They are used to construct the cells and tissues that form our bodies, provide sources of energy to power metabolism (as well as provide a mechanism for storing energy between meals), and are used to form the countless enzymes that drive our metabolism. Unlike the micronutrients (vitamins and minerals) which are needed in small amounts and are generally reused, macronutrients undergo a constant flux in our body, necessitating a consistent intake to provide enough energy for our survival and enough building blocks for the growth, maintenance, and repair of our bodies.
Each of the macronutrients is actually a complex of smaller building blocks with important nutritional roles. Proteins are constructed of amino acids, carbohydrates of sugars or monosaccharides, and fats of fatty acids. As macronutrients are absorbed from a meal, they are broken down into their individual constituents, which are used for the various purposes in metabolism. The great adaptability of our body chemistry gives us the ability to take these individual building blocks (amino acids, sugars, and fatty acids) and reassemble them or even interconvert them to satisfy our metabolic needs. This explains how an Innuit can consume a diet predominantly of fish protein and fat, yet still have enough carbohydrate (glucose) in their blood to fuel the demand of their brains. Or how someone can adopt a completely fat-free diet, yet still become obese through the conversion of excessive dietary carbohydrates into body fat. Because of their interconversion in the body, macronutrients themselves are not nutritionally essential, although some of their building blocks may be (nine of the amino acids and two of the fatty acids are considered essential). Evidence also suggests that although macronutrients can be readily interconverted in the body, each may have additional benefits when present in the diet as a particular percentage of total dietary intake.
To gain an appreciation of the function of each of the macronutrients and ultimately understand how much we may need, it is really necessary to discuss each individually. In this first part of the series, we will examine the roles and requirements for dietary protein.
Out of all the macronutrients, protein has the most diverse set of roles in metabolism. It forms the connective tissues that hold our organs together; it is a significant portion of our bones, and the predominant component of muscle fibers. Moreover, protein forms the thousands of enzymes which carry out critical chemical reactions, the various transporters that move nutrients in and out of cells and throughout the body, the antibodies of our immune system, and many of the hormones that direct our growth, energy utilization, and homeostasis. Due to its ubiquitous nature in the function of all living things, dietary protein occurs in most whole food sources. Muscle meats are the most concentrated sources (muscle fibers are constructed of protein filaments); milk and eggs are also good sources. Legumes, exotic grains, and many vegetables are good protein sources, such that a balanced vegetarian or vegan diet can provide adequate protein for the body’s needs. Once consumed, dietary protein is broken down by the action of stomach acid and several digestive enzymes secreted from the stomach and pancreas (called proteases). From here, the resulting individual amino acids or small protein fragments (called peptides) are absorbed from the small intestine, and distributed throughout the body to satisfy various roles. We generally don’t absorb enzymes or other proteins intact.
ROLES OF DIETARY PROTEIN IN NORMAL METABOLISM
Dietary protein has several fates in human metabolism:
New Protein Synthesis. The amino acids liberated from dietary protein can be used to make other proteins in the body. While organisms can make amino acids from other sources (such as fats or carbohydrates), making new proteins from dietary amino acids is the quickest and most energetically economical way. This is especially important for sustaining periods of rapid growth, such as during childhood development or intense weight training. Perhaps more importantly, dietary protein is the only source of essential amino acids for metabolism. Of the twenty different amino acids used to make proteins, humans have lost the ability to produce nine of them on their own (methionine, lysine, valine, tryptophan, phenylalanine, isoleucine, leucine, threonine, and histidine). Therefore, a minimal amount of dietary protein is required to supply enough of these essentials to maintain protein synthesis in the body.
Precursors to other biomolecules. Several of the essential amino acids from dietary protein are used to construct important “non-protein” chemicals for the body. For example, the hormones seratonin and melatonin, and vitamin B3 (niacin) are derived from the essential amino acid tryptophan; thyroxine (thyroid hormone), adrenaline, and the endorphins (natural analgesics) depend on intake of the essential amino acid phenylalanine. Thus, our ability to produce these hormones and neurotransmitters is heavily dependent on the presence of essential amino acids in the diet.
Dietary protein is also the predominant sources of the elements nitrogen and sulfur, both essential to metabolism. Nitrogen from dietary amino acids is redistributed in the body to make other amino acids, nucleotides (the building blocks of DNA, as well as the energy molecule ATP), and glycosaminoglycans (components of connective tissues, such as chondroitin, keratan, and hyaluronic acid). Sulfur is also used in the construction of glycosaminoglycans, as well as several important antioxidants (such as glutathione and alpha lipoic acid).
Fuel Source. Dietary protein can serve as an energy source. Following a meal, about 50 percent of the amino acids that have been released from dietary protein are metabolized by the liver into energy (ATP). Unlike carbohydrates or fats, excess amino acids from the diet are not stored in the body; if they are not immediately used to make new protein or energy, they are converted to carbohydrates or fats for storage. The liver can also metabolize most amino acids into glucose, to provide energy to the brain and other tissues. Proteins are less efficient at raising blood glucose than are carbohydrates; while they provide the same number of calories on a gram-for-gram basis (4 calories/gram), they only raise blood glucose 50–80 percent as much as an equivalent amount of carbohydrates.
Skeletal muscles depend heavily on amino acids for energy. The essential amino acids leucine, isoleucine, and valine (called the branched chain amino acids or BCAAs) are preferentially taken up by muscle cells after a meal to be burned as fuel. Healthy individuals will metabolize upwards of 10 grams/ day of BCAAs in their muscles if they are available in the diet.
SPECIFIC HEALTH BENEFITS OF DIETARY PROTEIN
In addition to their standard roles in metabolism, dietary protein has been associated with specific health effects, including:
Weight Loss and Satiety. High protein diets have been associated with better glycemic control, and have been shown to promote greater fat reduction than high carbohydrate or high fat diets that provide the same number of calories. Dietary proteins are more difficult to convert into energy than the carbohydrates or fat; diets high in protein have been reported to have a greater thermogenic effect and expend more energy than lower protein diets. Randomized, controlled trials comparing low and high-protein diets studies have shown that diets higher in protein are more effective at preserving lean muscle, reducing body fat, and maintaining lower insulin levels after a meal.
There is convincing evidence that proteins are more satiating than the other macronutrients, and that the satiating ability of proteins may be related to their amino acid composition and how quickly they are digested. Much of this effect has been attributed to the rapid appearance of the essential branched chain amino acids in the blood; leucine, one of the BCAAs, has been shown to influence the metabolic pathways in the brain that regulate food intake, at least in animal models. Evidence also suggests that higher protein intake at one meal may significantly decrease appetite at the next meal, although the studies in this area are not consistent.
Promoting Healthy Levels of Blood Lipids. Dietary protein, particularly dietary soy protein, has been studied for its ability to lower cholesterol levels by either increasing the removal of low-density lipoprotein cholesterol (LDL or “bad” cholesterol) from the blood, or as a replacement for other high fat/ high cholesterol protein sources. Over 60 controlled trials of soy protein consumption in humans have been performed, many in hypercholesterolemic patients. Taken together, these studies revealed that an average intake of 47 g/day of soy protein resulted in significant improvements in blood lipid/lipoprotein parameters, with average reductions in total cholesterol of 9 percent and LDL cholesterol of 12.9 percent. These data were the foundation for the FDA approved health claim for soy protein in the prevention of cardiovascular disease.
Promoting Healthy Blood Pressure. Several human trials and epidemiological studies have indicated an inverse associate between dietary protein intake and blood pressure. Forty-six human studies of protein intake and blood pressure (20 clinical trials, 15 observational studies and 13 biomarker studies) have demonstrated a clear, beneficial effect for plant protein on reductions in blood pressure. The reductions averaged up to a 1.4 mm Hg reduction in systolic blood pressure and a 1 mm Hg reduction in diastolic blood pressure for every 11 g of plant protein consumed per day, based on observational studies. The mechanism by which protein may reduce blood pressure is unclear; it may be helping to rid the body of sodium, it may increase insulin sensitivity, or it may increase the blood concentration of the amino acid arginine, the precursor to the blood-pressure lowering hormone nitric oxide.
HOW MUCH DIETARY PROTEIN SHOULD I BE GETTING?
As with most macronutrients, the required amount of dietary protein depends on individual needs. Discussion of the research regarding the merits of diets differing in the relative ratios of protein, carbohydrate, and fats will be the subject of a future article, but some general considerations on dietary protein content bear mentioning here.
The Food and Nutrition Board of the National Institute of Medicine has established a dietary reference intake of 56 g/day for adult men, 46 g/day for adult women, based on metabolic studies. These figures are based on a reference (“average”) body weight; a more individualized assessment of daily protein requirements for a healthy individual would be 0.8 g/kg (about 0.36 g/lb) body weight. Under circumstances of increased metabolic demand, higher protein intakes may be warranted. In pregnant and lactating women, for example, daily protein requirements increase to approximately 1.1 g/kg of body weight, or 71 g/day for the “average” woman. The dietary protein requirements of athletic individuals who wish to increase their lean body mass has been the subject of considerable debate, but is generally believed to exceed the reference values. Studies have suggested protein requirements of 1.1 g/ kg per day for endurance athletes and 1.3 g/kg per day in strength-trained athletes.
There are some circumstances where reduced protein intake may be warranted. Since excess dietary protein is not stored in the body, it must be immediately used up (to make new proteins), converted into energy, or converted to carbohydrates or fat for storage. In the latter two cases, amino acids are broken down, and the nitrogen they carry is eliminated from the body (as urea). The breakdown of amino acids and excretion of nitrogen are fundamental functions of the liver and kidneys, but for individuals with kidney or liver disease, excessive protein consumption can be problematic. Low-protein diets (below 0.8 g/kg per day) may be beneficial in these cases. Dietary protein requirements should also be carefully considered in individuals with hyperuricemia, a condition of excessive uric acid in the blood. Hyperuricemia affects an estimated 21 percent of Americans, and is a primary risk factor for gout, a type of arthritis typified by a rapid onset of inflammation, usually in the joints of the extremities. Elevated blood uric acid is also a risk for kidney and cardiovascular diseases, and diabetes. While dietary protein itself does not elevate blood uric acid, compounds found in some sources of animal protein (called purines) do increase hyperuricemia risk. Therefore, individuals with elevated blood uric acid should limit their intake of protein from meats (other animal proteins, such from milk or eggs, as well as vegetable protein, do not appear to be associated with hyperuricemia risk and may actually reduce it).
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Kevin M. Connolly, PhD
Kevin M. Connolly, PhD received his bachelor’s degree in anthropology from Brown University, and doctorate in biochemistry and molecular biology from UCLA. Before consulting for the dietary supplement industry, he spent 15 years in basic biochemistry research elucidating such diverse mechanisms as bacterial antibiotic resistance and collagen synthesis. He contributes to several online and print publications, and is a frequent guest on radio health programs throughout the country. When not writing, he teaches undergraduate biochemistry.