Metabolism refers to the chemical processes that are continuously occurring in the human body to generate energy or to produce more complex molecules, such as proteins, that are essential for life.
Most Americans get about 50% of calories from carbohydrates (about 1200 calories each day), 35% from fats (about 900 calories) and 15% from protein (about 325 calories).
Resting Energy Expenditure (REE)
Resting Energy Expenditure is the work your body (or metabolism) is doing while you sleep - basic metabolic processes which enable the body to stay warm, breathe, pump blood, etc.
Resting Energy Expenditure (REE) and Basal Metabolic Rate (BMR) are interchangeable terms. (They are measured differently, but differ by no more than 10%.)
REE accounts for burning around 1,500 calories each day, but the range is approximately 1,300 to 1,800.
Your heart is the major contributor to REE followed by the kidneys, brain, and liver.
In total, organs account for approximately 75% of REE. The other significant contributor to REE is skeletal muscle which accounts for 20 to 25%. Together, organs and muscle, which we often call lean body mass (LBM), account for nearly 95% of REE.
REE accounts for between 40% and 80% of your total daily calorie needs. The biggest variable in daily calorie needs is muscle.
Up into your 20’s, the body is likely still adding muscle and REE is still increasing; but later in life into your 60’s, muscle mass and REE begin to decline.
REE is your baseline for daily calories needs. REE is an important part of your daily energy expenditure, but the part you can control is the daily energy expenditure associated with voluntary movement … exercise … and muscle becomes the key to healthy metabolism.
Glucose is essential as a fuel for the brain and nervous system but in excess is extremely damaging to fine blood vessels in the eyes, heart, kidneys, and legs. This glucose excess or toxicity is the disease of diabetes.
Because glucose is both essential and toxic, the amount of glucose in the blood is very carefully controlled mostly by the liver and the pancreas (ie. insulin).
The Food & Nutrition Board (FNB) of the National Academy of Sciences, who establishes nutritional requirements, has stated that, contrary to protein and fat, there is no dietary requirement for carbohydrates.
But along with that statement, the FNB also states that they believe the diet is healthier with a minimum daily intake of 130 grams of carbohydrates plus at least 20 grams of fiber. This amount of carbohydrates allows for creating diets with a mixture of vegetables, fruits, and whole grains.
The unfortunate thing is that most Americans consume more than 300 grams of carbohydrates and less than 10 grams of fiber mostly from highly refined grains in breads and cereals plus potatoes and sugar.
Most researchers and physicians believe that this excessive intake of carbohydrates is a major factor (if not the most significant factor) contributing to epidemic levels of obesity, diabetes, and other preventable chronic diseases.
The only purpose for carbs is providing fuel. Fuel use falls into two categories: 1) obligatory needs to fuel the brain, nerve cells, and red blood cells (RBC). These cells can only use glucose (blood sugar) as a fuel and together they require about 80 grams of glucose each day; and 2) use by skeletal muscle.
Skeletal muscle can use carbohydrates, fats, and protein for fuel. The actual amounts and combinations of these fuels used by muscle depends on your physical activity and what you eat. At rest, muscle is using glucose at 2 to 3 grams/hour, so the daily total use would be between 45 to 70 grams. The combination of the obligatory needs for the brain plus the resting needs for muscle add up to the 130 grams recommended by the FNB.
Beyond these basic needs for the brain, RBC, and muscle, any additional carbohydrates that enter the body must get used or stored rapidly. After a meal containing carbohydrates, blood sugar will begin to rise. The rise in blood sugar triggers the pancreas to release insulin and the insulin increases the movement of the glucose from the blood into cells, particularly the liver, muscles, and body fat.
If the increase in blood sugar gets too high (above 140 mg/dL) or remains elevated too long (beyond 2 hours after the meal), these are the definitions of diabetes. The cause of diabetes is generally assumed to be related to insulin, but the alternative explanation is simply consuming more carbohydrates than the body can use.
One of the confusing things about carbohydrate needs is that the body makes glucose from protein or more specifically amino acids. For every 10 grams of dietary protein consumed, the body will eventually convert the amino acids into 6 grams of glucose. This means that glucose needs are at least partially met by conversion of dietary protein.
Protein: The Essential Macronutrient
Protein is the food source of amino acids. We consume protein in food; the protein is digested in our intestines; and the amino acids are then absorbed into our body.
The 20 amino acids we get from protein have numerous metabolic roles within the body. The most fundamental and basic role is providing the building blocks to make new proteins within our bodies. Your skin, bones, liver, hair, and muscles are all made up of different proteins and each protein has a different combination of the 20 amino acids.
Among the 20 amino acids in food, we have an absolute daily requirement for 9 of these amino acids. These 9 amino acids are defined as essential or indispensable because the body cannot make them.
This is reflected in the minimum requirement for protein defined as the RDA which is 0.8 grams of protein per kilogram of body weight (~ 0.36 grams/pound). But each amino acid has one or more additional metabolic roles, and the success of these roles is dependent on consuming more protein than the minimum RDA.
The dietary protein requirement necessary to optimize the metabolic roles associated with each of these amino acids is approximately 1.6 grams/kilogram body weight (~ 0.72 grams/pound) or twice the minimum RDA.
Leucine provides a great example to understand the difference between the minimum requirement for building new proteins versus the optimum need for metabolism. The minimum RDA for leucine is set at 2.5 grams PER DAY, but we now know that optimizing MPS requires that EACH MEAL contains approximately 3.0 grams of leucine.
Higher protein diets have multiple positive effects on body composition and especially your muscle metabolism. That is why higher protein diets are particularly beneficial during weight loss, they accelerate fat loss while protecting lean muscle tissues, including organs and muscles.
Higher protein diets enhance satiety. They reduce feelings of hunger and the desire for snacking making it easier to control your calorie intake. Protein also generates higher energy expenditure compared with carbohydrates or fats and substituting protein for carbohydrates reduces stress on insulin production and improves regulation of blood sugar, which is critical for controlling and preventing diabetes.
Underlying each of these benefits of protein is the essential amino acids is called leucine. Leucine is an important amino acid making up 8% to 10% of virtually every protein in the body. Beyond its role in the structure of proteins, it has important metabolic roles in energy metabolism, regulation of insulin, use of blood glucose, and stimulation of protein synthesis in muscle.
Maintenance of muscle is determined by exercise and dietary protein. We now know that triggering muscle protein sythesis (MPS) requires meals that contain approximately 3.0 grams of leucine. This amount of leucine will cause a rapid increase in the amount of leucine within muscle and that serves as a signal to the muscle that the diet has enough protein to support MPS.
After a meal containing protein, the body begins to digest the protein and absorb the individual amino acids. The leucine present in the protein begins to appear in the blood. The amount of leucine in the blood increases, and the amount of the increase is directly related to the amount of protein in the meal. In most meals, leucine accounts for 8% to 10% of the protein. The increased level of leucine will trigger insulin release by the pancreas which helps all cells use both blood sugar and amino acids. In muscle, the leucine triggers a unique regulatory protein called “mTOR” which signals the muscle to build new proteins. The leucine also signals the mitochondria to burn more fatty acids which supplies the ATP fuel necessary for muscle protein synthesis (MPS). This meal effect of leucine produces an anabolic response in muscle that helps muscle repair, replace, and remodel existing proteins. This process of is essential for your muscles to remain healthy.
It is also important to understand that this anabolic period only occurs after a meal that contains at least 3.0 grams of leucine and it only lasts for about 2 hours. To translate the 3.0 grams of leucine into your need for dietary protein, remember protein typically contains 8% to 10% leucine, so you would need 30 to 36 grams of protein at a meal to provide enough leucine to trigger the MPS response. This short-term meal-response also means that to maintain muscle health, we need at least two or three protein meals each day.
One additional benefit of the meal-response to protein is the energy expenditure. MPS requires significant amounts of calories or ATP. After a protein-rich meal, you often feel warmer. This is called a Thermic Effect for Food (TEF). This TEF is generally thought to be associated with the digestion, absorption, and metabolism of food, but for protein it directly relates to the energy costs of MPS. Because of the unique metabolic roles of protein, the TEF for protein is 15% to 20% while TEF for carbohydrates and fats is only 5%. That means, if the nutrient label says the food has 100 calories from protein, you lose 20 calories as heat (or TEF) and only really get 80 calories. But with carbohydrates, 100 calories yields 95 calories. This is an important benefit of protein in a weight loss diet. Simply by replacing carbohydrates or fats with protein you will increase calorie expenditure and help with weight loss.
Skeletal muscle is the largest metabolic organ in our body. It makes up about 40% of total body weight and accounts for nearly 50% of all protein in our body.
Muscle is the most important organ of your body for energy expenditure and the only component of energy expenditure that is really in your can control. Resting Energy Expenditure (REE) or your resting metabolism is involuntary. Your heart, kidney, and other organs are running while you sleep. While at rest, muscles account for about 20% of REE or 300 calories.
Skeletal muscle uses both fatty acids (fat) and glucose (a type of carb) as fuel. The exact proportions relate to your physical activity and what you eat. Healthy muscles in young, active individuals get 80% to 85% of their fuel from fatty acids.
Fat is the preferred fuel for virtually all the body (except for the brain and RBC).
In muscles, fatty acids are broken down or “burned” in the mitochondria. Mitochondria are often referred to as the furnace of the cell because they use oxygen to essentially burn the carbon contained in the macronutrients – fatty acids, glucose, and amino acids.
This process is called aerobic metabolism because it uses oxygen. Aerobic metabolism is ultimately limited by the number of mitochondria in your muscles and your ability to get enough oxygen to your muscles (i.e. breathing and circulation). During sleep and during normal daily activities, the predominate fuel for muscles is fat. Exercise training increases the number of fat-burning mitochondria in muscle.
Skeletal muscle can also use glucose as a fuel. Carbohydrate use depends on physical exertion and diet.
The relative proportion of fuel coming from fatty acids shifts to glucose as the intensity of the activity or your level of exertion increases. Fatty acids are the predominate fuel for muscles up to a heart rate of about 100 to 130 beats per minute (estimated as 60% of maximum exertion). At this point, the amount of ATP required to perform the activity exceeds the mitochondria capacity and your ability to get oxygen to cells. Muscles then shift to what is called anaerobic metabolism, meaning muscles are generating ATP from glucose without oxygen. During anaerobic metabolism, muscles quickly generate the essential ATP for movement, but they also produce lactic acid and are in oxygen-debt. If this oxygen-debt and accumulation of lactic acid continue, muscles will fatigue and may experience cramping.
Your diet also affects the balance of fatty acids or glucose used by your muscles. Remember blood sugar is both essential for the brain but potentially toxic. If blood sugar gets too high (from too many carbs), it causes diabetes. Because blood glucose must be maintained within a narrow range (not too low; not too high), the body will always elect to eliminate glucose first after a high carbohydrate meal.
Typically, carbohydrates in a meal are digested and absorbed within one to two hours. These carbohydrates must be eliminated by burning as fuel or storage as glycogen. If those two options don’t eliminate the glucose, the body must begin to alter other aspects of metabolism. The first change after a high carbohydrate meal will cause the body to stop burning fats and shift to remove the excess glucose.
The body shifts fats into storage and forces muscles to shift from 85% of fuel coming from burning fats to only 10% coming from fats and 90% coming from the carbohydrates. This shift in metabolism and fuel use requires a well-controlled and highly effective insulin response. Over time and with repeated exposure to high carbohydrate meals, the insulin response will become less effective (i.e. insulin resistance) and the body begins to exhibit signs of pre-diabetes.
If the shift in muscle metabolism from fat to glucose is not enough to stabilize blood sugar after the meal, the liver will begin to convert carbohydrates into fat. These new fats that are made in the liver are then packaged into transport units called triglycerides which move these new fats through the blood to be stored in your body fat. If you are exceeding your Carbohydrate Tolerance, you will develop the pre-diabetes symptom of elevated blood triglycerides (TG), meaning you will have TG > 150 mg/dL and ultimately may develop the serious condition known as fatty liver. These are conditions leading to diabetes.
If your blood value for fasting blood glucose (FBG) is > 100 mg/dL, or your triglycerides (TG) are > 150 mg/dL, or your A1c is > 5.7, or if your blood value during the two hours after a meal exceeds 140 mg/dL, these are all indications of abnormal glucose metabolism, and that you are regularly exceeding your body’s ideal carbohydrate tolerance.
Match Your Carb Intake to Your Activity Level
Your Carbohydrate Tolerance requires that you learn to match your dietary carbohydrate intake with your exercise needs for carbohydrate fuel. The range for carbohydrate use is from about 20 grams per hour for mild exercise up to about 70 grams per hour for intense exercise.
NOTE: Most of the information on this page has been sourced from Dr. Donald Layman's site Metabolic Transformation