Macronutrient Metabolism
Let me just say, this was a really aggressive topic for one blog post and I take full responsibility for that. Oops!
WHAT IS MACRONUTRIENT METABOLISM?
Macronutrient or energy metabolism involves the various pathways underlying food intake, food breakdown to release energy and the storage of excess energy. Utilization involves additional mechanisms for converting stored energy (fat/triglycerides/adipose tissue, carbohydrate/glycogen, and when underfed, protein/muscle) into usable energy or ATP. Ideally, it takes multiple textbooks and literature reviews to explain all there is to know about biochemistry and energy metabolism/utilization… Our bodies are these incredible and amazingly complex energy factories!
I am going to cover some of the basics today. I definitely don’t want to bore you with metabolic pathways/enzymes/intermediates or anything of that nature. Next week, I’ll cover more about fuel utilization at rest and during different types of exercise.
Without further ado….
Macronutrients are the nutrients that supply our bodies with the energy we need to survive and thrive:
Carbohydrates(4 kcals/gram) are an important source of energy during REST and EXERCISE. Carbs are important for all activity, but especially during higher intensity exercises like running and lifting. As a general rule of thumb, if you are breathing hard during exercise, carbohydrate is the preferred energy source. The main sources of carbohydrates in our diet include grains/flours, beans/legumes, starchy vegetables, fruits and sugars. ALL carbohydrates are ultimately degraded into sugars and absorbed from your small intestine into your blood stream to either be used as an immediate fuel source or stored in your liver/muscles. Carbohydrate storage is limited. Therefore, when carbohydrate intake exceeds energy needs and storage capacity, they will be converted to and stored as fat.
Fat(9 kcals/gram) is also an important energy source both at REST and during EXERCISE. Although we do store small amounts of fat in our liver and muscle, the majority of this energy reserve is in adipose tissue. Fat takes longer to absorb into blood than carbohydrates and plays an important role in satiety. In a healthy individual, fat reserves are still quite large!
Protein (4 kcals/gram) is comprised of amino acids linked together by peptide bonds. There are 20 total amino acids, 9 of which are called essential because the body cannot make them and requires them from diet. A “complete” protein is used to refer to a food/supplement that contains all of the essential amino acids in high amounts. Complementing proteins are proteins that, when eaten throughout the day, provide a full complement of all the essential amino acids. Whole grains and beans are a good example of complementing proteins. When protein intake exceeds protein excretion, there is a positive nitrogen balance and new lean tissue can be produced. Eating adequate carbohydrate and protein combined supports a positive nitrogen balance. A neutral or positive nitrogen balance is preferred!
Alcohol(7 kcals/gram) is also considered a macronutrient, but I am not going to talk about alcohol today, because hopefully we’re not getting much of our energy from there anyway, right? : )
ENERGY METABOLISM
Energy metabolism is largely regulated by energy availability and hormones. Hormones help regulate what energy goes to storage and what energy is broken down for fuel. I’m not going to discuss specific hormones today, but in general, when we eat more than our body needs for energy (intake > output), we will store the excess energy for later use. Different organs have different capacity for fuel use and storage, so let’s take a look at the unique metabolic profiles of the major organs involved in converting the metabolites from food into usable energy.
The Liver: Most chemical compounds that are absorbed from the food in our intestine travel to the liver where its fate is determined. The liver is the hub of energy metabolism, meaning it regulates how much of each metabolite is in our blood at a given time. For example, the liver removes metabolites like glucose from our blood, converts them into the needed form of energy or storage, and releases or stores the products. Our liver can store up to 400kcals of carbohydrate(glycogen) before the excess(assuming muscle stores are full) is stored as fat. The liver also absorbs dietary amino acids and synthesizes proteins. In general, when adequate carbohydrate is available, protein is not broken down for fuel. However, especially when carbohydrate availability is inadequate, amino acids can be used as a fuel source. The liver can do all sorts of creative energy conversions!
The Brain: Your brain runs exclusively on carbohydrate. This organ alone consumes about 120 g (~420kcal) of carbohydrate to function properly on a daily basis. That is the equivalent of about 4 slices of whole wheat bread to fuel your brain alone. Fatty acids are bound to albumin and cannot pass the blood-brain barrier. Your brain also lacks all energy storage, so it requires a steady supply of glucose (sugar/broken down carbohydrate). Does this mean that you need to be eating bites of carby foods continuously throughout the day? No, your liver can break down its stored glycogen to supply your brain with glucose, but it DOES mean that adequate carbohydrate intake is important for proper brain functioning. The liver can only store about 300 calories of glycogen. Recommendations for carbohydrate will vary quite a bit from person to person based largely on activity level, but if you are feeling mentally fatigued, it might be worth asking yourself (or a dietitian) if you’re including enough carbohydrate in your diet. I’ll have to talk about glycemic index and glycemic response and simple vs. complex carbs another time!
*During prolonged starvation, the body can adapt to using ketones to fuel the brain. Our bodies can adapt to a lot of things but that definitely doesn’t mean it’s conducive to proper functioning and performance.
Muscle: Skeletal muscle is very different from the brain in that it has the largest capacity for glycogen(stored carbohydrate) at about 150-300g (600-1200 kcals) or 75% of all glycogen in the body. Specific storage capacity varies person to person based on muscle mass and other factors. This stored glycogen can be readily converted into energy for use within muscle cells. Unlike the liver and brain, muscle cells cannot export glucose. This glycogen becomes very important for athletes, but I will get into that more in my next post! Muscle can also store fatty acids, the major fuel source of resting muscle.
*Cardiac muscle is unique in that it functions aerobically (with oxygen), and therefore, functions mainly on fatty acids. Like the brain, the heart is not storing glycogen. The heart can also use ketone bodies and lactate as fuel.
Adipose tissue(connective fatty tissue): This is our largest energy reservoir, about 77% of total body energy is stored in adipose tissue. Here, we store on average 120,000-135,000 kcals, or about 2 months worth of energy from triacylglycerols (stored fatty acids). Fat storage was originally a survival mechanism in case of long bouts of starvation. Most of us are fortunate not to have to worry about that today.
KEY TAKEAWAYS
1. Carbohydrates, fat and protein each plays an important role in human health and performance.
2. Carbohydrate storage is limited which is why it’s important to include a sufficient amount of carbohydrate in your diet on a daily basis (especially following long bouts of exercise when glycogen stores have been depleted).
3. Based on daily activity, body composition/size among other things, macronutrient needs vary on an individualized basis.
4. The body is incredible at adapting. For example, there are pathways to make carbohydrate from amino acids and other substrates when the body is carbohydrate deficient. Most people do not need to count their macros down to the gram. Rather, strive to eat an average of 20-35% of your calories from fat, 45-65% from carbs(at least half complex) and 10-35% from protein. These are large ranges for a reason!
Next post, I’ll be talking about macronutrient utilization, one of my favorite topics and what I did my research on in grad school!!!