition to cardiometabolic risk alter the association between this risk and dietary pattern?

Without answering these and many other questions generated by a body of literature that now includes this paper, it's hard to counsel patients on the best dietary pattern for reducing cardiometabolic risk.

Several options have led the list of dietary patterns that promote health, including the Dietary Approaches to Stop Hypertension (DASH) diet, which is low in fat, and the Mediterranean diet, which is high in fat.

How can we distinguish which one of these two diets, for example, is best for a typical patient when there is evidence that both diets are healthy and reduce cardiometabolic risk?

The answer may be that we haven't assessed the other variables in the mix adequately, and one of these variables is genetic predisposition. Other variables include alcohol use, physical activity, smoking and drug use, socioeconomic status, and sleep and stress patterns.

Also weighing in is BMI and cardiometabolic risk, which is not a linear relationship. Some patients (about 25%) with a high BMI don't develop metabolic syndrome from excess adipose tissue. What protects them? Could it be a genetic marker? Or is it dependent on where the adipose tissue is deposited? Is that genetically determined?

In summary, this study attempted to distinguish the macronutrient content of the diet that corroborated best with metabolic syndrome risk in thousands of US adults. The authors concluded that a certain macronutrient mix of low-carbohydrate and high-fat content was associated with high risk for metabolic syndrome compared with those who met the recommended intake of carbohydrate. However, the nuances enumerated above led to a less than satisfactory settling of this conclusion and led to many more questions than were answered.

In the end, it may not be possible to assess optimal diet without accounting for many other factors.

We may not understand the true risk for metabolic syndrome and cardiometabolic risk until there is a full reckoning of our interaction with not only animal and plant foods but also the entire environment around us, including our microbiome and genes; sleep and circadian rhythms; social interactions, depression, and mood; physical activity and interaction of muscle and adipose tissues with the environment; and endocrine disruptors in the environs that affect metabolism.

On a philosophical note: We are truly at one with the universe, and time is the fourth dimension. The NHANES study is dated between 1999 and 2018; since then, the universe around us has changed and is different — as is mankind.

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