• Pre-Collegiate Global Health Review

The Obesity Epidemic: Why and How

John Voudouris and Alexander Janczewski, Upper Canada College, Toronto, Ontario


This statistic may shock you–or perhaps not–but approximately 72% of American adults are overweight (Harvard School of Public Health, 2020). 42% of American adults are obese, with 9.2% experiencing imminent life-threatening concerns (CDC, 2018). It is estimated that one out of five adults in the world will be obese by 2030 (Sulivan, 2016). This problem is only growing, but why?

More than half the calories in the average American diet come from ultra-processed foods. Similarly, ultra-processed foods make up around 48% of the Canadian diet (Laborer’s Health and Safety Fund of North America, 2019). One is constantly told to eat healthy foods to live a healthy life. However, one must understand why exactly diets play such a significant role in overall nutrition to comprehend why obesity occurs and contribute to its mitigation in our society through education. Similarly, one must comprehend why diets with high amounts of processed foods, sugars, and fats are deemed unhealthy.

Processed Foods

Processed foods contain highly refined ingredients, mainly carbohydrates, that are geared toward creating a low-cost product with maximum gustatory enjoyment for the consumer. This is done by adding increased amounts of fats and sugars. However, the main problem is that these molecules are in their purest form possible, meaning that they are not tied up in cells (they are acellular). To better understand this, let us take the example of eating an apple versus eating a package of Skittles. As the apple enters your gastrointestinal tract, it is broken apart by various enzymes that remove the nutrients located in each cell of that apple. It takes time for these enzymes to break apart the cell membranes, isolate the starch, break it down into glucose and send it through the bloodstream to the cells. Contrary to what one would think, rapid digestion is actually detrimental to the body, as the nutrients are ingested in such a way that they overwhelm the metabolic processes of the body (Ciulla, 2021). Next, let us look at the Skittles. As the Skittles, which are almost completely made of glucose and fructose, enter the gastrointestinal tract, they are almost immediately digested by the enzymes as there is no cell to break apart. (Fructose is converted into glucose by the liver once digested). The nutrients are already isolated. Suddenly, the digestion time decreases significantly, and these nutrients are absorbed simultaneously into the bloodstream. As you will understand after reading the following sections, this sudden influx of glucose will cause an insulin spike, leading to a drop in blood sugar, stimulating further consumption by the body.

Figure 1: Distribution of processed vs unprocessed foods in our diet

Figure 2: How Insulin Works

Sugar and Insulin

One of the most detrimental parts of processed foods, as mentioned, is sugar. The natural bodily response to any glucose in the bloodstream is the secretion of the insulin hormone from the pancreas. Insulin stimulates the transfer of glucose from the bloodstream to somatic cells around the body which require glucose for energy. The glucose molecule is polar and too large to cross the cell membrane by simple diffusion, so facilitated diffusion is necessary. Cells have insulin receptor enzymes that, when attached to insulin, activate the transporter proteins in the cell membrane to allow for movement across the cell membrane (Figure 2) (Lönnroth, 1991). However, when excessive amounts of sugar are eaten within a certain amount of time, the pancreas over-secretes insulin to compensate for the rising blood glucose level (Ciulla, 2021). This excess of insulin leads to a rapid decrease in blood glucose that stimulates an urge for more food shortly after eating. This is known as an insulin spike and can lead to a “snowball effect” if the individual continues to eat food high in refined sugar, leading to repeated insulin spikes (Ciulla, 2021). This can have detrimental effects including insulin resistance, hyperglycemia, and diabetes, which will be covered under “Consequences of a High-Sugar Diet.”

Lipoprotein Hypothesis

The lipoprotein hypothesis is the generally accepted idea that sugar damages low-density lipoprotein (LDL) in the bloodstream, activating the immune system to pull the LDL into the endothelium of the blood vessels (Jarcho et al., 2015). In effect, this leads to the buildup of plaque in the arteries, contributing to atherosclerosis and heart disease.

LDL is a sphere-shaped package containing mostly cholesterol that transports cholesterol from the digestive tract to cells in need of this crucial molecule. All cells use cholesterol as a primary component of the cellular membrane and its phospholipid bilayer. It is also used as the main constituent in the myelin coating of nerves. When the cells have an excess of cholesterol, they package it into high-density lipoprotein (HDL) and send it to the liver for destruction or delivery to other cells in need (Ciulla, 2021).

When in the bloodstream together, glucose and LDL react due to a sole valence electron that can be donated from the glucose aldehyde group. When glucose reacts with LDL, it glycates the molecule–simply meaning the attachment of glucose molecules–which can lead to the destabilization of the molecule. Additionally, while normal cells can recognize and absorb the LDL, they are unable to do so with glycated LDL (Zimmermann et al., 2021). Due to the increased reactivity from glycation, the LDL reacts with the lining of the arteries (Moll, 2021). This further stimulates a response from the macrophages (white blood cells) that absorb this threat in the bloodstream itself.

Furthermore, glycated/oxidized LDL is highly invasive to the artery walls, where it attaches to macrophages, provoking their attack on the LDL. In doing so, the macrophage produces pro-inflammatory cytokines to summon more white blood cells and – after engulfing an excessive amount of LDL – it morphs into a foam-like mass, killing itself in the process (UT Southwestern Medical Center, 2019). This macrophage attaches to the endothelial cells of the artery (Patricia et al., 1999). HDL does not display the same ability to become a radical, and it has the ability to absorb excess cholesterol in the cells and bloodstream. Hence, LDL can be considered the “bad” cholesterol, while HDL can be considered the “good” cholesterol.

Irritation, as well as the secretion of cytokines, can lead to serious inflammation, causing a surge in platelets (red blood cells that clot blood), and could cause damage to the surrounding organs.

Consequences of a High Sugar Diet

Having a diet that is consistently high in sugar, especially refined sugar (not stored in cells), can lead to a variety of health concerns including insulin resistance, diabetes, heart disease and hyperglycemia.

The immune system recognizes these radicalized LDL molecules and sends white blood cells to remove them from the bloodstream by pulling them into the lining of the arteries. When significant amounts of these foam-like macrophages are transported to the endothelium, the buildup can restrict the flow of blood and contribute to heart disease (Patricia et al., 1999). In Figure 3, the effect of atherosclerotic plaque is illustrated. When the buildup under the endothelium is immense, the artery could end up getting plugged entirely, causing a heart attack. Additionally, pieces of this plaque can also break off, clogging the artery at a different point and causing a heart attack. These molecules harden after a certain amount of time and become incredibly difficult to remove, even with exercise and habitual changes. Hence, a diet with elevated levels of trans fats and sugars can increase the amount of glycated LDL, and lead to atherosclerosis.

Insulin resistance occurs after years of insulin spikes followed by decreases in blood glucose. Cells around the body, specifically muscle, fat, and liver cells, do not respond to insulin when it binds to their insulin receptors. This prevents the cells from accepting glucose through their membranes. While the specific mechanisms that lead to insulin resistance are unknown, scientists hypothesize that an enzyme in the mitochondria acts as an antioxidant defense enzyme when the cell is chronically exposed to glucose. This enzyme, called superoxide dismutase, shuts off the insulin response to halt the absorption of glucose by the cell (Hoehn et al., 2009). Since cells are unable to remove the glucose from the bloodstream without an insulin response, this sugar remains in the blood and contributes to hyperglycemia (elevates blood sugar). With a higher concentration of molecules in the blood, the viscosity will increase, putting stress on the heart and other organs in need of nutrients. Because of the difficulty with moving viscous blood, hyperglycemia can also lead to the necrosis of certain parts of the body, namely the feet, arms, and other extremities. Many diabetic patients experience similar conditions, although there is not a scientifically supported link between an elevated consumption of sugar and Type 2 diabetes. More research is necessary to make this evaluation, although some scientists speculate a correlation between a high-sugar diet and diabetes (Hughes, 2019).

Figure 3: Atherosclerosis


It is relatively common knowledge that excessive amounts of sugar, processed foods and fats contribute to obesity and a variety of other medical concerns, with heart disease being the number one killer of both men and women. However, the importance of understanding why these foods cause such a damaging bodily reaction is paramount for both consumers and medical professionals. Change can only come through understanding. Hence, it is hoped that with a proper understanding of the detrimental impacts of a poor diet, consumers of all ages will be able to make better choices. It is one of the few ways that the obesity epidemic can be vanquished.



Harvard School of Public Health. An Epidemic of Obesity: U.S. Obesity Trends

Centers for Disease Control and Prevention (CDC). (2018). “National Health and Nutrition Examination Survey, 2017-2018”

Sulivan, K. (2021). What are processed foods and why are they so bad for metabolic health?


Laborer’s Health and Safety Fund of North America. (2019). The Many Health Risks of Processed Foods. 15(12)


Ciulla, F. (2021). Modern Biology and Its Real World Applications – Columbia University High School Summer Immersion.

Lönnroth P. (1991). Regulation of insulin action at the cellular level. J Intern Med Suppl. 1991;735:23-9. PMID: 2043221


Ciulla, F. (2021). Modern Biology and Its Real World Applications – Columbia University High School Summer Immersion.

John A. Jarcho, M.D., and John F. Keaney, Jr., M.D. (2015). Proof That Lower Is Better — LDL Cholesterol and IMPROVE-IT. June 18, 2015 N Engl J Med; 372:2448-2450. DOI: 10.1056/NEJMe1507041


Ciulla, F. (2021). Modern Biology and Its Real World Applications – Columbia University High School Summer Immersion.

Zimmermann, R., Panzenbock, U., Wintersperger, A., Levak-Frank, S., Graier, W., & Glatter, O. et al. (2001). Lipoprotein Lipase Mediates the Uptake of Glycated LDL in Fibroblasts, Endothelial Cells, and Macrophages. Diabetes, 50(7), 1643-1653. doi: 10.2337/diabetes.50.7.1643


Moll, J. (2021). Understand How Oxidized LDL Cholesterol Affects the Body. Retrieved 12 September 2021, from https://www.verywellhealth.com/what-is-oxidized-ldl-698079

UT Southwestern Medical Center. (2019, April 24). How 'bad cholesterol' enters artery walls. ScienceDaily. Retrieved September 11, 2021 from www.sciencedaily.com/releases/2019/04/190424153655.htm

Patricia MK, Kim JA, Harper CM, Shih PT, Berliner JA, Natarajan R, Nadler JL, Hedrick CC. Lipoxygenase products increase monocyte adhesion to human aortic endothelial cells. Arterioscler Thromb Vasc Biol. 1999 Nov;19(11):2615-22. doi: 10.1161/01.atv.19.11.2615. PMID: 10559003.

Hoehn, K. L., Salmon, A. B., Hohnen-Behrens, C., Turner, N., Hoy, A. J., Maghzal, G. J., Stocker, R., Van Remmen, H., Kraegen, E. W., Cooney, G. J., Richardson, A. R., & James, D. E. (2009). Insulin resistance is a cellular antioxidant defense mechanism. Proceedings of the National Academy of Sciences of the United States of America, 106(42), 17787–17792. https://doi.org/10.1073/pnas.0902380106

Locke Hughes, reviewed by Brunilda Nazario, MD. (2019). How Does Too Much Sugar Affect Your Body?. https://www.webmd.com/diabetes/features/how-sugar-affects-your-body