Current Issues in Nutrition Research
As in years past, the American Dietetic Association (ADA) Conference, held October 19 through 22, 2002, provided attendees plenty of food for thought. The following session topics were among those of most interest to health and fitness professionals.
Rarely a day goes by without the media reporting on the findings from yet another new research study. When that study involves nutrition or exercise, our job as fitness professionals is to help our clients understand the bottom line from a 10-second sound bite. But more and more frequently, even the experts among us have trouble deciphering the real story. To assist clients in locating, comparing and interpreting study findings, fitness professionals need a solid understanding of research practices and methods.
When reviewing and comparing research, it’s helpful to keep in mind the following hierarchy of scientific evidence, shown in order of most to least reliable:
- multicenter, randomized control clinical trials
- longitudinal prospective cohort studies
- case/control studies
- case studies
- anecdotal studies
Each of these types of studies has its advantages and disadvantages, according to ADA session presenter Rachel Johnson, PhD, MPH, RD. For example, clinical trials, the “gold standard” of research studies, are typically experiments conducted on human subjects to evaluate the benefits and risks of a given treatment. In clinical trials, treatment groups must be randomized, a control group is required and investigators must be blinded/masked; also, the sample size must be large enough to allow differences to be detected. Advantages of this type of research are that study conditions are at least partially controlled and a cause-and-effect relationship can be observed. Disadvantages include the challenges involved in maintaining an adequate number of participants and the extraordinary costs of conducting such large-scale studies. An example of a clinical trial is the ongoing Women’s Health Initiative, which recently reported its findings on the risks of taking hormone replacement therapy that combines estrogen and progestin.
Cohort studies are observational examinations that compare the outcome of one group that is exposed to a potential hazard or disease with the outcome of another group that is not exposed. Unlike clinical trials, which focus on the risks/benefits of experimental treatments, cohort studies focus on hazard exposure versus nonexposure. The advantage of cohort studies is that they focus on hazard exposure before a disease develops; an example would be studying a group of smokers prior to the onset of lung disease. The disadvantages of cohort studies are that they require a large number of subjects; the follow-up period must be lengthy so that diseases have time to develop; and they are costly. One example of a cohort study is the Nurses’ Health Study, which recently released a follow-up report on body mass index (BMI) and its relationship to stroke and heart disease.
Like cohort studies, case/control studies are observational in nature; however, in case/control studies the focus is on subjects who have already developed a certain disease. Researchers then compare the afflicted participants’ past exposure to suspected risk factors with the exposure of controls who do not have the disease. An advantage of case/control studies is that they are useful for identifying rare exposures that may be important causes of disease. The disadvantage is that their design is subject to many potential biases.
Case studies are detailed analyses of persons or groups and usually focus on social, psychological or medical phenomena. The advantages of case studies are that they serve as catalysts for critical thinking and as springboards for framing quantitative questions. The disadvantage is that these studies are not considered as scientifically rigorous as other forms of research.
Finally, anecdotal studies provide information from personal experiences but are not based on fact or proper study design. The advantage is that many similar anecdotal reports may spur on future research. The disadvantage is that the results of anecdotal studies are not reliable.
During her session, Johnson reminded attendees that nutrition research involves a unique component: the recording of dietary intake data to establish the links between nutrition and disease. Researchers use several methods to collect dietary intake data: food records, food-frequency questionnaires, 24-hour recall and diet histories. Because it is common for research subjects to underreport their dietary intakes, it is critical that researchers validate the intake methodologies employed in a given study. Food underreporting occurs for several reasons. First, tracking intake requires a high degree of subject cooperation. Second, subjects often want to appear to be eating less than they really are. (According to Johnson, “The best way to get overweight people to lose weight is to put them on the diet they tell you they eat!) Subjects also underreport food intake because of psychological factors, such as body dissatisfaction or an unwillingness to mention “bad” foods. Conventional wisdom among nutrition experts is that underreporting does not provide a realistic picture of what clients are eating, so any conclusions based on underrreported data may not be reliable.
To help consumers make sense of media health reports, presenter David Frankenfield, MS, RD, recommended that health and fitness professionals
- stay current on the body of literature for important health issues
- make time to read the latest research
- know the most valid sources of information (evidence-based practice journals, credible online resources, etc.)
- learn to read efficiently and effectively (For example, systematic reviews or meta-analyses, which summarize the results of many related studies and present a consensus among experts, are more time-effective and scientifically rigorous than individual reports
- determine the level of confidence to give to a particular study or author (Is the journal peer-reviewed? Does the author have a bias or conflict of interest?)
For years, Americans have been urged to choose a diet moderate in sugars. The government’s current Dietary Reference Intakes for macronutrients suggest that added sugars should make up no more than 25 percent of total daily caloric intake (National Research Council 1989). But many people are still confused as to how much and what kind of sugar to consume. As is often the case with nutrition information, there is no simple answer, according to presenter Suzanne Murphy, PhD, RD. To illustrate her point, she posed and then attempted to answer the following questions:
Should Added Sugars Be Distinguished From Naturally Occurring Sugars? One thing the public is confused about is the difference between sugar that is “added” to the diet and sugar that occurs naturally in foods such as fruit and milk. Nutrition experts advise clients to moderate their intake of the former but not the latter. However, because “total sugars” (i.e., both added and naturally occurring sugars) are shown as a single value on Nutrition Facts labels, consumers are unable to distinguish between the two.
What Health Effects Can Be Directly Attributed to Sugars in the Diet? The only health effect of sugars that has been conclusively demonstrated is their association with dental cavities. However, copresenter Julie Miller Jones, PhD, stated that some preliminary data support a link between high sucrose intake and increased triglyceride levels in obese subjects.
Could Added Sugars Play a Role in the Obesity Epidemic? You may be surprised to learn that there is no strong evidence to support a relationship between sugars intake and BMI. Some population studies have even found an inverse relationship between sugars intake and BMI, but that’s probably because overweight individuals typically underreport the sugars they consume. However, we do know for certain that consuming too many calories contributes to obesity, and we also know that many foods high in sugar are also high in fat.
Does Sugars Intake Negatively Affect Nutrient Adequacy? Additional intervention studies need to be conducted to determine if excessive sugars intake results in poorer food choices in general. For example, we know that replacing milk with soft drinks decreases the amount of calcium consumed. But we don’t know whether decreasing sugars intake results in higher consumption of more nutritious foods.
Answering Your Clients’ Sugar Questions. Copresenter Diane Quagliani, MBA, RD, stressed that, when conveying messages about sweets to consumers, it is important to take the ADA’s position that there are no “good” or “bad” foods and that eating should be pleasurable. To help clients moderate their sugars intake without excessive worry, here are some tips shared by the presenters:
- Split a dessert with a friend or take half home to enjoy the next day.
- Don’t ban sweets from kids’ diets; by occasionally being permitted a few sweet treats, kids are less likely to overdo it.
- Order a small or medium soft drink instead of the extra large size, and skip the refills.
We all know that the baby boomers are rapidly aging and people in general have a longer life expectancy. But in order to help clients maintain their athletic abilities as they age, we need to understand the challenges specific to older adults who exercise and compete. Presenter Christine Rosenbloom, PhD, RD, shed insight into seniors’ unique needs with regard to energy (calories), carbohydrates, protein, fat, micronutrients, minerals, hydration and special medical concerns.
Energy (Calories). The most important factor when determining anyone’s energy needs is energy expenditure. The energy needs of older athletes are higher than those of sedentary older adults. However, the energy intakes of older athletes tend to be lower than those observed in younger athletes with similar training. This could be attributed to age-related decreases in muscle mass, reduced training volumes or reduced leisure-time activities.
Carbohydrates. Glycogen storage per unit of muscle is lower in older endurance athletes than it is in similarly trained younger athletes, whereas utilization of glycogen stores is higher. However, older athletes are able to increase their muscle glycogen levels and renew stores effectively through proper training and postexercise refueling.
Protein. For older athletes, as for their younger counterparts, protein needs depend on energy intake, the ability to repair tissue, and the mode and intensity of the exercise. Evidence suggests that all people need a bit more protein as they get older.
Fat. The ability to digest, absorb and use fat doesn’t change as people age. However, it is prudent to recommend that all older adults reduce their intake of saturated and trans fats, while increasing their intake of omega-3 fatty acids.
Micronutrients. Because they eat less and physiological changes affect their digestion/absorption of food, older athletes may be at greater risk of micronutrient deficiencies than younger athletes.
Minerals. Preliminary research indicates that older athletes may have a greater need for calcium, zinc and magnesium than their younger competitors.
Hydration. Clinical data suggest that older adults tend to maintain a state of dehydration. With age, kidney function decreases, thirst signals become dulled and the body retains less water. According to copresenter Robert Kenefick, PhD, dehydration can affect an older athlete’s ability to thermoregulate, which ultimately impairs performance. To ensure that older athletes are properly hydrated before, during and after competitive events, share the recommendations outlined for all exercisers in the American College of Sports Medicine’s (ACSM) position stand on fluid replacement (see “Water: The Neglected Nutrient” on page 40.)
It’s no secret that childhood obesity is on the rise. According to presenter Rebecca Kirby, MD, MS, RD, the number of overweight/ obese children and adolescents has more than doubled since 1976 in the United States, and this trend is being seen in other industrialized and developing countries as well. At particular risk are boys and African-American and Hispanic children of both genders. (Overweight/obesity in children is defined as having a BMI in the 95th percentile or higher for age and sex.)
Many factors contribute to childhood obesity. Influences include poor nutrition; inadequate or no physical activity; psychosocial and environmental factors; genetics; and factor X—what we do not know or cannot define. According to Kirby, it’s time for a call to action, and she believes federal agencies, state and local governments, private foundations and industry should all get involved.
Unfortunately, even with the support of federal agencies and private foundations, economic considerations often block or slow progress in combating childhood obesity, said copresenter Sylvia Moore, PhD, RD. For example, the Centers for Disease Control and Prevention’s (CDC) Division of Nutrition and Physical Activity spent $27.6 million in 2002, but this was allotted to only 12 state-based core programs. The CDC’s Youth Media Campaign’s 2002 budget was $68.4 million, down from $125 million in 2001. And the National Cancer Institute’s “5 a Day” program, designed to encourage kids to increase their intake of fruits and vegetables, has an annual budget of just $1 million, which is dwarfed by the multibillion-dollar advertising budgets of fast-food and soft-drink corporations.
Why are so few dollars devoted to promoting kids’ health? Moore cited several reasons, including cost, the complexity of interventions and special-interest-group dynamics. She went on to recommend new ways to approach the childhood obesity problem—for example, emphasizing nutrition in conjunction with physical activity, implementing more community-based programs, creating new policy models and forging new links among different sectors. By way of example, Moore cited a program called “Wellness in the Rockies,” a community-based research, development and education project designed to improve health among Idaho, Montana and Wyoming residents. The program’s charter is to teach consumers to value health; to respect body size differences; to gain self-acceptance; and to enjoy both physical activity and healthful, pleasurable eating.
The glycemic index (GI) ranks carbohydrate foods based on how quickly the carbohydrates enter the bloodstream and elevate blood sugar levels. The GI is determined by comparing the blood-glucose-raising effects of 50 grams (g) of carbohydrate from a particular carbohydrate food to the blood-glucose-raising effects of 50 g of glucose (considered to be the standard) over a specific period of time.
Many factors affect the GI value of a particular food, including its variety, whether it was processed and how it was prepared. For example, there are many varieties of rice, each with its own type of starch. In the case of a boiled potato, the GI can be increased by as much as 25 percent if it is mashed as opposed to served in 1-inch cubes. Even subtle differences in how ripe a banana is can double its GI. GIs can vary widely, although the foods contained in the average American diet tend to be in the 60 to 100 range.
Presenter Jeanne Goldberg, PhD, RD, said that few consumers know about the GI and those who do are confused. “It is extremely seductive to make things as simple as ‘High GI foods are bad; low GI foods are good,’” she cautioned.
In fact, according to copresenter F. Xavier Pi-Sunyer, MD, a food’s GI may actually depend more on an individual’s response to the food than on the food itself. For example, every individual will have a different reaction to the physical form of the food, its degree of starch gelatinization, its amylose-to-amylopectin ratio and its fiber, sugar, acidity and fat mixture. Even the way carbohydrates are absorbed is affected by individual factors, such as the viscosity of a person’s gut contents and the rate of gastric emptying.
Owing to all these variables, many dietitians are reluctant to use the GI alone to determine the nutritional quality of carbohydrate foods. Pi-Sunyer stressed that further research on the link between GI and health is needed. In the meantime, he advised that instead of obsessing over a food’s GI, clients be advised to do the following:
- Achieve and maintain normal weight.
- Increase physical activity.
- Decrease total calories.
- Decrease LDL (bad) cholesterol levels.
- Decrease saturated fat.
- Decrease total cholesterol levels.
- Increase dietary fiber. ‰
Kathryn Kolasa, PhD, RD, opened her session on water recommendations by saying,“When I was a student, we spent 5 minutes talking about the nutrient water.” She then went on to discuss how the barrage of information available about water today has generated a lot of confusion. Recently, media reports have questioned even long-standing hydration advice, such as the recommendation to drink eight 8-ounce glasses of water a day.
According to copresenter Joe Garry, MD, much of what we know about hydration was gleaned by studying athletes. Drawing on this body of research, Garry addressed the following aspects of hydration:
Dehydration. Dehydration has a number of adverse consequences, least among them an increased risk of heat-related illness. The importance of maintaining adequate hydration during exercise cannot be stressed enough to clients, especially athletes, since as little as 2 percent dehydration can negatively affect performance. The degree of dehydration that can occur depends on the exercise type, duration and intensity; the climate; the client’s sweat rate; and his or her hydration status at the onset of exercise. According to ACSM, athletes should be advised to drink 16 to 20 ounces of noncarbonated, noncaffeinated fluid approximately 2 hours before physical activity (ACSM 1996). During activity, they should drink enough fluid to match sweat and urinary losses; this translates to 4 to 10 ounces (0.5-1.5 cups) of fluid every 15 to 20 minutes.
Rehydration. Complete rehydration requires complete fluid restoration. Sodium is necessary in facilitating fluid restoration, with most sodium ideally gleaned through food. According to ACSM, after a workout, exercisers should consume approximately 1 pint (2 cups) of fluid for every pound of body weight lost (ACSM 1996). Garry recommended upping this amount to 24 ounces, or 3 cups of replacement fluid.
Overhydration. While relatively uncommon, overhydration can result in fluid overload; this, in turn, can dilute the amount of sodium in the blood, resulting in hyponatremia (low sodium levels).
Water vs. Sports Drinks. According to ACSM, drinks containing carbohydrate may be beneficial to people exercising for longer than 60 minutes (ACSM 1996). During longer events, exercisers should consider drinking fluids that contain electrolytes, to decrease the risk of hyponatremia.
Copresenter Ann Grandjean, EdD, reminded attendees that the amount of sweat lost during exercise is affected by many personal factors, including fitness level, clothing and age, and by environmental factors such as air temperature, humidity, wind velocity and cloud cover. Even up to 72 hours after exercise, body weight can fluctuate as a result of water loss.
On a practical note, Grandjean discussed different sources of water available in the average American diet. She said most people don’t realize that, in addition to water, many foods—especially fruits and vegetables—can contribute to fluid intake. As far as what types of beverages count toward total daily intake, Grandjean cited research showing that caffeine-containing beverages do not have a dehydrating effect on younger adults. However, she emphasized that the opposite holds true for alcohol.
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- Small amounts of sodium have been shown to enhance fluid retention, so suggest that older athletes increase their sodium intake through diet or with a sports drink. Depending on physiological and environmental factors, exercisers can lose as much as 2,500 to 5,000 milligrams (mg) of sweat per hour; the resulting sodium loss can be problematic for those on a low-sodium (< 2,500 mg per day) diet. Good sources of sodium include one cup of tomato juice (882 mg sodium) or one cup of Gatorade® (110 mg sodium).
- Fluids cooler than the ambient temperature and flavored to enhance palatability can enhance seniors’ fluid replacement.
- Carbonated beverages can cause stomach bloating or throat burn, which may discourage older clients from consuming enough fluid to stay hydrated.
- Caffeine and alcohol can increase diuresis and fluid loss.
- Seniors typically have less thirst sensation than younger people and may have difficulty retaining fluid, so older adults should consume fluids on a regular schedule, regardless of thirst.
Special Medical Concerns. Seniors’ athletic performance can be affected by chronic diseases such as hypertension, cardiovascular disease, asthma and visual impairment. Medications may also have an effect, especially diuretics, cardiac glycosides and nonsteroidal anti-inflammatory drugs (NSAIDs).
Grandjean, A.C., et al. 2000. The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration. Journal of the American College of Nutrition, 19, 591-600.
McGinnis, J.M., et al. 2002. The case for more active policy attention to health promotion. Health Affairs, 21 (2), 78-91.
National Research Council. 1989. Recommended Dietary Allowances. National Academy of Sciences.
Noakes, T.D. 1993. Fluid replacement during exercise. Exercise Sports Science Review, 21, 297-330.
Parks, E.J., & Hellerstein, M.K. 2000. Carbohydrate-induced hypertriacylglycerolemia: Historical perspective and review of biological mechanisms. American Journal of Clinical Nutrition, 71, 412-33.
Saris, W.H., et al. 2002. The CARMEN Trial: Increased intake of carbohydrates—simple or complex—and unchanged blood lipids in overweight subjects. Ugeskr Laeger, 164 (14).
Volpi, E., et al. 1998. Exogenous amino acids stimulate net muscle protein synthesis in the elderly. Journal of Clinical Investigation, 101 (9), 2000-7.
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