What Affect Does Diet and BMI Have on Physical Fitness?


Recent studies have shown that the physical fitness of an individual can be a promising indicator in measuring health and risk for outcomes such as obesity, cardiovascular disease, diabetes, cancer, and skeletal health. (1) The World Health Organization (WHO) recommends children and adolescents between the ages of 5-17 should get at least 60 minutes a day of moderate-to vigorous-intensity physical activity. Regular physical activity is associated with many health benefits in children and can improve cardiorespiratory and muscular fitness, as well as bone health (4). It is noteworthy to promote regular physical activity because research shows that cardiorespiratory fitness levels are significantly associated with total body fat and abdominal adipose tissue. (1) Lower levels of cardiorespiratory and muscular fitness are associated with CVD risk factors. (1) And improvements in cardiorespiratory fitness have positive effects on things like depression, anxiety, self-esteem, and academic performance. (1)

Findings from a cross-sectional study done in South Carolina found that children who are obese generally spend less time in moderate and vigorous physical activity than non-obese children. (2) It also found that the energy density of a child or adolescent’s diet is directly associated with fat intake, and both energy dense high-fat diets are associated with obesity. (2) In past studies it has been suggested that reducing dietary ED by combining increased fruit and vegetable intake, as well as decreasing total fat intake, was seen to control hunger and be an effective strategy for weight loss. (3)

High-fat diets can easily turn into unhealthy diets that lead to high risk of CVD and insulin resistance, and high-fat diets generally have high energy densities. (5) According to the CDC, 1 in 6 children and adolescents is obese and obesity affects 12.7 million children and adolescents between the ages of 2-19 years old. There is a 75% predicted increase in obesity by 2018. Children who are overweight and obese are more likely to become overweight and obese as adults. (CDC) Studies have shown that for every hour of exercise a day, risk for obesity is decreased by 10%. (2) The measure of physical fitness in children and adolescents can display health as well as predict future health outcomes as an adult. (7)

The purpose of this study was to evaluate if diet and BMI of children affected physical fitness levels by using data from the NHANES National Youth Fitness Survey. Energy density and total fat in the diet, as well as the BMI of the participants, were the variables used to assess performance on three important physical fitness categories, measured by the outcomes of four different physical fitness tests. The objective was to determine if BMI, energy density, and fat intake was significantly associated with physical fitness levels, and what this could mean as an outcome.


Data Source & Inclusion Criteria

The National Health and Nutrition Examination Survey (NHANES) is a cross-sectional survey that assesses the health and nutritional status of children and adults in the US. This experiment used the NHANES National Youth Fitness Survey (NNYFS). The NNYS is a one-year, cross-sectional survey conducted by the National Center for Health Statistics in 2012. For the purpose of analysis, this was the main source of physical fitness data. It had the purpose of gathering nationally representative data that represented physical activity and fitness levels, as well as provided an evaluation of health and fitness of children and adolescents ages 3-15. Data was collected through fitness tests and interviews. The nutritional component of data in the NHANES comes from What We Eat In America (WWEIA), gathered through dietary recall from each of the participants.

This analysis included a study sample of all children and adolescents between the ages of 3-15, who participated in the 2012 NHANES National Youth Fitness Survey. However, many children between >6 years met the exclusion criteria and did not participate the physical fitness tests used in this study to evaluate fitness levels. This resulted in a final n of 1,224 participants between the ages of 6-15.

Outcome Measures

The outcome measures in this study included three categories of physical fitness. Physical fitness was evaluated through fitness tests as part of the NNYS. Participants 6-15 years old participated in fitness tests (summer 2012), which evaluated the health of each age group. The NNYFS contains examination data that evaluates body measures, cardiorespiratory endurance, cardiovascular fitness, lower body muscle strength, muscle strength, and gross motor development. For this analysis, physical fitness was measured using the following categories: cardiorespiratory endurance, core muscle strength, and upper body muscle strength.

Cardiorespiratory endurance was measured by examining fitness test results of heart rate at the end of the test (bpm) and maximal endurance time (in seconds). Core muscle strength was determined by the number of seconds plank position was held (in seconds). Upper body strength was evaluated by the number of correctly completed pull-ups the participant could do. Each exercise was assessed in regards to BMI, energy density, and total fat.

Demographic Characteristics and Potential Confounding Variables

In order to assess if physical fitness was affected, variables of BMI, energy density (kcal), and total fat (gm) were used. The NHANES gathered data of total nutrient intakes from dietary interviews given by well-trained professionals. The dietary intake data can be used to estimate the types and amounts of food (as well as beverages) consumed throughout the past 24-hours. In the NHANES, body mass index (BMI) was calculated as weight in kilograms divided by height in meters squared (rounded to one decimal place). In order to analyze BMI as a categorical variable (BMI Category), sex-specific BMI quartiles were created from body mass index data and cutoff criteria from the CDC’s sex-specific 2000 BMI-for-age growth charts. BMI category provided four quartiles: 1) Underweight, 2) Normal weight, 3) Overweight, and 4) Obese.

Energy density and total fat were variables used to measure diet of children and adolescents. Dietary intake for energy density and total fat was measured using 24-hour recalls. To account for confounding factors, which occur when the outcome is influenced by a third factor, data from the NHANES regarding age, gender, race and income were used as covariates and all models run were adjusted according to this. What was looked at was whether energy, total fat, and BMI were significantly (inversely) associated with a decrease in physical fitness of children and adolescents.

Statistical Measures Used

Data from the NHANES was analyzed using SAS University Edition (SAS Institute, Cary, NC). To examine if there was a significant association of physical fitness levels with BMI and energy density / total fat intake, the PROC REG procedure was used. PROC REG procedure was used to analyze significance, if any, in upper body muscle strength (pull-ups), core muscle strength (plank), and cardiorespiratory endurance (heart rate, maximal endurance time). These models were adjusted for age in years at exam, race, and gender, and significance was determined with a value of p<0.05. BMI Category was analyzed using the GLM procedure to predict an outcome based on a categorical variable. Graphical data shown below is the performance outcomes based on the data from results of the GLM procedure of BMI category and the specific physical fitness exercises.


The data obtained from this study indicates that there was a significant inverse relationship observed between diet / BMI and various aspects of physical fitness of children and adolescents. There was a significant negative association of energy density in pull-ups (p=0.0458) and heart rate at the end of test (p=0.0195). Total fat intake had a significant inverse affect on heart rate (p=0.0404).

BMI was the most significant factor in affecting physical fitness. Children who are overweight/obese have less upper body strength than non-obese children. The mean number of pull-ups was approximately 5. Children who are obese completed on average almost 4 less pull-ups than children who are of normal weight (see figure 1).

Figure 1.

Source: Julia Wood

Children who are overweight/obese exhibit lower levels of cardiorespiratory endurance than normal weight children. Maximal endurance time was measured in seconds and measures the amount of time the actual exercise test takes (does not include warm up or recovery). The mean maximal endurance time was 650 seconds. Children who were overweight/obese were not able to perform the exercise test as long as those of normal weight. Overweight children lasted about 632 seconds, while obese children only lasted about 551 seconds, compared to normal weight children who could last approximately 632 seconds.

Children with a higher BMI have a lower level of cardiorespiratory endurance. The mean heart rate at the end of the test was 220 beats per minute (bpm). A non-obese child of normal weight had a heart rate of 249 bpm, while an overweight child had a heart rate of 208 bpm and an obese child had a heart rate of 209 bpm.

Children with a higher BMI display lower levels of core muscle strength. The plank is an exercise that assesses muscular endurance and core strength around the trunk and pelvis (NNYFS). Children with a normal weight had a greater ability to hold the plank position. Almost 35 seconds longer than obese children and almost 15 seconds longer than children who are overweight (see Figure 2).

Figure 2.

Source: Julia Wood

Strengths and Limitations

 In light of the results from this analysis, it is important to note the strengths as well as limitations. Strengths of using the NNYFS include the fact that it is a cross-sectional study that represents physical fitness levels and health of US children and adolescents as a whole. This means that the results can be applied to the entire population of US children and adolescents. Results show that there is a prevalence of low physical fitness levels in children and adolescents who have high BMI and an increased intake of high-fat/energy dense diets. From this analysis, the simple promotion of increased physical activity as well has healthy diets can be put out into the public in hopes of slowing the obesity epidemic and better health in children.

There are some weaknesses to this research. Diet factors of energy density and total fat were used in this study. Data was acquired for these two factors by dietary recall, so there is a possibility of recall bias. Also, the NHANES National Youth Fitness survey is of a cross-sectional survey design, so although analysis can point out prevalence stemming from results, it cannot determine causality. This study also uses two physical fitness tests that somewhat depend on weight/body mass. Pull-ups as well as plank exercises may be subject to influence based on body weight, which could skew results.


Our findings from this study indicate that a child or adolescent’s BMI and diet affect his or her performance on physical fitness tests. Children and adolescents who are overweight or obese (85th-95th percentile or >95th percentile) are seen to have lower levels of cardiorespiratory endurance, upper body muscular strength, and core muscle strength. High BMI was seen to negatively affect physical fitness the most and was more significant than any other factor (p<.001).

There is a significant inverse association between energy dense / high-fat diets and various aspects of cardiorespiratory endurance and upper body strength. Physical fitness is a marker of health and can predict health as an adult. Regular physical activity of at least 60 minutes a day for children and adolescents promotes health and fitness and may help to prevent obesity. Strategies promoting healthy eating may also slow the obesity epidemic.


  1. Ortega, F. B., Ruiz, J. R., Castillo, M. J., & Sjöström, M. (2008). Physical fitness in childhood and adolescence: a powerful marker of health. International journal of obesity, 32(1), 1-11.
  2. Ebbeling, C. B., Pawlak, D. B., & Ludwig, D. S. (2002). Childhood obesity: public-health crisis, common sense cure. The lancet, 360(9331), 473-482.
  3. Ello-Martin, J. A., Roe, L. S., Ledikwe, J. H., Beach, A. M., & Rolls, B. J. (2007). Dietary energy density in the treatment of obesity: a year-long trial comparing 2 weight-loss diets. The American journal of clinical nutrition, 85(6), 1465-1477.
  4. Janssen, I., & LeBlanc, A. G. (2010). Systematic review of the health benefits of physical activity and fitness in school-aged children and youth. International journal of behavioral nutrition and physical activity, 7(1), 1.
  5. Guldstrand, M. C., & Simberg, C. L. (2007). High-fat diets: healthy or unhealthy?. Clinical Science, 113(10), 397-399.
  6. Schrauwen, P., & Westerterp, K. R. (2000). The role of high-fat diets and physical activity in the regulation of body weight. British Journal of Nutrition, 84(04), 417-427.
  7. Harper, M. G. (2006). Childhood obesity: strategies for prevention. Family & community health, 29(4), 288-298.

Julia Wood is a senior at Fairfield University in Connecticut where she is preparing to graduate in May 2017 with a degree in Biology and minor in Health Studies. Julia was a 4-year member of the Fairfield women’s D1 cross-country team.

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