Effects of macronutrient intake on fuel utilization: potential sex differences

In this week’s EXSS Impact post we highlight the work of Meredith Mock, who is currently a graduate student in the Exercise Physiology specialization area, and Dr. Abbie Smith-Ryan.  Many thanks to them both for contributing to this week’s post.

Why did you do this study?

The sports nutrition industry is forecasted to hit $37.7 billion by the year 2019, with growth mainly attributed to increasing health awareness and a broadening consumer base. As more and more people seek sound health advice, there is a growing push to tailor fitness and nutrition recommendations to the specific needs of different populations. Professional athletes and weekend warriors alike seek to optimize their diets to enhance performance. The “best” style of eating remains an ever-evolving, hotly debated topic in both popular media and scientific communities.

mock1Many studies have sought to examine the influence of dietary habits, particularly the amounts of carbohydrates, fat, and protein one consumes, on metabolism and exercise. One way to predict how our bodies utilize these different macronutrients as fuel involves indirect calorimetry, a technique that collects and analyzes the air one breathes to estimate energy expenditure. The ratio of carbon dioxide metabolized (exhaled) to the amount of oxygen consumed (inhaled) is called the Respiratory Exchange Ratio (RER), and ranges from 0.7 to 1.0. An RER of 0.7 represents only fat being burned for energy, while an RER of 1.0 represents only carbohydrate usage. At rest, we maintain an RER of ~0.8, primarily using free fatty acids as fuel. During exercise, RER rises as our bodies switch to carbohydrate for quick fuel to meet increasing energy demands. The longer we can keep our RER low using fat as fuel, the more we preserve our muscle’s glycogen stores for increased strength and stamina during an exercise event.

It is well understood that fitness status contributes to one’s typical fuel utilization, with trained individuals maintaining a lower RER during rest and steady-state activity. However, there is accumulating interest surrounding the important role that dietary intake plays in determining how the body uses energy. Research has demonstrated that habitually eating a lower-carbohydrate, higher-fat diet may result in a lower RER as your body acclimates to greater availability of circulating free fatty acids. It is also understood that women tend to have lower resting RER’s than men, likely due to hormonal differences and varying fat storage patterns. Sex differences in fuel use suggest the need for nutritional strategies specific to males and females, however further study is merited before broad recommendations can be made.

Previous research from our team in the Applied Physiology Laboratory of the Department of Exercise and Sport Science demonstrated that baseline macronutrient distribution impacts post-exercise energy metabolism in women. Furthermore, relative contributions of carbohydrate and fat to total caloric intake were related to strength performance and body composition. However, more exploration into the influence of diet on fuel selection in varying populations is needed. Therefore, the purpose of my study was to explore potential sex differences in the relationship between habitual macronutrient distribution and fuel utilization during exercise.

What did you do and what did you find in this study?

mock2All subjects completed six minutes of steady-state cycling at a resistance based on their maximal aerobic capacity that was determined in prior testing. While riding, subjects’ breathed through a mouthpiece and tube connected to a metabolic cart collecting and analyzing all respiratory gases to estimate respiratory exchange ratio. We also collected 3-day food logs from each subject and analyzed their macronutrient intake using dietary analysis software. Groups were stratified by sex and correlations were performed to evaluate the relationship between macronutrient distribution and RER. Higher carbohydrate intake exhibited a significant, positive correlation with RER in males, while associations between carbohydrate consumption and RER were equivocal in females. Contrary to previous results, a positive correlation between protein consumption and RER was shown in females, however may have been confounded by differences in overall caloric intake.

How do these findings impact the public?

It is well known that the food we eat influences how we look, feel, and perform. Instead of a “one size fits all” approach to nutrition, more scientific evidence is needed to validate nutritional practices that best fit the training goals of all segments of the population. Future studies should evaluate fuel utilization in longer, more moderate exercise bouts to ensure subjects remain in steady-state throughout the test for the most accurate RER measure. Additionally, more research involving nutritional intervention strategies in males and females is needed to explore the influence of diets with varying macronutrient compositions on exercise metabolism and performance.

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