Have you ever been uncertain or confused by something you heard or read about nutrition? Nutrition misinformation abounds in the popular media, since freedom of speech and of the press allows unqualified individuals to promote unsubstantiated ideas. In addition, scientific understanding of human nutritional needs continues to evolve, leading to seemingly conflicting recommendations. Where can you find help in separating fact from fiction in the field of nutrition?
A Registered Dietitian (R.D.) is a nutrition specialist with a credential from the American Dietetic Association, the largest organization of nutrition professionals in the United States. A Registered Dietitian must have a minimum of a Bachelor's degree from an accredited university, must complete an internship, and must pass a nationally-administered examination. In addition, R.D.s must maintain their credentials through continuing education. These nutrition experts specialize in translating the science of nutrition into practical advice. Seek out a Registered Dietitian for personalized nutrition counseling and individualized eating plans. You can locate an R.D. by looking in the phone book under “Nutritionists and Dietitians”, by contacting a local hospital or health center, or by consulting the American Dietetic Association.
Keep in mind that there are no editorial boards or regulatory agencies for nutrition information that appears on-line. Therefore, stick to the web sites of recognized organizations (.org), the government (.gov), or universities (.edu). Here are several good ones to check out:
American Dietetic Association This site allows you to access food and nutrition information or find a nutrition professional.
American Heart Association This site provides information on heart disease including healthy lifestyle/heart disease prevention recommendations.
U.S. Department of Agriculture Here you can look up the calories or nutrients in a food, learn about federal food assistance programs, and link to the my pyramid food guidance system.
Another USDA web site. This site provides information related to the food pyramid which guides individuals in making food choices consistent with the 2005 Dietary Guidelines for Americans. It includes information on related topics such as recommended amounts of physical activity and includes special sections directed at children.
National Heart, Lung, and Blood Institute This site provides information and publications that address a wide range of health issues, including heart disease and high blood pressure. There is also information regarding the We Can program - “Ways to Enhance Children's Activity and Nutrition”.
Articles that appear in professional journals (such as The Journal of the American Dietetic Association, The American Journal of Clinical Nutrition, The Journal of the American Medical Association, etc.) are subject to peer review. Thus, the information they provide tends to be very accurate, but is not presented in a very consumer friendly format. Among consumer periodicals, look for magazines that have credentialed staff (R.D.s, M.D.s, Ph.D.s) on their editorial boards. A couple of particularly good resources are Cooking Light and EatingWell magazines. Periodical newsletters, such as the Mayo Clinic Health Letter, Nutrition Action Healthletter, U.C. Berkeley Wellness Letter, and the Tufts University Health and Nutrition Letter provide a wealth of current information.
When it comes to books, check the credentials of the author(s) before taking their words to heart; the proliferation of titles (especially in the weight loss field) is an indication that many of these programs are not the miracles they may claim to be. Here are a few good references to get you started:
The American Dietetic Association Complete Food and Nutrition Guide Roberta Larson Duyff, MS,RD,FADA,CFCS. 2002. Hardbound ISBN: 0-471-22924-5; Softbound ISBN: 0-471-44144-9.
Nancy Clark's Sports Nutrition Guidebook Nancy Clark, MS, RD. 2003. ISBN: 0-73604602X.
Ultimate Sports Nutrition Ellen Coleman, RD, MA, MPH and Suzanne Nelson Steen DSc, RD. 2000. ISBN: 0-923521-56-9.
Remember, good nutrition isn't magic; it's the application of common sense and good judgment to the best available information. If you come across something that promises miracles, it probably isn't true. The resources described above can guide you on the path towards better health and physical performance.
What is the most essential nutrient for active individuals? Is it protein? Carbohydrates? Vitamins? Well, in fact, it's water, the most abundant compound in the human body (making up 60% or more of body weight) and the milieu in which all the biochemical reactions required for life and physical performance take place. As little as a 2% loss in body weight through dehydration (loss of fluid) can have a negative impact on health and especially on the ability to train or compete in an athletic activity.
For normal, healthy individuals, thirst is usually an adequate indicator of the need for fluid. As long as an individual has access to a variety of fluids (water, milk, juice, coffee, tea, etc.) and foods that contain water (fruits, vegetables, soups, etc.), he/she is likely to stay adequately hydrated. Interestingly, recent reviews of scientific studies suggest that even caffeine-containing beverages contribute to an individual's hydration status, in spite of the fact that caffeine is known to act as a weak diuretic (chemical that causes water loss).
Activity increases water loss in the form of sweat. The amount of fluid lost depends on the individual's tendency to sweat, the environmental conditions (temperature and humidity) under which the activity is performed, and the intensity and duration of the activity. Since hydration status is critical for optimal performance and the regulation of body temperature, competitive athletes are encouraged to drink according to a schedule:
Hydrate before a practice or competition
2-3 hours before: ~16 fluid ounces (2 cups)
10-20 minutes before: ~8 fluid ounces (1 cup)
Drink during workouts or competition
Every 15-30 minutes: ~4 fluid ounces (1/2 cup) or amount determined by training experience
Replace sweat losses after competition
Drink ~ 16 fluid ounces (2 cups) for every pound of body weight lost during activity (or approximately 1 fluid ounce per ounce of body weight lost as determined by subtracting weight after exercise from weight prior to exercise).
In general, for activity lasting less than 1 hour, water is a good choice for fluid replacement. However, formulated sports drinks may be a good option for those who are more likely to drink enough of a flavored beverage (e.g. very active children). For activity lasting longer than an hour, formulated sports drinks are advisable because they contain some carbohydrates for energy and minerals to replace those lost in sweat.
The dangers of dehydration, which can lead to hyperthermia or excessive body temperature, cardiovascular system failure, and death, have been well publicized. More recently, reports of excessive drinking leading to a condition called hyponatremia (low blood sodium levels) have come to light. This is a less frequent but equally serious problem, since hyponatremia causes swelling in the brain that can lead to seizures, coma, and even death. This condition is most likely in individuals who exercise for extended periods of time (4 hours or more such as hikers, long-distance cyclists, and marathoners) if they drink excessively while exercising, compounded by losing sodium in their sweat. To prevent hyponatremia, athletes need to become accustomed to their typical fluid needs during activity by weighing themselves before and after training sessions. In addition to avoiding over-drinking during activity, they should also consume sodium in the form of food or a sports drink during activity lasting longer than 2 hours.
Fluid needs are highly individual. In most cases, thirst and plain water are sufficient for keeping an individual well hydrated. However, the more active an individual is, and the longer and more intense the activity, the more important it becomes to use body weight changes as an indicator of fluid needs, and the more useful specially formulated sports drinks become.
For further information, go to the website of the Gatorade Sports Science Institute, or consult the chapter “Fluid, Electrolytes, and Exercise” in Sports Nutrition, A Practice Manual for Professionals, 4th edition (ISBN: 0-88091-411-4), edited by Marie Dunford, PhD, RD, published by the American Dietetic Association.
Body fat is a major source of fuel for daily energy needs, helps maintain body temperature, and protects vital organs from injury. Thus, a certain amount of body fat is essential, whereas too much body fat is associated with an increased risk of developing high blood pressure, diabetes, high blood cholesterol levels and heart disease.
Fat in food also plays several important roles: it serves as a concentrated source of energy, improves the palatability (taste and texture) of some foods, increases the satiety value of food (increases the amount of time needed to digest and absorb food, thus delaying the recurrence of hunger), helps with the transport and absorption of fat soluble vitamins, and provides essential fatty acids (fatty acids that humans can't synthesize but must have to form other essential compounds). Therefore, a certain amount of dietary fat is necessary, but too much can be harmful as it frequently leads to the over-consumption of energy and the development of obesity and high blood cholesterol levels.
Both body fat and food fats exist primarily as chemical compounds called triglycerides, also known as triacylglycerols. These compounds consist of three fatty acids attached to a glycerol molecule. Fatty acids are mainly chains of carbon and hydrogen atoms. They differ in length (the number of carbon atoms in the chain) and the presence and position of double bonds between the carbon atoms. These attributes in turn determine the physical characteristics of the fat (for example, its melting point or how hard it is at room temperature) and its physiological effect (how it's used in the body and whether or not it raises blood cholesterol levels). All fats contain a mixture of fatty acids, but generally one type tends to predominate.
A saturated fatty acid has no double bonds in its hydrocarbon chain. Saturated fatty acids have higher melting points and thus tend to be solid at room temperature. Examples of foods containing mostly saturated fatty acids include meat, poultry, and dairy products, as well as coconut, palm, and palm kernel oils. Eating too much saturated fat tends to raise blood cholesterol levels.
A monounsaturated fatty acid has one double bond in its hydrocarbon chain. These fatty acids have a lower melting point than the saturated fatty acids, and thus tend to be liquid at room temperature. Examples of foods high in monounsaturated fat include canola oil, olive oil, and peanuts/peanut oil. Eating monounsaturated fat in place of more saturated fat appears to have beneficial effects on blood cholesterol levels (lowering LDL or low density lipoprotein levels while preserving HDL or high density lipoprotein levels) as long as energy needs are not exceeded.
A polyunsaturated fatty acid has more than one double bond in its hydrocarbon chain. Polyunsaturated fats have relatively low melting points and thus are liquid at room temperature. They are named according to the position of one of their double bonds.
Omega-6 fatty acids have their first double bond on the 6th carbon atom from one end of the chain. Omega-6 fatty acids predominate in vegetable oils such as corn, safflower, soybean, and sunflower. Consuming this type of fat appears to lower total cholesterol levels, both the harmful LDL-cholesterol and the beneficial HDL-cholesterol. One particular omega-6 polyunsaturated fatty acid, linoleic acid, is the primary essential fatty acid - a fatty acid that must be obtained from food. In the body, linoleic acid is converted into longer chain polyunsaturated fatty acids that are important components of cell membranes and other compounds.
Omega-3 fatty acids have their first double bond on the 3rd carbon atom from one end of the hydrocarbon chain. Omega-3 fatty acids occur primarily in fish, especially high-fat types like salmon and sardines. They are also found in some plant foods like walnuts and flax seed. Eating omega-3 fatty acids appears to have beneficial effects beyond cholesterol lowering alone. For example, dietary omega-3 fat appears to reduce inflammation and the clumping of platelets in the blood, factors which may contribute to the development of heart disease.
Trans fatty acids are monounsaturated or polyunsaturated fatty acids in which the hydrocarbon chain of a fatty acid is arranged on opposite sides of a double bond (trans means "across"). This chemical configuration, or shape, is different from most naturally occurring mono- or polyunsaturated fats in which the hydrocarbon chain is arranged on the same side of a double bond (the cis configuration; cis means "same"). While some trans fats occur naturally in food, most are created by hydrogenation, a process applied to liquid vegetable oils to make them more solid to improve their cooking characteristics and keeping qualities. Unfortunately, trans fats tend to raise blood LDL-cholesterol levels, acting more like saturated fats than the unsaturated fats from which they were made. Foods that contain trans fats include shortening, margarine, baked goods, and snack foods. Recently, many manufacturers have reformulated products to reduce or eliminate trans fat and, starting in January, 2006, must list the trans fat contents of products on their Nutrition Facts labels.
Cholesterol is a waxy, fat-like substance that occurs only in foods of animal origin. The richest food sources are egg yolks and organ meats like liver. Cholesterol in food does not automatically become cholesterol in blood, and cholesterol is manufactured by body cells even in the absence of dietary cholesterol. The liver processes cholesterol absorbed from food, some of which may be used for important functions like cell membrane components and steroid hormones. High intakes of dietary cholesterol may raise blood cholesterol levels, but are not as strong of an influence as saturated and trans fats intakes.
General guidelines for fat consumption promoted by the American Heart Association and the United States Department of Agriculture are:
In practical terms, this translates into:
For more information regarding fats and their relationship to health, visit the web sites of the American Heart Association and the American Dietetic Association.
People often wonder what to eat right before they exercise or compete. What many fail to recognize is that the fuel sources their muscles rely on are dependent upon what they ate (and how they trained) for days and weeks ahead of time. In broad terms, muscle cells need energy from food to provide fuel for muscular work along with vitamins and minerals to participate in and regulate the biochemical reactions that yield energy.
People obtain energy from the food nutrients protein, carbohydrate, and fat. The building block components of these nutrients are absorbed from digested food and used or stored as needed.
Dietary protein is digested into building blocks known as amino acids. Absorbed amino acids are used to create proteins that the body needs such as new or replacement tissues and regulatory hormones. If the amino acids are not needed for protein synthesis at the time they are available, nitrogen is removed from them in the kidneys, and the remaining hydrocarbon skeletons may be used for energy or stored as fat. Thus, excess dietary protein is not stored in a form in which it can be retrieved as protein. It follows, then, that eating a lot of protein at one time is wasteful since it can't be saved for later use as protein, and that the body requires a fresh supply of amino acids from dietary protein at regular intervals. Dietary proteins are not a major source of energy for muscular work, but an adequate intake is important for optimal physical performance over the long run to provide the raw materials needed to synthesize muscle and other vital tissues and biochemicals.
Dietary carbohydrates are digested into simple sugar units called monosaccharides: glucose, fructose, and galactose. In the liver, fructose and galactose are converted into glucose, the form of carbohydrate that circulates in the blood to be taken up as needed by body tissues. Glucose can be burned for energy immediately, or stored to a limited extent as chains of glucose molecules called glycogen in the liver and muscle tissues. The amount of glucose that can be stored as glycogen depends upon an individual's inherent capacity and level of training. Glycogen stores can be maximized by diet and training modifications. Carbohydrates eaten in excess of immediate energy and glycogen storage needs are converted into fatty acids and stored as triglycerides (fat) in adipose (fat) tissue.
Dietary fats (triglycerides) are digested into fatty acids, absorbed, and transported in the bloodstream as part of lipoproteins or fat particles called chylomicrons. Fatty acids from these particles are taken up by tissues as needed and either used to make cell components (e.g. cell membranes), burned for energy, or synthesized into triglycerides that are stored within skeletal muscles and, to a much greater extent, in adipose (fat) tissue. When needed for energy, these stored triglycerides are again broken down into fatty acids that can be used as fuel for muscular activity.
Muscular work requires energy to fuel the contraction of muscle cells. Energy is stored in three forms in muscle cells: as the chemical compound creatine phosphate, as glycogen, and as fat. Energy stored within the chemical bonds of these substances is released and used to make adenosine triphosphate or ATP. There are three different systems used by cells to produce ATP: the phosphagen system, anaerobic glycolysis, and the aerobic system which consists of the Krebs cycle and the electron transport chain. What type of fuel is used and how the energy is produced depends upon the intensity and duration of the activity.
For very short bouts of very intense activity lasting less than 30 seconds (e.g. weight lifts, sprints and throws), the phosphagen system is the primary energy producer. This system uses creatine phosphate stored within the muscle cells as fuel. Creatine phosphate stores in muscle cells are very limited, so this system is the primary energy producer only for very short (several seconds) activities. Creatine phosphate is synthesized by the body, but is also available in foods, especially those of animal origin. There is some evidence that creatine supplements can increase the amount of creatine stored within muscle cells and improve performance in a laboratory setting, but not yet enough evidence to prove that this translates into improved performance in the field.
Short duration high intensity activities lasting somewhat longer (up to about 2 minutes such as longer sprints or the bursts of activity required in sports like basketball, soccer, etc.) use anaerobic glycolysis (breakdown of glucose without oxygen) as the primary mode of energy production. This system uses only glucose as fuel. Because anaerobic glycolysis creates energy faster than oxygen can be delivered to the muscle cell, not all the chemical energy of the glucose molecule is used, and the glucose molecule is converted into the waste product lactic acid in the process. Thus, there are two limits to performance that relies upon anaerobic glycolysis: the availability of glucose for fuel, and the ability to clear lactic acid from the working muscles. The amount of glucose available to fuel anaerobic glycolysis depends on the amount of glycogen stored within the muscle tissue, which in turn is influenced by dietary carbohydrate intake. Individuals who exercise or train strenuously, particularly on successive days, need to consume a relatively high carbohydrate diet to maximize their glycogen stores. Diet/training modifications to super-load glycogen stores are usually not recommended for short duration activities because water is stored with glycogen, which can lead to feelings of stiffness or heaviness that may interfere with performance. More important for this type of exercise is training to improve the body's capacity to clear lactic acid build-up from the muscles.
Longer duration low and moderate intensity activities depend mainly on energy generated by the aerobic system. During aerobic activity, the presence of oxygen enables muscle cells to utilize all three energy nutrients (carbohydrate, fat, and protein) for fuel, and to metabolize these nutrients completely to the end waste products of carbon dioxide and water. The rate of energy production via aerobic metabolism is slower than via anaerobic metabolism or the phosphagen system, but the total amount of energy generated per molecule of glucose is much greater, and, since fatty acids can also be utilized as a fuel source, the overall amount of energy that can be produced is very large. However, a compound derived from glucose must be present in order for fatty acids to be metabolized completely via the aerobic system. Thus, most of the energy used to fuel aerobic activity comes from fatty acids, but the depletion (using up) of glycogen stores can limit aerobic performance because it is associated with feelings of fatigue. Those who participate in aerobic activities such as bicycling, running, and swimming long distances benefit from eating a relatively high carbohydrate diet to maximize their glycogen stores.
In addition, training to improve cardiorespiratory conditioning is important. As cardiorespiratory conditioning improves, the ability to deliver oxygen to working muscles increases, enabling a greater reliance on the relatively unlimited energy resource of fatty acids which spares the utilization of limited glycogen stores and delays the onset of fatigue. Specialized diet and training regimens can be used to enhance glycogen stores, but have some drawbacks. For example, water stored in muscles along with glycogen may cause feelings of stiffness and heaviness that interfere with performance. In addition, many competitive athletes resist the tapering or reduction in training that is necessary to maximize glycogen stores.
Many vitamins and minerals play important roles in energy production. For example, the B vitamins (thiamin, riboflavin and niacin) participate as coenzymes in glycolysis, the Krebs cycle, and the electron transport chain. Iron is essential as part of hemoglobin in blood and myoglobin in muscle tissue and thus is needed for oxygen transport and aerobic metabolism. Vitamin C is involved in collagen (connective tissue) synthesis and functions as an antioxidant.
As critical as various vitamins and minerals are to optimum performance, it is generally easier for active individuals to meet their needs than non-active, because their greater energy expenditure allows them to eat more food. Generally, exercise does not appear to increase vitamin and mineral needs, with the possible exceptions of increased vitamin C and vitamin E requirements for those who participate in a lot of endurance activity. Even for these vitamins, the additional amount is easily supplied by food. A good rule of thumb for active individuals is to take a daily multivitamin/mineral supplement that provides approximately 100% of the “Daily Value” of a broad range of vitamins and minerals. Making good food choices is more important than taking supplements since the proportions of dietary carbohydrate, fat, and protein influence overall health and the risk of developing chronic diseases as well as fuel supplies for physical performance.
Since the fuel (glycogen and fat) needed for exercise was stored on preceding days, eating what feels best is the most important consideration. Carbohydrates empty relatively quickly from the stomach, whereas fat slows down the rate of emptying, so a high carbohydrate, low fat meal is a good choice two to four hours prior to exercising. Fluid for hydration is also advisable. Some examples of good pre-exercise meals include cereal with fat free milk and fruit, pasta with tomato sauce (easy on the meat or cheese), a bagel with a little low fat cream cheese and fruit juice, a lean meat (e.g. turkey or ham) sandwich (hold the mayo) or pancakes (no butter) and syrup with fat free milk or juice. Meal replacement bars are another possibility, although they are generally more expensive and less nutritious than real foods.
Eating after exercise is important for replenishing depleted glycogen stores. Conditions during the 30 - 60 minutes immediately following strenuous exercise are especially receptive for this. In addition, it appears that mixed protein and carbohydrate feedings promote the best replenishment. Prompt refueling can make a big difference in how an active individual feels and performs the rest of the day as well as later in the week.
Physical performance depends on many factors: genetic make-up, mental preparation, physical training, and nutritional status. In general, active individuals need to pay attention to consuming enough carbohydrates and fluid, and to distributing food intake throughout the day. A multivitamin/mineral supplement is good nutritional insurance, but not a substitute for good food choices. Individuals who want specific recommendations based on their food preferences and activity patterns can consult a registered dietitian for an individualized eating plan. Contact the American Dietetic Association or the Sports, Cardiovascular and Wellness Nutritionists Dietetic Practice Group of the American Dietetic Association for referral to a local R.D.