Decomposing Fats & Carbs
Calories are not interchangeable. To understand why, you first need to understand the basic macronutrient subtypes.
Not medical advice.
Metabolic Dysfunction: past, present, future
As I’ve written previously, rates of metabolic dysfunction (obesity, diabetes) have been rising since the 1970s. This trend has continued from 2000 to the present even though total calorie intake (how much we eat) has been flat or declining since then, and active energy expenditure (physical activity) has increased. This throws a wrench in knee-jerk explanations for the obesity epidemic: “People are eating too much!” and “People aren’t exercising enough!”
Here’s what obesity and type II diabetes rates in the US have looked like since the 1960s in the US:
Notice the broad trends:
Both obesity and diabetes have been rising since the 1960s.
The rise in obesity rates started accelerating by 1980.
The rise is diabetes started accelerating in the mid-1990s.
The rise in metabolic dysfunction has hit all major demographics. While there may be differences in the precise pattern of change over time for different groups, pretty much everyone—old and young, male and female, black and white—is moving in the same direction. This suggests that the major factors at play are likely systemic—common dietary patterns driving metabolic dysfunction across all groups.
Total calories are not the answer.
Total calorie consumption may be part of the picture, but we can’t account for the rise in obesity and diabetes by simply saying, “People are just eating more.” We started consuming a lot more calories in the 1970s, a trend that continued through the 1990s. This trend stopped and even reversed by 2000 but obesity and diabetes continued climbing. Furthermore, people did not simply eat more of the same things while caloric intake was rising. They ate more calories and different calories—the macronutrient composition of diets was changing throughout the timeline.
Notice the basic trends in macronutrient consumption over time:
Protein intake is remarkably flat over time;
Total fat intake has trended up over time;
Carbohydrate intake has gone down and up over time, rising with increases in obesity from the 1970s to 2000;
Fat and carb intake did not continue climbing from ~2000 onward, despite continued rises in obesity.
A major theme we will frequently revisit: calories are not interchangeable. Different types of foods with the same caloric content can have distinct metabolic consequences. A gram of carbs from whole oranges will not generate the same change in blood glucose or insulin release as a gram of orange juice; a gram of protein from chicken eggs will not be incorporated into tissue to the same extent as a gram from peanut butter; and so on.
It’s not enough to simply look at consumption patterns of the major macronutrients (fats, carbs, and protein). There are many different subtypes of each, with some showing more dynamic consumption patterns over time. Here’s an example—among other things, the graph shows how two different subcategories of carbohydrate have changed over time (sugars generally and high-fructose corn syrup specifically):
And here’s one showing total fat availability together with the major classes of dietary fats: saturated, monounsaturated, and polyunsaturated.
In order to think about these patterns, we have to understand something about the biology of each macronutrient subtype. How do the major types of dietary fat differ? Does the body utilize each one the same way? How do different carbohydrates, including various forms of sugar, differ in their metabolic effects?
We will focus here on fats and carbs, as they have displayed more dynamic patterns of consumption change over time compared to protein. (We’ll explore protein in a later post dissecting plant- vs. animal-based protein digestibility).
Below I will break down the major types of dietary fats, followed by carbohydrates, reviewing some basic biology. In parallel, we will describe the “Standard Dogma” of healthy eating for fats and carbs—the recommendations we get from official institutions like the United States Department of Agriculture (USDA), which are what your physician is most likely to recite.
Once we have a basic foundation in dietary fats and carbs, we can revisit consumption trends in more detail, at the level of macronutrient subtypes and specific, widely consumed foods. This will help us start to zero in on the major dietary drivers of obesity and diabetes.
Where are we going?
To give away the punchline a bit, here are some general trends of dietary change that I believe are significant contributors to the rise in obesity and diabetes (keep these in the back of your mind as we go along):
Intake of processed carbohydrates has increased, especially processed sugars. Certain forms of sugar found in ultra-processed foods, such as fructose and fructose derivatives, play an outsized role in metabolic dysfunction;
The composition of dietary fats we consume has changed dramatically over time—large increases in omega-6 polyunsaturated fatty acids (PUFAs) driven by the consumption of cheap cooking oils and ultra-processed foods. This is tied to decreases in the consumption of other fats, saturated fat and omega-3 PUFAs, traditionally obtained from animal sources;
A continued shift toward plant-based diets may make it harder to reverse trends in metabolic dysfunction, as the nutrients in plant-based foods are often less digestible by the human body than animal-based foods. This is true for macronutrients like protein as well as many micronutrients.
The first two points represent entrenched consumption patterns arising from the food environment our civilization has evolved. That these things are major drivers of metabolic dysfunction does not fit well into the Standard Dogma of healthy eating (described below). Certain experts largely disagree that these are major factors. Others largely agree. I will not be proposing anything novel—just curating available information to explain why I align with this basic perspective.
The third point is a more recent acceleration of a historical trend: moving people toward plant- and away from animal-based food sources. I suspect this emphasis will retard our ability to become more metabolically healthy in the future. Members of various dieting factions—carnivores, vegans, and so on—argue about this topic with an almost religious fervor. In later posts, I will summarize how I think about plant- vs. animal-based nutrition in terms of basic biology, as well as why dieting devotees are often animated by a kind of religious zeal.
But first, we need to cover some basic biology.
After we understand more about the major types of dietary fats and carbohydrates, we can revisit the trends in obesity and diabetes over time, asking a simple question: how did the macronutrient composition of our diet change at key points along the timeline? What macronutrients were people consuming before, during, and after obesity and diabetes rates began to climb?
This article draws on information from these podcast episodes:
M&M #132: Obesity Epidemic, Diet, Metabolism, Saturated Fat vs. PUFAs, Energy Expenditure, Weight Gain & Feeding Behavior | John Speakman
M&M #131: Dietary Fat, Cholesterol, Cardiovascular Health & Disease, Carbohydrates, Dietary Guidlines, Food Industry & Diet Research | Ronald Krauss
M&M #115: Exercise Science, Nutrition, Plant vs. Animal Protein, Muscle Physiology, Sleep, Endurance vs. Resistance Training, Fat, Carbs, Amino Acids | Luc van Loon
Decomposing Fat: Basic Types of Dietary Fatty Acids
“Fat” is a broad term generally referring to lipids, a class of organic compounds insoluble in water. Body fat is found in adipose tissue, composed of cells specialized for storing and releasing fat molecules. This fat is stored in the form of triglycerides, which contain three fatty acid molecules. Triglycerides are also the most common form of dietary fat. Here’s what they look like:
In addition to dietary fats, which are used for energy and energy storage in the body, lipids (fats) come in other forms: phospholipids, which our cell membranes are made of, as well as steroid molecules (e.g. cholesterol, hormones).
The types of dietary fat you see on nutrition labels, such as saturated or unsaturated fats, refer to the types of fatty acids found in triglycerides. There are four types, classified by their chemical structure—the number and placement of double bonds, which determines their geometry. The fatty acid composition of the triglycerides you consume and store in adipose tissue has important metabolic consequences, as does the fatty acid composition of your cell membranes.
Here’s a diagram showing the four main classes of dietary fatty acids, together with the standard consumption recommendations commonly given by health professionals—the recommendations you’re most likely to get from a doctor.
The Standard Dogma of healthy eating: good & bad fats
The far right column represents what we’ll call the Standard Dogma of healthy eating. According to the Standard Dogma: Saturated and trans fats are bad. Unsaturated fats are good.
We will ignore trans fats, as there is universal agreement that they are, in fact, bad. I’m not aware of anyone serious who argues otherwise, which is why their use in foods has been limited in the US and elsewhere. Obesity rates have continued to climb despite the decreases in trans fat consumption that followed changes to food regulations.
We will focus on saturated, monounstratured, and polyunsaturated fats, keeping the Standard Dogma in mind—the recommendations we get from major institutions like the USDA.
The Standard Dogma says: limit saturated fat intake and get lots of “heart healthy” unsaturated fat. Many experts agree with this advice, but many do not, arguing that it’s based on limited or outdated evidence. (We’ll have much more to say about this disagreement and its origins later on).
Here’s a simple breakdown of the fatty acids types we will consider, with some basic facts and examples. This is the approximate level of detail I believe is worth knowing for the average person curious about dietary fat and its health consequences:
Saturated: No carbon-carbon double bonds. Linear molecules. Solid at room temperature. Individual saturated fatty acids (SFAs) have different lengths.
Prototypical foods: Beef, butter, coconut oil
Example fatty acid: Palmitic acid
Monounsaturated: One carbon-carbon double bond. Bent molecules. Liquid at room temperature. Individual monounsaturated fatty acids (MUFAs) differ based on where the double bond is located.
Prototypical foods: Olive oil, avocado, certain nuts
Example fatty acid: Oleic acid
Polyunsaturated: Multiple carbon-carbon double bonds. Bent molecules. Liquid at room temperature. Further classified based on the number and placement of double bonds. Individual PUFAs differ based on the number and location of their double bonds.
Omega-3
Prototypical foods: Fatty fish (e.g. salmon), walnuts, flaxseeds
Example fatty acids: EPA, DHA
Omega-6:
Prototypical foods: Vegetable oils (e.g. soybean, corn, sunflower oil); various nuts, seeds, and grains.
Example fatty acid: Linoleic acid
You need to consume some amount of saturated, monounsaturated, and polyunsaturated fat—all are essential. Where things get tricky, and there’s widespread disagreement, is exactly how much of each. Some experts will tell you unsaturated fats are “heart healthy” and to limit saturated fat intake. Others will tell you saturated fat has been unfairly demonized and overconsumption of omega-6 PUFAs drives inflammation and metabolic dysfunction.
We next need to establish a basic understanding of dietary carbohydrates, the other macronutrient class that has shown significant historical shifts in human consumption. With that knowledge base, we can start dissecting human dietary consumption trends while considering the metabolic consequences of eating specific foods with different nutrient profiles.
Decomposing Carbs: Basic Biology of Carbohydrate Types
Like fats, carbohydrates are macronutrients that can provide energy to the body. By metabolizing (breaking down) carbs, we generate energy to power our body. Proteins can also be used for energy, but the body doesn’t “like” to use them for this—it does so only when absolutely necessary. While fats and carbs both consist of carbon, hydrogen, and oxygen atoms, the body handles them differently due to differences in their chemical structure.
Carbs consist of sugar molecules. These can one or two sugar molecules (“simple sugars”) or multiple sugar molecules stitched together in more complex structures (“complex sugars”).
Certain carbs are the “preferred” source of energy for the body, in the sense that, given the choice between carbs and fats, our cells will generally use carbs for energy first, breaking them down into single sugar molecules. These simple sugars provide a quick and readily available source of fuel, making them appropriate for high-intensity activities and those requiring quick bursts of energy.
Carbs (sugars) are typically thought as “energy for now,” fats as “energy for later” (storage). Both types of macronutrient can be used for fuel or storage—sugars can be stored as glycogen in the liver, fats go to adipose tissue.
While carbs and fats can both be used to generate energy, fats are more energy-dense than carbs, yielding about twice the energy per gram that carbs do. This is probably why we evolved the tendency to store energy for later in the form of fat and burn sugars to get energy now. In a time of famine, it’s best to have as much energy-dense material saved up as possible.
Below are some of the most common forms of dietary carbs, with examples and basic facts for each. In addition to these carbs, all found in nature, there are many artificial carbohydrates produced from human processing. We will discuss things like high-fructose corn syrup more in later posts.
Simple sugars: Single sugar molecules or a combination of two. Simple sugars are a source of energy. Different types also have distinct metabolic effects and are therefore not equivalent (“a sugar is not a sugar”).
Glucose: The primary dietary energy source of the body. Found in many natural and processed foods.
Fructose: Found in natural foods like fruits. Used in many processed foods due to it’s sweetness and reinforcing (addictive) effects.
Sucrose: One glucose attached to one fructose molecule. Commonly known as table sugar.
Complex sugars
Starch: Many simple sugars attached together. Broken down into simple sugars by the body. Found mainly in plant foods, e.g. grains, legumes, etc.
Fiber: A non-digestible carb found in many plant foods—our body cannot break these down into simple sugars for energy. Serves as food for the gut microbiome.
Soluble: Dissolves in water to form a gel-like substance. Found in oats, legumes, fruits (especially citrus), and vegetables (e.g. carrots).
Insoluble: Does not dissolve in water, retaining its structure throughout digestion. Found in whole wheat, nuts & seeds, fruit skins, and vegetables (especially dark leafy greens).
These are the major dietary carbs we will focus on. With the exception of fiber, our bodies turn carbohydrates into simple sugars to be used for energy—sucrose and starches are quickly broken down into glucose or fructose, which then circulate through the body.
We will dive into more detail later, but “a carb is not a carb.” One gram of carbohydrate from one source is not metabolically equivalent to one from another. Specific carbs differ in their ability to be absorbed and broken down into simple sugars. As we will learn, glucose and fructose—the two most common single-molecule dietary sugars—have very different metabolic effects.
Scientists employed by the food industry have also invented processed carbohydrates, engineered for mass production and consumption—cheap to produce on a large scale and designed to be maximally reinforcing (addictive). A key example is high-fructose corn syrup, a solution with comparable amounts of glucose and fructose. With sucrose (table sugar), glucose and sucrose molecules are stitched together. With high-fructose corn syrup, they are separate. These kinds of structural and compositional differences are why calories are not interchangeable—your body won’t have an identical response to sucrose and high-fructose corn syrup, even if they contain the same number of glucose and fructose molecules.
Past, Present & Future of the Western Diet
We already know the general macronutrient profile of our diet had changed over time. Now we need a more detailed comparison of diets before, during, and after the onset of the obesity epidemic. We don’t simply want to know whether fat or carb content has been higher or lower—we want to describe diet composition at the level of individual carbohydrate and fat subtypes. How has the balance of glucose vs. fructose shifted? How have omega-6:omega-3 polyunsaturated fatty acid ratios changed over time, and how do they compare to other fats?
Once we describe the evolution of the Western diet in more detail, at the level of specific macronutrients and food types, we can dive further into the metabolic consequences of these dietary shifts—how different foods, with distinct macronutrient profiles, impact our metabolism and health. See this post for a dissection of how patterns of dietary fat consumption have changed over time.
To learn more about the topics covered in this essay, try these episodes of the Mind & Matter podcast:
M&M #132: Obesity Epidemic, Diet, Metabolism, Saturated Fat vs. PUFAs, Energy Expenditure, Weight Gain & Feeding Behavior | John Speakman
M&M #131: Dietary Fat, Cholesterol, Cardiovascular Health & Disease, Carbohydrates, Dietary Guidlines, Food Industry & Diet Research | Ronald Krauss
M&M #115: Exercise Science, Nutrition, Plant vs. Animal Protein, Muscle Physiology, Sleep, Endurance vs. Resistance Training, Fat, Carbs, Amino Acids | Luc van Loon
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This is fantastic. Thank you for putting this together. I find that for me, it’s easier to understand and digest information in written form, rather than podcast, so I truly appreciate this series of essays.
Damn. This is beautiful! You explained so succinctly what I've read in an ad-hoc manner for months now. I'm taken aback, really good explanation so far!