By Robert Santana MS, RD, SSC
Carbohydrates are perhaps the most misunderstood macronutrient. If I had to be a macronutrient, I’d probably be a carbohydrate. Since I am not, I’ll settle for being an author who defends their honor, palatability, and function in human physiology. The term carbohydrate originates from the root word carbo-, denoting carbon, and hydrate, denoting a compound that is produced when chemical substances combine with water. Put simply, they are hydrates of carbon, although structurally they are more complex than this.
Carbohydrates are classified as either simple carbohydrates or complex carbohydrates. This is where my carbohydrate lesson used to end as a teenager learning about nutrition from high school physical education teachers and sales representatives at commercial gyms or “sports nutrition” stores. Online coaches do a better job of explaining this today, but the sea of misinformation continues to ebb and flow.
Simple carbohydrates include monosaccharides and disaccharides. Mono- refers to single, di- refers to two, and saccharide originates from the Latin term sacchurum, which means sugar, and -ide is the suffix specific to these chemical compounds. Therefore, monosaccharides consist of a single sugar unit, a saccharide. The three most abundant monosaccharides are glucose, fructose, and galactose, with glucose being the most abundant. The disaccharides consist of double sugar units, and include sucrose, lactose, and maltose. Sucrose is table sugar, and is comprised of a fructose and glucose molecule. Lactose is found in dairy products and is comprised of galactose and glucose. Maltose is a sugar that forms when starch is broken down (more on this later) and is comprised of two glucose molecules. Maltose is formed when grains are sprouted in water and dried, which process activates enzymes that release maltose from stored starch, as well as other sugars and proteins. It’s found in beer, malted grains, and malt candy.
Complex carbohydrates include oligosaccharides (oligo- meaning “few”), which contain 3-10 saccharide units and polysaccharides (poly- meaning “many”), which contain more than 10 saccharide units. Most oligosaccharides are not broken down by human-produced digestive enzymes, so we rely on our “gut flora” – our intestinal bacteria – to digest them for us. The most common oligosaccharide is raffinose, which is found in beans and other legumes. Other oligosaccharides include stachyose and verbascose, which are found in peas, bran, and whole grains.
Another common category of oligosaccharides are dextrins, which are added to many commercially produced foods and supplements. Dextrins are essentially glucose units extracted from starch molecules (see below) bonded together in short chains, hence their popularity in many carbohydrate supplements. Dextrins are often listed on the food labels as maltodextrin, corn syrup solids, and hydrolyzed corn starch.
Polysaccharides, on the other hand, can consist of thousands of glucose units. The most common polysaccharides are glycogen, which is the stored form of glucose in animals, cellulose, which is the major structural component of plant cell walls, and starch, which is the storage form of glucose in plants. Starch comes in two forms, amylose and amylopectin, and both contain many chains of glucose. Amylopectin accounts for 80-85% and amylose accounts for 15-20% of the starch found in food.
The structural difference between them is in the degree of branching: amylose exists as a straight chain of glucose molecules lined up in ⍺(1,4) bonds. Amylopectin and glycogen are both highly branched, where they have ⍺(1,4) bonds to link individual glucose molecules together and ⍺(1,6) bonds that form linkages that “branch out” and create a nonlinear chain. In other words, unlike amylose which exists in a single linear chain of glucose, amylopectin (as well as glycogen) contains multiple branches of glucose chains, which makes the glucose easier to access.
You can think of this as a “bushy” tree with many branches vs. a straight tree trunk with short and perfectly horizontal branches. The advantage to branching is that there are many “ends” that glucose can be accessed from. This is in contrast to the linear chain of amylose, which has only two defined ends, and can only be accessed from one of those ends (the “non-reducing” end). During digestion, each glucose unit is hydrolyzed from “end to end” until the final unit is reached.
Imagine a tree with apples hanging from it. If you climb a ladder you can pick from several branches at the same time. If these apples hung from the tree in a straight vertical line you would have to extend the ladder vertically and pick one at a time. Thus, branching provides a metabolic advantage when glucose is needed quickly. Such a situation occurs when performing intense activity, such as squatting and deadlifting for several reps and/or sets. There are other factors involved, but this is one structural feature that makes glycogen a viable energy source during intense activity.
Cellulose is another plant form of carbohydrate that is considered a dietary fiber and not an energy source for humans. This is because the glucose molecules are linked by β (1,4) bonds, which are resistant to ⍺-amylase and can only be digested through the action of cellulase. Since mammals do not produce the cellulase enzyme themselves, grazing animals rely on the more robust gut flora in their more complex digestive systems to access cellulose for energy utilization.
Since “carbohydrate intolerance” has become a fashionable diagnosis recently, a discussion of carbohydrate digestion is in order. This is also useful in understanding the human digestive processes from a general standpoint. In general, protein digestion begins in the stomach and is finished in the small intestine, carbohydrate digestion begins in the mouth with enzymes in your saliva and is finished in the small intestine, fat digestion occurs in the small intestine, and the absorption of all three occurs in the small intestine.
As previously stated, monosaccharides are the smallest unit of sugar and are ready for immediate absorption in the small intestine. Most foods consumed contain di- and polysaccharides, which need to be broken down, or hydrolyzed, to their respective monosaccharide units for absorption in the small intestine. All dietary carbohydrates travel through the GI tract and are eventually absorbed into the bloodstream after hydrolysis. Since glucose and galactose transport differs from fructose transport, they will be explained separately.
All ingested carbohydrates travel down the esophagus and into the stomach, where gastric juice is released to form chyme, a semi-liquid mass of partially digested food. Gastric juice primarily consists of hydrochloric acid and facilitates the denaturation and digestion of proteins, the release of micronutrients, and the killing of ingested bacteria. Therefore chyme is a very acidic substance. After chyme is formed in the stomach, it is released into the duodenum (the upper portion the small intestine), while the pancreas simultaneously releases pancreatic juice (notably containing bicarbonate and digestive enzymes) to buffer the hydrochloric acid to protect the intestinal walls and continue the digestion of polysaccharides if needed.
Upon entry into the small intestine, glucose and galactose are absorbed primarily via secondary active transport, and to a lesser extent facilitated diffusion. Secondary active transport, in this instance, occurs when glucose binds to sodium to travel down its concentration gradient, and into the cell. In other words, glucose is like Marty McFly riding his skateboard grabbing onto cars to get to school, only the road has to stay reasonably empty or he’ll be stopped in traffic. Facilitated transport is similar in that the glucose molecule is bound to a transport protein but does not require a second substance to latch onto. In this case, Marty just drives the DeLorean instead of skateboarding behind cars.
Main point: All carbohydrates must broken down into their respective monosaccharide units for absorption. For polysaccharide digestion, this begins in the mouth and continues in the small intestine. Monosaccharide units enter the intestines through transport proteins, are released for absorption, then exit the intestine using protein transporters.
Disaccharide Digestion and Lactose Intolerance
Virtually all disaccharide digestion takes place in the microvilli, or brush border, of the upper small intestine. The hydrolysis of disaccharides requires disaccharidases, which are enzymes that hydrolyze disaccharides to their respective monosaccharides. For example, lactose is hydrolyzed into galactose and glucose, sucrose is hydrolyzed into fructose and glucose, and maltose is hydrolyzed into two units of glucose. The resulting monosaccharides then enter the enterocytes.
Since we are on the topic of lactose and lactase let’s touch on lactase activity in humans. Lactase activity is highest in human infants and decreases steadily after weaning. This may result in lactose intolerance in some individuals. Approximately 2/3rd of the world population is somewhat lactose intolerant, with the highest frequency of lactose intolerance seen in Native Americans, Asians, and individuals of Middle Eastern descent. It is much less prominent in individuals of Northern European descent. In other words, it’s not a widespread problem that applies to every single human on the planet. Additionally, with modern advances in food technology you can now purchase lactase treated milk or do it the old-fashioned way and add lactase enzymes to regular milk. The idea that milk is this terrible thing that only cows should consume is fashionable, but not based on fact in most situations. Sorry to disappoint, but cow’s milk is not cyanide or any other type of toxic substance, and other mammals such as cats and dogs also consume it without problems.
There are plenty of academic textbooks that cover the intricacies of glucose metabolism. This article is not written for academics – they already know everything – so I’ll let them argue about the superiority of one term over the next. Since those of you with a science degree heard it, read it, and saw it in 25 courses and those of you without science degrees will feel like you are reading Chinese, I’ll just dial down the details to the main points, with a few diagrams for the enthusiasts.
After entering the small intestine, approximately 15% of glucose leaks back through the brush border into the intestinal lumen, 25% enters into circulation passively (instead of latching onto cars or driving the DeLorean, Marty just walks), about 60% is transported from the cell into circulation bound to a carrier protein, and a small fraction may be used by the enterocytes to fuel their own energy needs. Since glucose is a polar molecule, it cannot readily cross the phospholipid bilayer of the plasma membrane.
As stated earlier, glucose is transported out of the enterocyte and to the liver for glycogen storage or release. As glucose enters the liver, a phosphate group is attached to carbon number 6 of the sugar via phosphorylation, making it glucose-6-phosphate via the enzyme glucokinase. The glucose-6-phosphate can be used for energy via glycolysis, or it can be stored as liver glycogen. Glucokinase is stimulated by insulin, which is the hormone responsible for lowering blood glucose levels. Thus, as glucose levels rise, insulin is released, glucokinase is expressed, and the liver converts glucose to glucose-6-phosphate for conversion into glycogen or into pyruvate or lactate for energy use (more on this later). Glucokinase has a low affinity for glucose, which means that at higher glucose levels, glucokinase operates at a higher velocity, which allows the liver to rapidly remove glucose from the blood. This is practical, because at normal or low glucose levels it would not make a whole lot of sense to clear glucose quickly unless we were physiologically suicidal. The process described above is part of what makes the liver the key regulator of glucose homeostasis.
Glucose is also stored in the muscle, kidney, and adipose tissue as well. Since muscles account for most of our bodyweight, they store most of our glycogen (approximately 500 grams vs 100 grams in the liver vs marginal amounts in the kidney and adipose tissue). Muscle glycogen storage follows a similar pathway as liver glycogen, only that hexokinase (instead of glucokinase) is the enzyme that converts glucose into glucose-6-phosphate. The key factor is that hexokinase is inhibited by high glucose-6-phosphate concentrations, which means that when the muscle has stored enough glycogen, glucose-6-phosphate is no longer converted to glycogen, leading to a reduction in glucose uptake into the muscle.
Additionally, muscle glycogen can only be used to fuel the muscle itself and cannot be broken down into glucose for release into the bloodstream. This is due to the lack of the glucose-6-phosphatase enzyme in the muscle cells, which is the enzyme responsible for converting glucose-6-phosphate into glucose. Alternatively, muscle glycogen can be converted into alanine, via the glucose-alanine cycle, or lactate, both of which can be converted to pyruvate, which can convert back into glucose in the liver. In short, glucose entry into the muscle is a one way trip and thus does not contribute to blood glucose levels.
Now that we understand how glucose is digested, absorbed, and metabolized, let’s move onto to its role in energy production. As stated in my Calories for Barbell Training article, humans expend energy simply being alive, as well as by digesting and absorbing nutrients. The small intestine uses glucose for energy to perform the functions described above. This can change with dietary manipulations, but if you take a human that is naive to nutrition fads and left him with food to eat from all major food groups, the diet would likely be mixed. So, everything we discuss moving forward assumes a mixed diet and normal physiology.
The vast majority of our physiological functions preferentially use glucose for energy. By preferentially, I mean that if the option to choose between macros is present, as it is when consuming a mixed diet, the human body will opt for glucose over fats and protein. Our intestines, our nerves, our heart, and our brain all use glucose to perform the major vital functions necessary for us to stay alive. Proteins and fats can also be used for energy but when given the option, the human body prefers glucose because it is lazy and likes efficiency. In fact, it prefers glucose so much that our liver will convert amino acids from proteins and glycerol from fats to glucose via a process called gluconeogenesis if carbohydrate intake is insufficient. More on this later.
There are three major energy systems in human metabolism: 1.) Phosphocreatine (ATP/PC), 2.) glycolysis, and 3.) oxidative phosphorylation of fatty acids. All three of these energy systems are working simultaneously regardless of the activity. However, the intensity of the activity performed will dictate which energy system predominates during the respective activity. At rest, most of our energy production comes from oxidative phosphorylation, which means we theoretically burn more dietary fat assuming that all we are doing is resting. Now, what is interesting about carbohydrates is that if we eat more carbohydrates we use more carbohydrates. So, if we were to measure the respiratory exchange ratio (RER) at rest, consume a carbohydrate rich meal, and then measure it again, you’d see an increase, which is reflective of higher carbohydrate oxidation.
This also concurrently suppresses fatty acid oxidation if carbohydrates are overfed, which means that we oxidize less fat on a high carbohydrate diet. In contrast, fat consumption has a minimal effect on fat oxidation, and excess fat is easily stored if calorie intake is sufficiently high. This is because dietary fat is already in its triglyceride form and can easily be sequestered into fat cells without additional metabolic steps. So, while we should theoretically burn mostly fat at rest or during light activity, if we consume a mixed diet, we’re likely burning an even split of fat and carbohydrates.
This discussion is a general overview of what happens when we are lying in bed resting or if we decide to eat some donuts. Now what happens when we start moving around? If activity is light, oxidative phosphorylation continues to predominate. When we start moving around vigorously – running, sprinting, or squatting and deadlifting – things change. If we are squatting heavy or sprinting, our ATP/PC system predominates and we rely primarily on stored creatine phosphate and ATP for energy. The issue with this is that we do not store very much ATP; we continuously recycle it, and creatine stores are very limited. This is why creatine monohydrate supplementation is useful for lifters. The more creatine we have available the more high intensity muscle contractions we can produce. Even with that said, our ATP/PC system is only useful for activities that last 5-10 seconds.
As previously stated, the liver converts glucose into muscle glycogen. The liver also converts glucose into usable energy via a metabolic pathway referred to as glycolysis. Glycolysis begins after the conversion of glucose to glucose-6-phosphate to one of two possible end products: pyruvate or lactate. For our purposes all that you need to know is that in the presence of oxygen (i.e. under “aerobic” conditions) glucose converts to pyruvate, which converts into acetyl coenzyme A (“acetyl CoA”) for entry into the Kreb’s Cycle. Both glycolysis and the Kreb’s cycle release electrons via the electron carriers NADH and FADH2. The electrons are sent to the electron transport chain, where ATP is produced.
In the absence of oxygen (i.e. under “anaerobic” conditions) glucose converts into lactate. Since strength training is an anaerobic activity, the majority of glucose is converted to lactate. Lactate can then be 1.) shuttled to the liver to be converted back into glucose (referred to as the Cori Cycle), can be 2.) used by the muscles themselves as an energy source, and can be 3.) shuttled to other organs in the human body for use (e.g. the brain, heart, etc).
Lactate can also act as a buffer, which is why it is not the cause of soreness during or post-exercise or the cause of exercise-induced acidosis. The reason that muscles “burn” during exercise is because during anaerobic glycolysis the rate of ATP resynthesis increases, resulting in a greater release of hydrogen ions, which leads to a reduction of intramuscular pH, thus causing “the burn.” As a result, lactate can be a useful indicator of exercise induced acidosis, but it’s not the cause.
Anaerobic glycolysis predominates for 15 seconds to 2 minutes, with glycogen serving as the main source of glucose under these conditions. The end product of aerobic glycolysis is pyruvate, which can be converted to acetyl CoA for entry to the Kreb’s cycle, where electrons are passed to the electron transport chain for ATP production. Keep in mind that all three systems are working simultaneously, so although creatine phosphate predominates during heavy barbell exercises, we are also breaking down muscle glycogen during our 5 sets of 5 squats and even burning off trace amounts of fat. This is where carbohydrates become essential in the lifter’s diet. The key point is that heavy lifting happens relatively quick, so the fuel source needs to be accessed relatively quickly. Creatine and glucose are much easier to access than fat, which takes much longer to oxidize (more on that in the fat paper).
Macronutrient Partitioning, Weight Management, and Carbohydrate Goals
Now my watered-down attempt to be “academic” has concluded, so let’s talk about What You Care About: the role of carbohydrates in weight management and training performance. Optimizing macronutrients for lifters is essential to make sure that we are properly fueled and recovered. There are many diet fads that restrict entire macronutrient food groups (e.g. low fat, low carb) with various claims of their effectiveness. We have established that carbohydrates are essential for performance under the barbell, so “eating keto” typically results in fatigue and excessive soreness for most. The proponents of low-carb dieting argue that it is superior because we are better at burning fat and become more insulin sensitive. Here are some factors to consider with this argument.
If “eating keto” (<20 g carbohydrate) makes us better fat burners, we therefore will burn more fat, correct? Well yes, because in the near absence of dietary carbohydrates, and presence of high dietary fat intakes, the body has to adjust and oxidize more dietary fat for energy because it still needs ATP to perform basic physiological functions. It does this by breaking apart the glycerol molecule from triglyceride (the storage form of fat in adipose tissue, three fatty acid molecules and a glycerol group) so that it can be used for gluconeogenesis. Amino acids can also be converted to glucose via this pathway as well.
Now we become more efficient at using dietary fat for energy but this does not necessarily mean that we will burn more stored body fat. Mobilizing stored body fat is primarily a math problem. Sure, there are genetic limitations as well as psychological and hormonal influences, but for our purposes let’s assume normal physiology. I’ve already discussed why the “3500 calorie rule” does not always apply, but it can be useful for illustrative purposes. Assuming a pound of body fat requires a 3500-calorie deficit, one would have to subtract 500 calories per day for 7 consecutive days to lose 1 lb of body fat/week, 1000 calories per day to lose 2 lb/week, 1500 calories per day to lose 3 lb/week, etc. This applies whether you are in ketogenesis or not, and we are all subject to the limitations of our metabolic rate.
Since most individuals have a total daily energy expenditure of 1500-4000 calories per day, they will run out of room to subtract food real quick, thus resulting in a 1-2 lb fat loss per week for most individuals. This is regardless of macronutrient ratios. The human body stores fat like we save money – it’s our reserve in the event of a recession, which in this case would be a famine. We only use it if we are in a net energy deficit. Therefore, if a lifter has weight to lose, calorie restriction should be primarily through fat restriction while keeping carbohydrates and protein as high as possible for as long as possible.
If a lifter has to gain weight, similar ratios are still advantageous. It is well established that high-carbohydrate diets are superior for performance at all sports for the reasons described above. There are several situations where lifters need to gain weight for various reasons. These reasons include, but are not limited to, underweight status, filling out a weight class, moving up a weight class, or simply to gain muscle mass if the goal is aesthetic. This is done by eating at a caloric surplus. That caloric surplus will be high in carbohydrates for most people. As we stated, carbohydrates are necessary for glycogen storage, and carbohydrates also stimulate insulin release. Insulin is the most anabolic hormone in the human body and facilitates deposition of both glucose and amino acids into the muscles. This results in better training performance and better recovery from those training sessions.
Since we are trying to skew weight gain towards lean mass and away from fat mass, it only makes logical sense to keep fats at moderate levels when eating to gain weight since it contributes the least to performance for reasons described above. So how many carbohydrates should you eat a day? My practical experience as an RD, the published literature, and the established guidelines have shown that self-reported carbohydrate intakes of 300-400 g/day of carbohydrates for adult males and 150-250 g/day of carbohydrates for adult females tends to work well for getting strong under isocaloric or hypercaloric conditions. This results in 1200-1600 calories/day and 600-1000 calories/day for males and females respectively. This lines up with 45-65% of calories from carbohydrates assuming a 2500-3500 and 1500-2500 calorie diet for males and females respectively.
These should be comprised mostly of fruits, vegetables, legumes, and whole grains, with simple carbohydrates reserved to peri-workout meals. Additionally, ~40 g/day and ~30 g/day of these carbohydrates should be in the form of fiber for males and females, respectively. This is a general guideline and serves as a good starting point, but it will ultimately need to be adjusted for the individual. Older adults often need less than this because we become more insulin resistant with age, and we are also not training as much.
Carbohydrate timing need not be complicated, but there are some physiological considerations worth mentioning. Glucose and insulin follow a diurnal variation, and general guidelines have been proposed based on this. Glucose is typically lowest in the morning with insulin sensitivity the highest. Glucose peaks in the late evening/early morning hours (i.e. ~1:00 am), with greater insulin resistance. This diurnal flux has also been observed in individuals who work graveyard shifts, which means it is independent of sleep.
This serves as the premise of recommending that carbohydrates, especially simple sugars, be consumed earlier in the day, with minimal starchy carbohydrates in the evening. These meals should also be spaced out in evening time intervals to promote more steady changes in glucose and insulin throughout the day. If it is a training day and you train in the evening, it is best to shift your carbohydrates to the afternoon hours, with most of them consumed before and after training sessions. Although the insulin sensitivity tends to decline in the evening, a phenomenon known as exercise-mediated glucose transport occurs independently of insulin. This means that training alone triggers an increase in glucose transport to the muscles both during and after exercise. Additionally, post-exercise carbohydrate has also been shown to reduce markers of muscle damage.
So although insulin sensitivity isn’t optimal in the evening, we don’t need as much insulin to transport glucose during and after workouts. Training earlier in the day theoretically takes advantage of the more optimal insulin response in the morning hours. However, it is unclear if a superior time of day exists other than the time that works best for the lifter. The theories were worth mentioning here since they do come up quite often.
It is quite fashionable to discredit carbohydrates as toxic waste, unnatural, disease causing, baby seal-killing nutrients that we should steer clear of. Fashion does not always line up with reality. Athletes have been consuming carbohydrate-rich diets for centuries, and the approach continues to work today. I wish that I had some complicated voodoo to sell to you here, but unfortunately I am clearly on the path of avoiding extreme wealth by not being a very good crook. Now, some individuals may fall outside of these recommendations, and that’s fine. Remember, this paper assumes normal physiology and thus clinical conditions (e.g. diabetes, epilepsy, various autoimmune diseases) are something to refer to a RD for further discussion. But this material is quite simple, and the information has been available for several decades. However, just like 5 exercises can’t possibly be enough to get strong and build muscle, neither can a well-balanced diet including 5 major food groups, right?