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.

Four Things We Need to Focus More on

It can be a challenging process for many when attempting to follow the latest diet plan or exercise program claiming various health benefits. There is an easy way to cut through all the rhetoric regarding these types of diet plans and so-called healthy programs by simply getting more of the following each day: sleep, spinach, steps and strength. Here are some of the health benefits when you get more of each on a regular basis.

GET MORE SLEEP: Sleep should be first on everyone’s list of things to try to get more of because when you’re deficient in it, everything from how you feel to what you eat is affected negatively. Research has shown that people who get less than six hours of sleep a night have higher blood levels of inflammatory proteins than those who get more than 6 hours of sleep.

This is important because inflammation is linked to diabetes, stroke, heart disease, arthritis, and premature aging, according to data published in the Centers for Disease and Control and Morbidity and Mortality Report.

Research conducted in 2004 has shown that sleep deprivation can enhance the release of specific peptides in the body that produce hunger. Men that slept only four hours each night for two days witnessed a decrease in specific hormones such as leptin and an increase in ghrelin compared with men who slept ten hours during that same time period. Leptin is an appetite suppressant hormone that is produced by adipose (fat) tissue, and ghrelin is released from the stomach in response to someone fasting and promotes the feeling of hunger. The hormone leptin acts on the central nervous system, most notably the hypothalamus, by not only suppressing food intake but stimulating energy expenditure as well. Ghrelin levels typically increase before meals and decrease after meals. This particular hormone stimulates appetite as well as fat production and can lead to increased food intake and a gain in body weight. A second study from the University of Pennsylvania Sleep and Chronobiology Laboratory looked at the sleeping and eating behavior of 225 people. They reported in the journal Sleep, when you’re awake between the hours of 10 p.m. and 4 a.m., you’re more likely to consume extra calories. The group ate, on average, an additional 553 calories typically choosing foods higher in fat when they were kept awake until the early morning hours. So  you may want to start getting more Z’s beginning tonight. A good tip is to eliminate all caffeinated products by early afternoon if you typically have trouble sleeping.

Recommended reading: The Promise of Sleep by William Dement, MD, Dell Publishers, 2000.

Research study: A Prospective Study of Change in Sleep Duration: Associations with Mortality in the Whitehall II Cohort, Sleep, 2007.

Photo Credit: http://www.12keysrehab.com

EAT MORE SPINACH: Eating more of a plant-based diet, including spinach, would in fact be a good thing for all of us. There is a 32 percent less chance of getting heart disease on a plant-based diet. Spinach is considered at the top of the healthiest vegetable list for nutrient richness. Not only is it rich in vitamins and minerals, it also has an abundance of health-promoting phytonutrients such as carotenoids and flavonoids to provide you with powerful antioxidant protection. In one study on the relationship between the risk of prostate cancer and vegetable intake — including the vegetables spinach, broccoli, cauliflower, cabbage, brussels sprouts, collards, and kale — “only spinach showed evidence of significant protection against the occurrence of aggressive prostate cancer.” If you’re interested in trying to eat cleaner, healthier and more of a plant-based diet, like we were, than the Boston-based food delivery service Purple Carrot may be just what the doctor ordered. Visit Purple Carrot and use the promo code “koko” for a $25 discount on your first order.

Recommended reading: What to Eat by Marion Nestle, North Point Press, 2006 and Always Hungry? David Ludwig, MD, PhD, 2016.

Research study: Nutritional Update for Physicians: Plant-Based Diets, 2013.

Photo Credit: Purple Carrot (https://purplecarrot.com). Use promo code: “koko” for a $25 discount on your first delivery.

GET MORE STEPS: The odds are that you have heard the old adage “use it or lose it” more than once. It can be a good idea to do a reset, especially before the upcoming Holiday season that is about to take over your life and have a goal of changing your mindset regarding daily activity. The fact is if you’re not finding the time to stay active as you age, your body will slowly begin to “shut down.” When this begins to happens over time – everything from energy levels to metabolism to aerobic capacity to strength – are effected and slowly begin to decrease. Simply getting out for a walk/run/hike will help offset those areas and more. Hippocrates once said, “walking is a man’s best medicine.” To find out if his 2,400 year-old remark was actually valid, two scientists from University College London performed a meta-analysis of research published between 1970 and 2007 in peer-reviewed journals. After studying more than 4,000 research papers, they identified 18 studies that met their high standards for quality. These overall studies evaluated 459,833 test-subjects who were absent of cardiovascular disease at the start of the investigation. The subjects were followed for an average of 11.3 years, during which cardiovascular events (i.e. heart attacks and deaths) were recorded. Their meta-analysis makes a strong case for the benefits of good old walking. The group of studies showed that walking reduced the risk of cardiovascular events by 31 percent, and decreased the risk of dying during the time of the study by 32 percent. A good tool for your tool box is to wear a pedometer to make you more aware of your daily activity via daily steps. Find your 3-day average for steps and start adding 500-1000 steps/week until you’re in the 7,500 to 10,000 steps ball park. For additional information on the benefits of pedometers look into my TBC4 Plan.

Recommended reading: The Step Diet, by James Hill, PhD et al., Workman Publishing, 2004 and Move a Little, Lose a Lot by James Levine, MD, PhD, Three Rivers Press, 2009.

GET STRONGER: There is no way around it, you need to get stronger and maintain strength as you age. Getting and staying stronger as you age will help you hold onto your functional ability. The only way to “hold on” to your strength is work on getting stronger now by becoming more active and sticking with strength training for the rest of your life! An area that is often neglected, when it comes to strength, is grip strength. Research has shown for a long time now that elevated grip strength level is associated with increased longevity. In addition to grip strength, focus on getting your big muscle groups stronger, like your back, buttock and legs.

According to research, individuals who did not strength train lost about 5 to 7 pounds of muscle every ten years and the by-product of this was a reduction in their metabolism by about 50 calories a day. As you grow older, the loss of muscle becomes more pronounced and by the time you reach the age of 70, the muscular system has experienced a 40 percent loss of muscle mass and a 30 percent decrease in strength. With the loss of muscle mass comes the loss of strength and power. A person’s balance, mobility and functionality are also compromised. Strength appears to peak between the ages of 25 and 35 and is maintained or slightly lower between ages 40 and 59 and then declines by 12-14 percent per decade after 60 years of age, according to research published by Doherty in 2001.

Research study: Prognostic value of grip strength: findings from the Prospective Urban Rural Epidemiology (PURE) study, Lancet, 2015.


Doherty TJ, (2001). The influence of aging and sex on skeletal muscle mass and strength. Curr Opin Clin Nutr Metab Care 4:503-508.

Centers for Disease and Control and Morbidity and Mortality Report.

Nedeltcheva AV, et al., (2010). Insufficient Sleep Undermines Dietary Efforts to Reduce Adiposity. Annals Internal Medicine 153, 435-441.

Spiegel, K. et. al, (2004). Sleep Curtailment in Healthy Young Men is Associated with Decreased Leptin Levels, Elevated Ghrelin Levels, and Increased Hunger and Appetite. Annals of Internal Medicine, 141: (11) 846-85.

Rogers, M. A. and Evans, W. J. (1993). Changes in skeletal muscle with aging: Effects of exercise training. In J. O. Holloszy (Ed), Exercise and Sport Science Reviews. Baltimore: Williams and Wilkins.

The Total Body Conditioning Plan, Wood, M, (2016).

3 Key Factors Needed to Build Muscle and Strength

imagesThere are three key ingredients that you need as part of your recipe to successfully build muscle and increase strength.

Adequate Training Stimulus. Without this training stimulus it just won’t happen and if you’re not getting results, the odds are that you – like most people – will stop exercising within the first six months of starting. You need to focus on overloading your muscles in a progressive manner, pushing your muscles to momentary failure with each set of exercise. This happens only after you develop a strong base level of strength. The same hold true if you’re looking to improve aerobic capacity on the cardio side. Try adding in a few days of high-intensity interval training into the mix, using protocols like Tabata (20 seconds of hard work, 10 seconds of recovery x 8 rounds) or a Gibala protocol, 30 seconds of high intensity work followed by four minutes of recovery repeated x 4 rounds. This can be done on a bike, rowing machine, elliptical, sprint work etc.

Adequate Recovery and Protein Intake. This is where many drop the ball. It’s very difficult to get plenty of recovery between workouts while making sure your body is getting the required amount of protein to maximize protein synthesis. See how your body responds to 1 gram of protein/kilogram of body weight and slowly progress to 1 gram of protein/pound of body weight if needed. Here is what that might look like when I plug-in my own body weight:

1 gram/kilogram of body weight (228 lbs/2.2 = 104 kilograms or 104 grams of protein/day).

1 gram/pound of body weight (228 x 1 gram = 228 grams of protein/day).

To make this happens you will probably need to take in 20-30 grams of protein with each meal and snack. A good way to ensure this happens is to drink a whey protein drink especially post workout and before bed.

Adequate Sleep. Another difficult area for many people. Your goal is 7-8 hours of uninterrupted sleep each night. If not the body’s hormonal system can get out of whack. Hormones like cortisol (known as the stress hormone) can increase with insufficient sleep. Read the following study here and article here. Researcher and author, Charles Poliquin puts it nicely into perspective:

“lack of sleep is like the opposite of strength training”

All three of these variables are under your control. You can manage this and you can definitely have success with it, you just need to choose to “commit to get fit.”