Low-Carbohydrate Diets and their Effect on the Human Body
Anna and Hannah
University of North Dakota
This review explores the effectiveness and safety of a low-carbohydrate, high-fat diet as an alternative to a low-fat, high-carbohydrate diet, which is currently the standard for chronic disease prevention and weight loss. In comparison studies, the effects of a low-carbohydrate diet performed as well or better than comparable low-fat diets with regard to laboratory values (lipid levels, cholesterol levels, and blood pressure), overall satiety factors and weight loss in both overweight subjects with no chronic disease and those with health-related disorders. A low-carbohydrate diet has metabolic and hormonal effects that suggest an underlying scientific basis for considering it as a different approach to low-fat, high-carbohydrate recommendations in enhancing the lives of those who are overweight/obese or those with chronic disease. Low-carbohydrate diets may be a feasible option to help reverse prospective factors for heart disease, diabetes mellitus and obesity.
According to authors Gropper and Smith, carbohydrates are the major source of energy fuel in the human body; these macronutrients make up roughly half of the total caloric intake of the average person. The Dietary Guidelines for Americans 2015 has set the RDA for carbohydrates at a minimum of 130 grams each day, with the suggestion of providing 45% to 65% of total energy intake from carbohydrates. This translates to about 225 to 325 grams of carbohydrates each day, based on a 2,000 calorie diet (Mayo Clinic, 2014). In contrast, a low-carbohydrate diet is one which limits the daily intake of carbohydrates between 60 to 130 grams a day (Mayo Clinic, 2014); thus, a low-carbohydrate diet typically provides between 12% and 26% of total energy intake from carbohydrates based on a 2,000 calorie diet.
1.1 Physiological aspects of carbohydrates
In the body, the most nutritionally important monosaccharide is glucose, a six-carbon sugar (Gropper & Smith, 2013). Glucose is one of three monosaccharides; the other two are fructose and galactose. Disaccharides are those sugars which are made up of a combination of two monosaccharide sugar units. Lactose is made from glucose and galactose; maltose is made from two glucose subunits; and sucrose is made from fructose and glucose. Complex carbohydrates, such as polysaccharides, are made of more than ten sugar units in a single chain. Glycogen, the storage form of glucose, as well as starch and dietary fiber from plant foods are examples of polysaccharides. About half of the dietary carbohydrates consumed each day are in the form of polysaccharides, while the other half of dietary carbohydrates is supplied mainly by the simple sugars sucrose and lactose (Gropper & Smith, 2013).
1.2 Purpose of glucose
Glucose is important in the human body because of its ability to be converted into ATP, which is used to provide energy to cells. Blood glucose level is tightly regulated in the body, due to the implications it can have on metabolism and cellular homeostasis (Gropper & Smith, 2013). Glucose is also the primary fuel source of the brain and nervous tissue, because it has the ability to freely cross the blood brain barrier, and it is the only fuel source for red blood cells in the body (Tymoczko, Berg, & Stryer, 2015).
However, what happens if an individual consumes too much of this fuel source? Research in this area has increased in the last few decades due to the increasing incidence of obesity and other chronic diseases. It is believed that there may be a link between consumption of a high-carbohydrate diet and the increased risk for developing one or more chronic diseases.
1.3 Excess glucose
The human body has the capability to store glucose in the form of glycogen in the liver and skeletal muscle where it is readily available for use if the body requires a burst of quick energy (Gropper & Smith, 2013). However, if the meal being consumed has excess carbohydrates, not all of it will be converted to glucose and stored as glycogen. Excess glucose can be converted into fatty acids and stored in adipose tissue as triacylglycerols.
1.4 Association with disease
This extra fatty tissue deposit due to excess consumption of glucose in the form of carbohydrates is what may cause the increased risk of disease development in an individual. The consumption of a high-carbohydrate diet can increase body weight and increase the risk for developing cardiovascular disease, metabolic syndrome (Mayo Clinic, 2014), and type II diabetes (Harvard). The United States is a country in which the rates of chronic diseases are increasing; therefore, the widespread consumption of a low-carbohydrate diet might be the key to a healthier America. According to the Centers for Disease Control and Prevention (CDC, 2016), in 2012 it was estimated that nearly half of all US adults (117 million individuals) had one or more chronic health problems, while one in four US adults had two or more chronic health problems. In 2013, cardiovascular disease was the number one reported cause of death in the US, with stroke/cerebrovascular accident as number five and complications due to diabetes listed as number seven (Death and Mortality, 2016). Due to this alarming information, it is imperative that more research be done to determine the potential effects of a low-carbohydrate diet on the human body.
Thus far, it has been determined that low-carbohydrate diets have been shown to improve blood laboratory values, increase satiety during and after a meal, and can facilitate with weight loss. However, there may be many more benefits to consuming a low-carbohydrate diet that researchers have yet to discover.
2. Specific Laboratory Values
Low-carbohydrate diets have shown to improve laboratory values including decreased blood pressure, decreased triglyceride levels and decreased very-low-density lipoprotein cholesterol levels. In a study by Foster et al. (2010), the majority of the patient’s laboratory values improved during the time they were on the low-carbohydrate diet. In this study there were 307 patients. The average age was 45 years old and the average body mass index was 36.1 (classified: obese). The low-carbohydrate diet was defined as limiting carbohydrate intake to less than twenty grams every day for three consecutive months (Foster et al., 2010). After three months, they increased their carbohydrate intake (five grams/day per week) until a stable and desired weight was achieved (Foster et al., 2010). The low-fat diet was defined as limiting energy intake to 1,200 to 1,800 kilocalories every day and less than 30% of calories from fat for three months (Foster et al., 2010). The study focused on the food composition rather than calorie intake of carbohydrates. It was found that during the first six months, the low-carbohydrate diet group had greater reductions in diastolic blood pressure, triglyceride levels, and very-low-density lipoprotein cholesterol levels (Foster et al., 2010). The low-carbohydrate diet group also had greater increases in high-density lipoprotein cholesterol levels (Foster et al., 2010). Therefore, low-carbohydrate diets could be considered an effective method for individuals desiring to improve laboratory values.
2.1 Adverse outcomes
In the same study by Foster, there were adverse outcomes with the low-carbohydrate diet. There were lesser reductions in low-density lipoprotein cholesterol levels and more adverse symptoms than the low-fat diet group (Foster et al., 2010). The adverse symptoms of the low-carbohydrate group included bad breath, hair loss, constipation and dry mouth. Except for constipation, all of these differences were limited to the first six months of treatment for the two-year study. Despite these negative effects, it appears that the low-carbohydrate diet improved more laboratory values than the low-fat diet, and the adverse symptoms which accompanied the low-carbohydrate diet were temporary. Overall, the low-carbohydrate diet was associated with favorable changes in cardiovascular disease risk factors at two years (Foster et al., 2010), which means that the risk for cardiovascular disease for those individuals on the low-carbohydrate diet decreased as the study progressed.
2.1 Further evidence
In an article by Kiska (2011), the replacement of fats and oils for for refined sugar and refined sugar products is evaluated. As the US diet began leaning towards a more low-fat approach, the food industry responded by creating foods which were lower in fat, yet higher in refined sugar and carbohydrate products. According to Kiska, this added sugar can lead to increased body weight, body fat percentage and accelerated disease processes. Total cholesterol, low-density-lipoprotein cholesterol, triglyceride and sugar serum levels may also increase as a result. It is estimated that the average American consumes 135 pounds of refined sugar products each year in this suggested low-fat diet. Replacing the fat in our diet with refined sugar products is not the answer, because these refined sugar products can contribute to an excess amount of glucose which is converted to fatty acids and stored as triacylglycerols in adipose tissue, leading to a possible increase in body weight and increased risk for chronic diseases.
Along with improving laboratory values, low-carbohydrate diets increase satiety and are comparatively more sustainable than other diets. A study completed by Martin et al. (2011), in the research journal “Obesity” revealed that a low-carbohydrate diet resulted in decreased cravings and decreased feelings of hunger. The study was completed with 270 obese adults, with a mean body mass index of 36 kg/m2. The exclusion criteria briefly included: the presence or history of a chronic disease that is known to affect appetite, food intake, or metabolism; history of cardiovascular or protein wasting disease (e.g., Cushing’s syndrome); and pregnant or lactating females. The participants were randomly assigned to a low-carbohydrate diet or a low-fat diet. The low-carbohydrate diet consisted of less than 20 grams of carbohydrates a day for the first three months of the study, with all carbohydrates coming from vegetables which have a glycemic-index score of less than fifty (Gropper & Smith, 2013).
The participants consuming the low-carbohydrate diet were encouraged to follow a diet plan similar to that of the Atkins diet, and they were told to eat foods rich in fat and protein until their hunger was satisfied. After three months, those with the low-carbohydrate diet could slowly add more carbohydrates to their diet (increased carbohydrate intake by five grams each week) through increased vegetable consumption, as well as limited consumption of fruits, whole grains and dairy products until the participant’s desired weight was achieved. The low-fat diet focused on lowering energy intake and consuming about 30% of energy from fat, 15% of energy from protein, and 55% of energy from carbohydrates. Women were instructed to consume 1,200-1,500 calories a day, and men were instructed to consume 1,500-1,800 calories a day (Martin et al., 2011).
The participants’ cravings were measured for specific types of foods (sweets, high-fats, fast-food fats and carbohydrates/starches), as well as their preference for high-sugar, high-carbohydrate and low-carbohydrate/high-protein foods and appetite (Martin et al., 2011). These food cravings were measured using the Food Craving Index, and the food preferences were recorded using a Food Preference Questionnaire. Recordings for the Food Craving Index were taken at the beginning of the experiment, and again at months three, six, twelve, eighteen, and twenty-four. The outcomes demonstrated that those consuming the low-carbohydrate diet had significantly larger decreases in cravings for carbohydrates and starches, as well as a decrease in preferences for high-carbohydrate and high-sugar foods (Martin et al., 2011). Additionally, the low-carbohydrate diet group reported being less bothered by hunger compared to the low-fat diet group (Martin et al., 2011). These results encourage the belief that low-carbohydrate diets do in fact increase satiety.
4. Weight loss
Low-carbohydrate diets have been touted as a popular method of facilitating weight loss. A study by Bazzano et al. (2014), was completed with 148 individuals who did not have a clinical diagnosis of cardiovascular disease. The participants were obese men and women aged 22 to 75 years with a body mass index of 30 to 45 kg/m2 (Bazzano et al., 2014). Sixty participants were put on a low-fat diet, while 59 participants were put on a low-carbohydrate diet (Bazzano et al., 2014). The low-carbohydrate diet consisted of <40g/day of carbohydrates. The low-fat diet consisted of <30% of daily energy intake from total fat and <7% from saturated fat (Bazzano et al., 2014). Neither diet included a specific calorie or energy goal. Both groups received dietary counseling regularly throughout the trial. This dietary counseling included meeting with a dietitian and attending small group counseling sessions, occurring first weekly and then monthly as the study continued. The low-fat and low-carbohydrate groups met separately but followed a common behavioral curriculum (Bazzano et al., 2014).
The study was conducted for twelve months and data for weight, cardiovascular risk factors and dietary composition were gathered at zero (initial data collection), three, six, and twelve months. During follow-up, the total energy intake and physical activity was similar between groups. At twelve months, participants on the low-carbohydrate diet had a greater decrease in weight (-3.5 kg), fat mass (-1.5%), ratio of total-high-density lipoprotein cholesterol (-0.44), and triglyceride levels (-.16 mmol/L) and a greater increase in high-density lipoprotein cholesterol level (0.18 mmol/L) than those on the low-fat diet (Bazzano et al., 2014). However, serum levels of total and low-density lipoprotein cholesterol did not significantly change between participants in either groups (Bazzano et al., 2014).
The conclusions of this study may be subject to certain limitations. The dietary intake was self-reported and therefore may have been subject to memory and recall issues. However, the intake was collected within 24 hours of consumption and multiple 24 hour dietary recalls were taken to reflect weekday and weekend patterns. Also, the dietitians who completed the recalls were not blinded to the study hypothesis. Detailed and specific scripts were used for all counseling sessions. Lastly, conclusions were limited by the lack of cardiovascular disease end points. Therefore, statistically significant results should be interpreted with caution. However, cardiovascular disease risk factors were assessed extensively. Overall, this randomized, twelve-month trial showed that a low-carbohydrate diet resulted in greater weight loss and reduction in cardiovascular disease risk factors than a low-fat diet between obese adults (Bazzano et al., 2014).
Low-carbohydrate diets have shown to improve blood laboratory values, increase satiety during and after a meal, and can facilitate with weight loss. The Academy of Nutrition and Dietetics recently released comments regarding the Dietary Guidelines for Americans for 2015-2020. In discussing saturated fat, even though there has been much evidence documenting the significance of saturated fat on the modulation of LDL and other circulating lipoprotein concentrations, the evidence of the effect on the tested disease endpoints (the occurrence rate) of cardiovascular disease due to saturated fat consumption is unclear. The Dietary Guidelines for Americans Committee (DGAC) suggested that carbohydrates contribute a greater amount of risk for cardiovascular disease than saturated fat, so the replacement of carbohydrate in the diet can possibly result in a greater decrease for chronic disease risk.
Therefore, the DGAC has suggested that, through the summarization of the Dietary Guidelines, the most effective recommendation for the reduction of cardiovascular disease would be a reduction in carbohydrate intake. The energy deficit caused by the reduction of carbohydrate in the diet should be made up by food sources of polyunsaturated fat (Tuma, 2015). This is conclusive with a low-carbohydrate diet: replacing high-carbohydrate foods with healthy mono- and polyunsaturated fats, while including moderate protein and low-carbohydrate, vitamin-rich foods. Following such a diet, in addition to a healthy lifestyle and moderate physical activity can help to reduce one’s risk for chronic disease development (Tuma, 2015).
Bazzano, L. A., Hu, T., Reynolds, K., Yao, L., Bunol, C., Liu, Y., Chen, C., Klag, M. J., Whelton, P. K., He, J. (2014). Effects of Low-Carbohydrate and Low-Fat Diets. Annals of Internal Medicine Ann Intern Med, 161(5), 309. doi:10.7326/M14-0180
Carbohydrates and Blood Sugar. (n.d.). Retrieved from http://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/
Chronic Disease Overview. (2016, February 23). Retrieved from http://www.cdc.gov/chronicdisease/overview/
Deaths and Mortality. (2016, February 25). Retrieved from http://www.cdc.gov/nchs/fastats/deaths.htm
Foster, G. D., Wyatt, H. R., Hill, J. O., Makris, A. P., Rosenbaum, D. L., Brill, C., Stein, R.I., Mohammed, S., Miller, B., Rader, D.J., Zemel, B., Wadden, T., Tenhave, T., Newcomb, C., Klein, S. (2010). Weight and Metabolic Outcomes After 2 Years on a Low-Carbohydrate Versus Low-Fat Diet. Ann Intern Med. 2010;153:147-157. doi:10.7326/0003-4819-153-3-201008030-00005.
Gropper, S. A., & Smith, J. L. (2013). Advanced Nutrition and Human Metabolism (6th ed.) (63-72, 79-83). Belmont, CA: Wadsworth/Cengage Learning.
Healthy Living: Weight loss. (2014, September 20). Retrieved from http://www.mayoclinic.org/healthy-lifestyle/weight-loss/in-depth/low-carb-diet
Hu, T., Yao, L., Reynolds, K., Niu, T., Li, S., Whelton, P., He, J., Bazzano, L. (2015). The effects of a low-carbohydrate diet on appetite: A randomized controlled trial. Nutrition, Metabolism and Cardiovascular Diseases. doi:10.1016/j.numecd.2015.11.011
Kiska, T.A. (2011). Refined sugar products and the ‘low fat diet’ effect on American’s serum cholesterol levels. Nutritional Perspectives: Journal of the Council on Nutrition, 34(3), 12-13 2p.
Martin, C.K., Rosenbaum, D., Han, H., Geiselman, P.J., Wyatt, H.R., Hill, J.O., Brill, C., Bailer, B., Miller-III, B.V., Stein, R., Klein, S., Foster, G.D. (2011). Change in food cravings, food preferences, and appetite during a low-carbohydrate and low-fat diet. Obesity, 19(10), 1963-1970. doi: 10.1038/oby.2011.62
McVay, M.A., Voils, C.I., Coffman, C.J., Geiselman, P.J., Kolotkin, R.L., Mayer, S.B., Smith, V.A., Gaillard, L., Turner, M., Yancy, W.S. (2014). Factors associated with choice of a low-fat or low-carbohydrate diet during a behavioral weight loss intervention. Appetite, 83, 117-124. doi:10.1016/j.appet.2014.08.023
Tuma, P. A. (2015, May 8). Academy Comments re. The DGAC Scientific Report. Retrieved from http://www.eatrightpro.org/resource/news-center/on-the-pulse-of-public-policy/regulatory-comments/dgac-scientific-report
Tymoczko, J. L., Berg, J. M., & Stryer, L. (2015). Biochemistry: A Short Course (3rd ed.) (168-176). New York, NY: W.H. Freeman & Company.