Is Fat or Sugar Making Us Fat?

We all know obesity and cardiovascular disease (also known as heart disease) are on the rise. According to the CDC, 35.9% of US adults were considered obese in 2009-2010. Years of research have been dedicated to determining why the population is getting fatter and what to do about it. Is fat or sugar making us fat? Here is a look at where research has taken us over the years.

Calories In, Calories Out: The calories in, calories out theory (see Beyond the Calories) concludes that if you eat too much and you move too little, you’ll gain weight (and vise versa). We know that 1 gram of fat contains 9 calories and 1 gram of either protein or carbohydrate contains 4 calories. So in an effort to get individuals to “eat less”, the recommendation to eat less fat (the most calorie-dense of the macronutrients) took flight and the low-fat diet was born. Check out the following video series Big Fat Lies by Tom Naughton, creator of the documentary Fat Head, about how bad science has shaped the nutrition world as we know it today.


The Lipid Hypothesis: In support of the low-fat craze, research was conducted to determine if dietary fat caused heart disease. In the 1950’s American scientist Ancel Keys studied fat and sugar and their association with cardiovascular disease (CVD). He studied what individuals in different countries ate and observed the presence of heart disease among the populations. The study revealed that the countries where fat consumption was the highest had the most heart disease. You could spend days reading about the flaws in his Seven Countries Study (cherry picking his data to fit his hypothesis, failing to do a multi-linear regression analysis correctly, disregarding confounding variables like the fact that some countries may not have had the tools to accurately record deaths from CVD or that his method for measuring food consumption may not have accurately portrayed what people actually ate or threw away, etc.). The bottom line is that correlation isn’t causation. Here’s what I’m getting at. If we know that low fat diets lower cholesterol, and lower cholesterol is linked to decreased heart disease, we assume that low-fat diets decrease heart disease. Unfortunately this type of teleoanalysis is highly criticized for being unscientific and invalid.

Is Fat or Sugar Making Us Fat?

Think about it. If A = B, and B=C, then A=C. Any logicians out there see a problem with this? The relationship between the variables isn’t transitive. The logic is faulty because the premise is wrong. If I told you that excess water intake is linked to frequent urination and frequent urination is linked to diabetes and therefore excess water intake causes diabetes – you’d think I was crazy. That’s because correlation isn’t the same as causation. Just because one variable is correlated or linked to another, doesn’t mean is causes it.

A Calorie is NOT a Calorie: Unfortunately, years of nutrition recommendations were been based on the above principles. And what was the result? We ate less calories and less fat. Foods like fat-free yogurt and salad dressing grew in popularity at the supermarket. Because these foods contained zero fat, their flavor suffered. Food manufacturers began adding sugar to these products to make them more palatable. In eating less fat we began eating MORE carbohydrates (primarily sugar).

Large vs. Small LDL Cholesterol: Another quick important point I want to make. There are 2 different types of LDL cholesterol in our body. Large LDL is fluffy and buoyant – it floats in the bloodstream. It’s too big to fit under the endothelial layer of your arteries to contribute to plaque formation. Small LDL on the other hand can. Small, dense LDL is the type of LDL linked to artery clogging heart disease. Many studies about LDL cholesterol and heart disease fail to include this important distinction. Dietary fat increases large LDL cholesterol, while carbohydrates (sugar) increase the small, dense harmful LDL cholesterol. You can predict what would happen to your lab values on a low fat, high sugar diet…

So what’s the Real Deal with Sugar? Let’s talk for a minute about the chemistry of sugar. The forms of sugar we are most familiar with are lactose, fructose, sucrose and glucose. Lactose is the sugar found in milk and isn’t very sweet. Fructose is the natural sugar found in fruit. Sucrose (also known as table sugar) is made up of both glucose and fructose (50% glucose and 50% fructose). High fructose corn syrup (HFCS), which hit the US market in the 1970’s, is also made up of both glucose and fructose, but with more fructose than glucose (45% glucose and 55% fructose) – hence the name. Because of the high fructose content, HFCS tastes sweeter than table sugar. It’s also cheaper to make. So as you can imagine, food manufacturers began substituting HFCS for sucrose because it was more economical. Why does all of this matter? Remember when I told you that the TYPE of foods we are eating matters for weight management. It’s all about how the foods you eat get metabolized in the body. And different types of sugar get metabolized DIFFERENTLY.

Is Fat or Sugar Making Us Fat?

Glucose Metabolism: Whenever you consume table sugar (sucrose) or HFCS, your body metabolizes the 2 components (glucose and sucrose) differently. Let’s start with glucose. Glucose is metabolized by your muscles. When you eat glucose, it goes into your blood stream, insulin increases and the glucose is used for energy, stored as glycogen (sugar storage) in the liver for later, and any small excess is stored as fat.

Is Fat or Sugar Making Us Fat?

Fructose Metabolism: Unlike glucose, fructose is metabolized ONLY in the liver. It is not utilized by your muscle cells. In the liver, fructose is stored as glycogen (sugar storage) for later, and the rest is stored as fat. Fructose can be turned into glycogen faster than glucose. This is beneficial for elite athletes who are looking to replete glycogen stores during or after a sporting event, but is not ideal for the average person. Additionally, a larger percentage of fructose ends up in fat storage than with glucose metabolism. So you can image what might happen if you increase the amount of fructose in the diet by replacing sucrose with HFCS…but this is only the beginning of the problem. Uric acid is a byproduct of the metabolic processing of fructose. Uric acid blocks nitric oxide (the body’s natural vasodilator – meaning it decreases blood pressure) and therefore causes blood pressure to increase. So increased fructose consumption can lead to increased fat storage (i.e. small, dense LDL cholesterol) and increased blood pressure – both familiar signs of heart disease.

Why are people eating more? You have 2 important hormones in the body that regulate hunger: ghrelin (the hunger hormone) and leptin (the full hormone). When your stomach is empty, your body produces ghrelin, which tells your brain, it’s time to eat. When your stomach is full, your body produces leptin, which tells your brain it’s time to stop eating. It takes at least 20 minutes for the leptin signal to reach the brain – hence the importance of eating slowly and giving these hormones time to do their job! When you consume glucose and your body produces insulin to metabolize it. Insulin signals leptin and you receive a loud and clear message that you are full. Fructose on the other hand has no affect on insulin and therefore doesn’t stimulate leptin. If your brain is never told that you are full, you will keep eating. Ever eaten 3 sugary donuts or 4 bowls of HFCS-containing cereal and not feel satisfied?

Is Fat or Sugar Making Us Fat?

HFCS vs. Real Sugar: While high fructose corn syrup contains 5% more fructose than sucrose (be it white sugar, brown sugar, honey or maple syrup), they both contain fructose, and they will both add to fat storage in the body. Think agave nectar is a healthier option? Agave can range from 55-90% fructose (more than HFCS) depending on the brand. This high-fructose containing agave is metabolized primarily in the liver and therefore has a limited affect on your blood sugar. While this might seem ideal for diabetics, think about what it’s doing for your waist line.

Fiber: So how can we minimize the affects that sugar and carbohydrates have on the body? Pair carbohydrate-rich foods with fiber – this is how sugar appears in nature (i.e. naturally occurring fructose in a fibrous piece of fruit). In doing so you can decrease the carbohydrate absorption rate (decreases the effect the glucose will have on our blood sugar), slow digestion time in the stomach (keeps you fuller longer) and increase transit time through the intestine (this helps your brain receive the leptin signal sooner).

So why are people eating more today than ever? It’s all about the biochemistry of the body. It’s important to recognize that bad science has pointed us in the wrong direction for years causing us to restrict calories, eat less fat, consume more sugar, and become sicker than ever. Read more about The Best Way to Fill Your Plate for tips and tricks around optimal nutrition and healthy eating. Finally, check out Sugar: The Bitter Truth below by Dr. Robert Lustig of the University of California if you really want to geek out on the biochemistry of this topic!


  1. Danny Grayson


    Can we not blame a world wide obesity epidemic on one thing? No food group or one food is fattening. We aren’t just eating more because of sugar consumption. Are we (as Americans) not more stressed/underslepped/distracted eaters too? At the end of the day CICO is fact. One cannot gain weight unless they’re in a caloric surplus/lose unless in a deficit.

    We’re eating less sugar ( However, we’re eating more. From 1970 – 2000 we began to eat 530 more cals (

    • Certainly there are additional factors to the equation. Stress and sleep both affect cortisol hormone levels which can lead to inappropriate hunger signals and blood sugar imbalances as well! Sugar consumption (which includes all carbohydrate sources) is a small (but extremely important) part of the very large picture that obesity in America is.

  2. Have you seen Lustig’s 2nd presentation? It’s even better.

  3. Great post Amari! I get a lot of questions regarding this topic and I couldn’t agree more that sugar consumption is a very important piece of the obesity puzzle. I love your diagrams – did you create them yourself?

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