Physiologists regularly extol the importance of carbohydrates as a vital fuel that drives exercise and sport performance. Before the Industrial Revolution, carbohydrates were the major source of nutrients and energy for people throughout the world. Carbohydrates that come primarily from plants in the form of vegetables, fruits and grains are a direct link to the earth’s food chain. However, evidence is mounting that carbs from added sugars in cookies and soft drinks present several health risks.
Which carbohydrates play an important role for exercise, and what are the health risks of Americans’ sweet-tooth craving for sugar?
What Are Carbohydrates?
Carbohydrates are biomolecules that contain oxygen, carbon and hydrogen atoms. They are made up of small building blocks referred to as monosaccharides (mono = one; saccharide = sugar). When we eat plants and digest them into monosaccharides, our bodies convert them to glucose, which muscle cells use to produce energy. Glucose is the main carbohydrate in blood and is stored in muscles and the liver in the form of glycogen.
Other animals besides humans can also make carbohydrates, usually for use in their own bodies. As a source of dietary carbohydrates, lactose—the natural sugar in milk—is the one meaningful exception.
Why Is Glucose the Preferred Fuel for Exercise?
The energy that drives life—and exercise—is released when chemical bonds in food substrates (macromolecules) are broken. The energy in the food’s molecular bonds is released in the body’s cells to produce adenosine triphosphate (ATP), an immediately available source of energy for practically all bodily functions, particularly muscular contractions during exercise.
At rest, the body’s energy demands are met by the breakdown of bonds in fats and carbohydrates in fairly equal proportions. During more intense, challenging exercise, glucose and glycogen are the preferred fuel, not only because of their availability (glucose in blood and glycogen in muscle), but also because muscle cells have a highly developed enzyme system that can chemically break carbs down rapidly to synthesize ATP.
Sugar Overload: The Perils of Too Much Added Sweetness
Cupcakes, cookies, muffins, doughnuts, chocolate, ice cream, soft drinks, fruit drinks, energy drinks and more—the list of sweet temptations we have developed as a society is quite exhaustive. In an invited commentary for a JAMA journal (2014), Laura Schmidt, PhD, emphasizes that overconsumption of added sugar, slipped into countless products during manufacturing, is cause for concern. (Problems are not due to eating too much fresh fruit, which has naturally occurring sugars, fiber, water, vitamins and antioxidants.)
Originally, as Schmidt explains, health concerns related to sugar were based in fears that eating too much of it would lead to obesity and dental cavities (which has been prodigiously confirmed). The new concerns are that added sugars—predominantly sucrose and high-fructose corn syrup—may be independent risk factors for cardiovascular disease (CVD), type 2 diabetes, abnormal lipids and hypertension.
THE SUGAR BURDEN ON CARDIOVASCULAR DISEASE
Yang and colleagues (2014) analyzed national data (1988–2010) on the influence of added sugar consumption on CVD. Their analysis shows that CVD risk becomes elevated once added sugar intake surpasses 15% of daily calories. The researchers highlight, from their analysis, that regular consumption of sugar-sweetened beverages (ÔëÑ7 servings/week) is linked with increased risk of CVD mortality. Woefully, the risk increases with an increase in sugar intake. Yang et al. conclude that people who consume approximately 17%–21% of calories from added sugar have a 38% higher risk of CVD mortality compared with those whose intake is just 8% of calories.
A LINK BETWEEN SUGAR INTAKE AND TYPE 2 DIABETES?
From a systematic review and meta-analysis of 17 studies, Imamura et al. (2015) conclude that habitual consumption of sugar-sweetened beverages is independently associated with a greater incidence of type 2 diabetes. The researchers suggest that consumption of sugar-sweetened beverages may be linked to 4%–13% of type 2 diabetes cases in the United States. Imamura and colleagues also state that artificially sweetened beverages and fruit juice are not healthy options for preventing type 2 diabetes. This data fully supports the recommendation to consume a wide variety of fruits, which have naturally occurring sugars that have not been linked to the incidence of type 2 diabetes.
SUGAR’S EFFECT ON BLOOD PRESSURE AND BLOOD LIPIDS
In a meta-analysis of randomized controlled trials involving 1,699 participants, Morenga et al. (2014) examined higher and lower intakes of dietary sugar in adults or children who had no acute illnesses. Study results show an association between higher intakes of sugar and harmful increased concentrations of triglycerides, total cholesterol, LDL (low-density lipoproteins) cholesterol and blood pressure.
The researchers suggest that the most likely explanation for this association is the fructose component of sugar. Overconsumption of dietary fructose, particularly from sugar-sweetened beverages, has been shown to increase liver fat synthesis, which results in higher concentrations of circulating triglycerides and cholesterol. In addition, elevated liver fructose metabolism may impede liver energy balance, leading to an increase in uric acid (a waste product from digestion of foods containing purines—e.g., dried beans, sardines and certain meats). Higher levels of uric acid can impair the function of endothelial cells, resulting in vasoconstriction of blood vessels and eventual high blood pressure.
SUGAR INTAKE VS. SUGAR RECOMMENDATIONS
The average American currently consumes about 19.5 teaspoons of sugar daily, which is approximately 66 pounds of added sugar over the course of a year. In contrast, 200 years ago, in 1790, the data suggest that the average yearly intake of sugar was 8 pounds (White 2018). As a reminder, the average dose of sugar in one 16-ounce soda is about 16 teaspoons.
The American Heart Association recommends that adult females consume Ôëñ6 teaspoons (╦£25 grams) and adult males consume Ôëñ9 teaspoons (╦£38 g) of added sugar daily. Presently, consumption is approximately 3–6 times higher than these recommendations.
Are There Unique Health Risks to High-Fructose Corn Syrup?
High-fructose corn syrup, which was first introduced in the 1970s, is produced by a process that converts glucose to fructose. HFCS makes up a very large proportion of added sweeteners in beverages and many packaged foods (i.e., cereals, baked goods, desserts, flavored dairy products and canned foods). Dornas et al. (2015) note that fructose is advantageous in processing because it is 1.5 times sweeter than sugar and inexpensive to produce. The researchers conclude from their review that HFCS definitely contributes to the development of several adverse health effects, such as insulin resistance, high blood fats, intra-abdominal fat accumulation, high blood pressure and elevated uric acid levels. However, more research is needed to determine how excess HFCS causes these problems and what doses produce the negative health outcomes. With reliable data, it may be possible to propose and recommend evidence-based guidelines for HFCS intake.
See also: Are All Sugars the Same?
Interestingly, adding sweeteners to foods, which began as a way to combat high fat intake (and obesity), is becoming an equally serious problem. The evidence clearly shows that added sugars are not just empty calories; they are very harmful calories. The negative health risks of eating too much added sucrose and fructose clearly indicate that we should choose whole foods, as opposed to processed foods, as much as possible.
Small changes can make a difference. For example, curbing the intake of sugar-sweetened beverages and sugar-laden yogurt would be a most healthful lifestyle change. Choosing breakfast cereals with little or no sugar is a positive strategy. It’s time to be mindful about all of the added sugars in foods, including those not commonly thought of as sweet, such as sauces, salad dressings, crackers and breads.
The bottom-line message for fitness pros to give clients is that healthy, unprocessed (or minimally processed) carbohydrates fuel our workouts and deliver beneficial vitamins, minerals, fiber and phytonutrients. Perhaps the first step for many clients in their “sugar restart” is not to avoid all sugar-sweetened products but to begin by limiting them in the daily diet.
Glossary of Sweet Terms
- glucose: sugar used by the muscle cells to produce energy; found in blood
- glycogen: principal storage form of glucose that is found primarily in muscles and the liver
- sucrose: table sugar primarily extracted from sugarcane and sugar beets
- fructose: sweetener primarily derived from sugarcane, sugar beets and corn; 1.5 times sweeter than other sugars
American Heart Association (AHA). 2018. Added sugars. Accessed Jan. 29, 2019: heart.org/en/healthy-living/healthy-eating/eat-smart/sugar/added-sugars.
Dornas, W.C., et al. 2015. Health implications of high-fructose intake and current research. Advanced Nutrition, 6, 729–37.
Imamura, F., et al. 2015. Consumption of sugar sweetened beverages, artificially sweetened beverages, and fruit juice and incidence of type 2 diabetes: Systematic review, meta-analysis, and estimation of population attributable fraction, BMJ, doi:10.1136/bmj.h576.
Schmidt, L.A. 2014. New unsweetened truths about sugar. JAMA Internal Medicine, 174 (4), 525–26
Te Morenga, L.A., et al. 2014. Dietary sugars and cardiometabolic risk: Systematic review and meta-analyses of randomized controlled trials of the effects on blood pressure and lipids. American Journal of Clinical Nutrition, 100, 65–79.
White, J.R. (2018). Sugar. Clinical Diabetes, 36 (1), 74–76. Accessed Jan. 9, 2019: clinical.diabetesjournals.org/content/36/1/74.
Yang, Q., et al. 2014. Added sugar intake and cardiovascular diseases mortality among US adults. JAMA Internal Medicine, 174 (4), 516–24.