Mary M. Fynn, Ph.D, RD, L.D.N

Nutrition Topics

Energy yielding nutrients

The energy yielding nutrients are carbohydrate, lipid, and protein. Alcohol supplies energy, but does not provide any other nutritional value and therefore will not be discussed here. For nutrition purposes, “energy” is the term used for what is commonly called “calorie.” The energy from a nutrient is the amount of heat released when the nutrient is burned. In the US, energy is commonly denoted as calorie (kcal). There has been an attempt to switch to the use of joules, which is the international unit of measure for energy. However, like the attempt to implement the metric system in the US, the change to joules has been met with resistance. Energy is expressed as kcal per gram of a nutrient:

Nutrient Kcal per gram
Carbohydrate 4
Protein 4
Lipid 9

To convert kjoules to calories: 1 calorie = 4.184 kJ. Then divide kJ by 4.184 in order to express kjoules as calories.

If you know the number of grams of an energy yielding nutrient, the way to determine the total energy in a food is calculated like this:

Snickers: 4 mini

Grams kcal per gram Kcals supplied by the nutrient
Cabohydrate 25 x 4 100
Protein 2 x 4 8
Lipid 8 x 9 72
Total 180

This food has an energy distribution of 100 kcals from carbohydrate, 8 kcals from protein, and 72 kcals from lipid. In food labeling, the manufacturer is allowed to round the numbers. Therefore, if the calories for 4 mini Snickers added up to, say, 177, the nutrition label of this particular food item could indicate that it has a total of 180 or even 175.

Dividing the kcals supplied by the nutrient by the total number of kcals gives you the percentage of nutrients in the food. Therefore, this product would have:This food has an energy distribution of 100 kcals from carbohydrate, 8 kcals from protein, and 72 kcals from lipid. In food labeling, the manufacturer is allowed to round the numbers. Therefore, if the calories for 4 mini Snickers added up to, say, 177, the nutrition label of this particular food item could indicate that it has a total of 180 or even 175.

Carbohydrate = 100/180 = 56%

Protein = 8/180 = 4%

Lipid = 72/180 = 40%


Functions of protein:

  • • essential component for many vital cell constituents
  • • carriers in blood for vitamins and other compounds
  • • hormones, cell membrane receptors, antibodies
  • • enzymes
  • • fluid balance
  • • supporting structures
  • • bull glucose synthesis (gluconeogenesis); this is a survival mechanism and will occur when carbohydrate intake is not sufficient to maintain blood glucose
  • • supply energy; this typically only occurs at the end of endurance exercise when blood glucose is low.

Protein has a wide variety of important functions. Although it does supply energy at times, protein does not typically do so, contrary to conventional wisdom. Protein, in contrast to carbs and lipids, contains nitrogen.

Proteins are composed of twenty amino acids. Ten of these amino acids are essential, which means they cannot be synthesized at an adequate rate and need to be re-obtained through one’s diet. Eight of these, in turn, are essential all the time. The remaining two, arginine and histidine, are known as being “conditionally essential” because we require them only in times of rapid growth. On the other hand, we can produce nonessential amino acids whenever we need them.

The content of essential amino acids determines whether we categorize foods rich in protein as being either “high” or “low” quality.

A high quality protein is also known as a complete protein because it contains all of the essential amino acids. Meat and dairy provide the majority of high quality proteins. Eggs are the highest quality proteins. Soybeans and quinoa are both plant and complete proteins.

Lower quality proteins, in contrast, are called incomplete proteins because they lack one or more of the essential amino acids. Plant protein and lower quality protein are one and the same, comprising all starch (such as grains, potatoes, legumes) and vegetables. With low quality proteins, we may receive all of the essential amino acids by combining two different types of food products. This is the only way to accomplish protein synthesis, for without all of the essential amino acids the synthesis will not take place. For this reason, there were those who worried about the need to combine sources of plant protein in order to verify that amino acids were being consumed at appropriate levels. Nonetheless, as the number of vegans and vegetarians has increased without any evidence of mass protein deficiency appearing, this anxiety has since been allayed.

Extra dietary proteins are not stored as protein, but as fat. Whenever we consume more protein than we need, those amino acids are catabolized and either become components for energy or are converted into triglycerides and then stored as fat. Consuming a large amount of protein, therefore, will contribute to excess body fat.

Dietary requirements for protein assume that one’s energy needs are met. If these needs are not met, then amino acids are used for energy. In addition, these requirements also assume that one has consumed a sufficient amount of carbohydrates. If not, then amino acids will be relied upon to make glucose through gluconeogenesis. Although protein requirements are based mainly on the content of lean tissue in the person, body weight is used more conventionally. If someone has a higher than average amount of body fat, then the “ideal” body weight for that person should be used. For a healthy adult, the protein required is about 0.8 g/kg.

Although the protein requirements for athletes may be slightly higher than for non-athletes, the literature on this subject is far from clear. Since the percentage of lean tissue in athletes is higher than in non-athletes, a larger requirement is possible. Yet many of the athletes who participated in these studies were college-aged males. Their protein intake may have been higher as a consequence of not being fully formed. Meanwhile, different food sources can yield different results.

In the event that there is a requirement for extra protein, it should be consumed shortly after an intensive workout, forty-five minutes being the typical window. During this period, muscle enzymes take amino acids from the diet up into the muscle tissue, thereby replenishing them. For an athlete, the typical requirement of proteins is thought to be about 1.2 to 1.6 g/kg.

Protein deficiency
When energy levels are adequate, but protein intake is low, especially when it is abruptly lowered, say, during a famine, then one can develops kwashiorkor. Kwashiorkor results in stunted growth, skin lesions, and decreased plasma albumin. In young children, it typically presents itself as a swollen belly on an otherwise emaciated body. A lack of protein, which prevents fluid from being balanced, causes the swelling.

Marasmus is a state in which overall energy is low for extended periods of time. During Marasmus, the reduction in energy is greater than the reduction in protein. It takes the more general form of starvation and was seen most hauntingly in patients with certain types of cancer as well as AIDS.

Although common in the US, taking excess amounts of protein is not without consequence. Excess protein increases fluid needs–itself determined by the dietary content of protein, sodium, and fiber. Consuming fluid insufficiently, on top of taking in excessive amounts of protein, increases kidney stone risk. As said before, the human body does not store additional proteins and amino acids as protein. Additional amino acids will be converted to fatty acids, then triglycerides, and finally stored as fat. Excessive catabolism of amino acids leads to more calcium to be lost through the urine, increasing the risk of both kidney stones and osteoporosis. Vegan diets contain sufficient protein. Therefore, excess protein intake would begin with adding animal products.

Common food sources of protein


Plant sources:


Vegetables       about 1 per ½ cup

Legumes          7 per ½ cup

Grains              2 to 3 per ounce

Nuts                 4 to 6 per ounce

Peanut butter    8 per oz (2 Tbs)

Animal source:

Milk                 8 per 8 floz

cheese  8 per oz.;

cottage ch. 15 per ½ cup

meat/poultry                 7 to 8 per ounce (leaner cuts with proportionally more protein)

Seafood                       5 to 7 per ounce


Carbohydrate is the major source of energy for both cellular metabolism and one’s diet. Carbohydrates can be either “simple” or “complex”.  Simple carbohydrates– for which food science gives the more colloquial name “sugar”–are small in size, making them also sweet to the taste.  The two classes of simple carbohydrates are called monosaccharides and disaccharides. As indicated by their prefixes, a monosaccharide comprises 1 sugar unit, while a disaccharide comprises 2.

Monosaccharides Main dietary source
Glucose Fruits, sweet corn, corn syrup, honey
Fructose Fruits, honey, high fructose corn syrup
Galactose Not found free in nature
Disaccharides Monosaccharide component Dietary source
Sucrose Glucose + fructose Sugar cane, sugar beets (both used to make “table sugar”), molasses, maple syrup
Lactose Glucose + galactose Dairy (milk, cheese)
Maltose Glucose + glucose Beer, malt liquor

In contrast, complex carbohydrates contain more than two sugar units. For this reason, they are called “polysaccharides” and are primarily chains of glucose.  Unlike monosaccharides, they are larger in structure and not sweet to the taste.  Plants store energy in the form of complex carbohydrates or what we commonly call “starch.” Starch is only found in plant products and some of its more familiar sources include all grains, potato, legumes (beans), and many vegetables.

Two major dietary sources of carbohydrate are the food groups “fruits” and “vegetables”. Although both contain large amounts of carbohydrates, the ripening process for each is different. In fact, they are inversely related. So, for instance, when a fruit first buds on a plant, it is primarily a complex carbohydrate. As it ripens, however, it transforms into a simple carbohydrate.  On the other hand, vegetables at the start of the ripening process are simple and then ripen to complex.  Thus, when they are ripe, fruits taste sweet, whereas vegetables taste sweet when they are younger and not ripened.

The one type of carbohydrate and polysaccharide that human beings do not digest is called fiber. Because we do not digest it, carbohydrate as fiber does not supply energy. For instance, some fibers are metabolized by the bacteria found in the large intestines. In this way, they are thought to be advantageous to the gut bacteria.  The amount of fiber in food tends to increase with age of plant product. “Dietary fiber” is fiber found naturally in food. For a variety of reasons, fiber is also added to foods where it is not normally found. For example, manufacturers can add fiber to non-fat yogurt, which makes the yogurt thicker than non-fat yogurt without fiber.

Fiber can either be insoluble or soluble, depending upon how it interacts with water.  Insoluble fiber does not dissolve in water and is therefore not metabolized by the bacteria in the large intestines. In contrast, soluble fiber dissolves in water, which leads to swelling and is metabolized by the bacteria in the large intestines.

The effect of the different fiber classes on health:

Fiber type Action in intestines Dietary source
Insoluble Increase in stool size, weight, and frequency; decrease in transit time (i.e., decrease in constipation) Wheat bran (breakfast cereals with > 10 grams/serving are most effective); whole grains
Soluble Forms gels so it slows digestion and absorption of nutrients (glucose, cholesterol, and minerals), which can decrease blood levels of the nutrient

Metabolized in the large intestines; helps to keep intestinal wall health

Pectin containing foods (fruits, legumes), gums (e.g., carrageenan), oat bran, psyllium

Diets containing large amounts of fiber have been associated with a lower risk of both heart disease and gastrointestinal disease. Nonetheless, studies testing fiber as a supplement have failed to show a decrease in the risk of diseases, such as colon cancer. Thus, it may perhaps be the case that some other component of the food item apart from the fiber is providing the disease protection, like the phytonutrient content of plant products.  A number of studies have shown whole grains to be associated with better health.  While whole grain products do contain fiber, they also contain other nutrients, such as trace minerals and phytonutrients.  People who frequently eat whole grain products have also been shown to weigh less and gain less weight over time compared to people who eat primarily refined grains.   Your writer thinks this is because whole grains have a more distinctive taste than refined grains.  As creatures of appetite, we invariably eat for taste. When foods have a more noticeable taste, we tend to eat less of them. Therefore, those who eat whole grains may be eating less of them than those who eat refined grains. Additionally, it may be that whole grains are related to an overall healthier lifestyle.

When carbohydrate is digested and the main carbohydrate in the blood is glucose.  We maintain a certain level of blood glucose.  What is typically measured in a lab is “fasting blood glucose” or FBG.  A healthy level of FBG, which may also be expressed as “FBS” for: fasting blood sugar, is up to 100 mg/dl.  When FBS is >100 and < 126 mg/dl, this indicates that the person is “insulin resistant” and is an indicator of pre-diabetes.

Our bodies store carbohydrates as glycogen, which is found in the liver and in skeletal muscle.  Glycogen stored in the liver is used to maintain blood glucose. Because the liver and skeletal muscles can only store so much glycogen, we need to eat foods rich in carbohydrates on a regular basis in order to maintain blood glucose.  Otherwise, in the absence of sufficient store of carbohydrates, our bodies will begin to use the amino acids in skeletal muscle to make glucose in a process called gluconeogenesis. Gluconeogenesis is a survival mechanism, but one that could potentially lead to skeletal muscle wasting over time. Glycogen stored in a muscle does not leave that muscle so it is not available to maintain blood glucose. During exercise or any other type of physical activity, the glycogen in skeletal muscle is used to fuel the muscle.  The amount of glycogen stored on skeletal muscle can be increased with training the muscle and eating a diet sufficiently high in carbohydrate.

Functions of carbohydrate:

  1. provision of energy is the primary function of glucose. Glucose is a primary energy source for most cell types.  Carbohydrate is also the largest dietary component of most natural diets.
  2. sparing protein – if insufficient carbohydrate is consumed, amino acids will be used to synthesis glucose by gluconeogenesis. We do not store protein, so the amino acids would come from skeletal muscle.
  3. prevention of ketosis (incomplete fat catabolism). Approximately 100 grams of carbohydrate are needed to prevent ketosis in the prototypical 70 kg male.

The glycemic index (GI) is the blood glucose response to a carbohydrate containing food compared to a standard (glucose or white bread).  The GI is influenced by the fiber content of the food, any processing of the food (particle size), physical structure, and other variables.  The GI could also vary depending on the age of a plant product.  For example, when fruit first forms on the plant it is more complex carbohydrate compared to simple carbohydrate; fruit ripens to simple carbohydrate; the reverse happens for most vegetables.  This is why ripe fruit (“older”) tastes sweeter than newly formed fruit, while young vegetables taste sweeter than “older” vegetables.  So young fruit would have a GI that is lower than ripe fruit and young vegetables would have a GI higher than old vegetables.

The GI for a food is also different depending on whether it is tested on its own versus part of a meal.  For example, fat in the meal delays the stomach emptying, so the GI of carbohydrate foods in the meal will be lower if there is fat in the meal.  The essential amino acids in meat cause insulin release so the GI is lower with a meal containing meat.  A lower GI does not mean that the food is healthy.  By examining a table with the GI of common foods, one would find that the GI of a baked potato is 48, potato chips 8, chocolate 9, and jelly beans 21.  Your writer does not find the GI very useful.  My approach is to encourage the use of whole grains, unlimited vegetables and limit fruit to 3 to 4 servings a day.  As fruit tends to be sweet, it is easier to overeat.  Also, a serving of both fruit and vegetable is about ½ cup with fruit having 60 calories per serving and vegetables have 5 to 15 calories.  While fruit contains fairly healthy components, calories are calories and overeating fruit can contribute to weight gain.

Phytonutrients are compounds in plant products that protect them from their environment.  Although we do not require them for either growth or development, many studies have made a strong case for phytonutrients containing properties that reduce the risk of chronic diseases. Thus, phytonutrients may explain why a diet rich in plant products is a guarantee of good health and a long life. The number of different phytonutrients is in the hundreds: they are as diverse as the plant products they are found in. Because their primary purpose is to offer protection, they tend to be concentrated on the outer part of the plant. For example, the outside kernel of grain contains many phytonutrients; this is at least partly why eating a whole grain product is healthier than eating refined grains.  Additionally, a diverse number of phytonutrients are produced when a plant is cut, crushed, or otherwise “injured.” There are many phytonutrients in red grapes, for example. But many more are created when the grapes are crushed, manipulated, and fermented in the wine-making process.


Lipids are a group of compounds that are insoluble in water.  From a dietary standpoint, “fat” refers to solid lipid and “oils” to liquid lipid.

The simplest form of lipid is the fatty acid, which is a chain of carbons surrounded by hydrogen.  One end contains a carboxyl group and is called the “alpha end”.  The other end contains a methyl group and is the “omega end”.  Fatty acids are of varying chain length and are named using the number of carbons in the chain.  The carbons form bonds with each other in the chain and hydrogens can fill the other bonds.  If all the carbon bonds are filled by hydrogen, the fatty acid is called “saturated”.  If two adjoining carbons are missing hydrogens, a double bond forms between the carbons and the fatty acid is called “unsaturated” with one bond being a monounsaturated fatty acid and two or more bond are polyunsaturated fatty acids.

The polyunsaturated fats are essential fatty acids (EFA), meaning humans can not make them.  We require approximately 5% of our energy from essential fatty acids.  The EFA families are named by the location of the first double bone counting from the methyl/ omega end.  There are two main families: the omega 3 and the omega 6 families.

The omega 3 family:  linolenic acid is the starting compound and has 18 carbons.  The 20 carbon member is eicosapentenoic acid (EPA) and the 22 carbon is docosahexaneoic acid (DHA).  Humans are very inefficient at elongating and desaturating linolenic acid to receive the 20 and 22 carbon fatty acids.  This means that we can not readily make the EPA or DHA.  Linolenic acid is found in canola oil, walnuts, and purslane.  The 20 and 22 carbon omega 3 fatty acids are found in fatty fish.

The omega 6 family:  linoleic acid is the starting carbon and has 18 carbons.  The 20 carbon member is arachidonic acid; humans can readily elongate and desaturate linoleic acid to synthesize arachidonic.  Linoleic acid is found mainly in vegetable seed oils (corn, soybean, safflower); the main dietary source of arachidonic is beef.

The omega 3 and omega 6 fatty acids of 20 carbons or longer are used to synthesize eicosanoids which are hormone-like compounds; they are called “hormone-like” as they can work like hormones but they work where they are made (and hormones travel by the blood from where they are made to where they work).  The omega 3 family makes eicosanoids that are anti-inflammatory, anti-aggregation, and vasodilators.  The omega 6 family makes eicosanoids that are proinflammatory, proaggregatory, and vasoconstrictive.  The 20 carbon members of the families compete at the starting enzyme so the predominant fatty acid will determine what set of eiconsanoids are made.   The health benefits from the omega fatty acids are related to the blood ratio of the two fatty acid families.  The typical American diet is high in omega- 6 due to the use of vegetables oils and their products (margarine, mayonnaise, commercial salad dressings).   Some years ago, health professionals in the US started to address this by suggesting patients consume fish oil pills.  Your writer does not support this practice as all polyunsaturated fats will increase oxidation, which will contribute to a list of diseases.  I suggest that people minimize their intake of vegetable oils; this would decrease the omega-6 fatty acid in the body and would allow any longer chain omega 3 fatty acids present to work without excessive hindrance.

Triglyceride is a lipid that has a glycerol backbone with three fatty acids attached to it.  Triglyceride is the lipid that supplies energy both in food and in us (triglyceride is what is stored in the adipose tissue).  The 3 fatty acids are a mixture of fatty acids with some saturated, some unsaturated.  The fatty acids that predominate give the naming to a food as being a source of saturated fat, monounsaturated fat, or polyunsaturated.  However, no food is purely 1 type of fatty acid.  Interestingly, although beef is labeled a source of saturated fat, it contains mainly monounsaturated fat.

Trans fats are produced primarily in an industrial process.  They are made by adding hydrogen to polyunsaturated fats (liquid vegetable oils).  The process is called “hydrogenation”.  This solidifies the oil, which is how margarine and vegetable shortenings are made.  hydrogenation also makes the oil less likely to oxidize (become rancid).  This increases the shelf-life of food.  Foods with a long-shelf life, like commercial baked goods, are high in trans fatty acids.

The health concern with trans fatty acids is that in the body they can replace cis or the natural fatty acids in phospholipids and other places where one would fine triglycerides.  The trans fatty acids are unnatural to the body and have been consistently linked to an increase risk of chronic diseases, especially cancer and heart disease.  Some of this research has involved measuring the amount of trans fatty acid present in the adipose tissue.  As these fatty acids are not naturally found in the body (i.e., we do not make them), any present in the adipose would be from the diet.  Your writer has long felt that as any food with trans fatty acids is not a healthy food, counseling people to eat a more healthy diet would minimize the intake of trans fatty acids.

Cholesterol is a lipid that is a sterol.  It is only found in animal foods and does not supple any energy.  Cholesterol is used to make some hormones (estrogen, testosterone); is a starting compound for vitamin D synthesis in the body; it is part of bile; it is an essential structural part of cell membranes; and it is part of lipoproteins.

Lipoproteins are made to carry lipid in the blood.  All lipoproteins contain lipids (cholesterol, triglycerides, phospholipids) and protein, which are called “apoproteins”.  The amount of each component varies by the lipoprotein.

Chylomicrons are made in the small intestines and are used to transport exogeneous (dietary) triglyceride.  The rest of the lipoproteins are named by density or weight; the weight being the apoprotein content.

Very low density lipoprotein cholesterol (VLDL) is made in the liver to carry primarily endogenous triglyceride which is made in the liver from extra calories we consume.  VLDL triglyceride is the triglyceride measured in fasting blood.  VLDL levels are increased with a low-fat, high carbohydrate diet; the carbohydrate consumed in excess of need or ability to store as glycogen is converted to fatty acids, which are then converted to triglyceride.  How high VLDL increases with a high carbohydrate diet depends on the starting level for fasting triglycerides and how well insulin is working; insulin resistance would make theVLDL increase greater compared to when there is not insulin resistance.

When sufficient triglyceride is removed from the VLDL (i.e., either stored in adipose or used for energy) there is a decrease in density, and the particle becomes a low-density lipoprotein (LDL). LDL particles are mainly cholesterol.

Effect of diet on LDL:

  1. saturated fats can increase LDL levels with the increase mainly dependent on the diet of comparison.  There is also thought to be a genetic effect on the change with some people being more responsive to diet than others.
  2. polyunsaturated fats can decrease LDL more than other dietary fats will; however, PUFA will become part of the lipoproteins, which leads to oxidation of the particle.
  3. monounsaturated fats tend to decrease LDL, but they definitely do not lead to oxidation.
  4. a low-fat diet will lower LDL because the VLDL is not being efficiently converted to LDL.  When a low-fat diet decreases LDL, you will see an increase in fasting triglycerides (VLDL).

High levels of LDL have been related to an increase risk of CHD, but it is the oxidized form of LDL that leads to an increase in risk of CHD.

High density lipoprotein cholesterol (HDL) has an inverse relationship to heart disease, however, the reason why is not known.  The possible reasons include:

  1. reverse cholesterol transport – HDL is thought to bring cholesterol from the peripheral back to the liver for disposal.  Your reader has not found the evidence for this being the main health benefit of HDL very convincing.
  2. block oxidation of LDL – HDL carries antioxidants and may prevent LDL from oxidizing.
  3. Marker for efficient triglyceride catabolism – triglyceride levels have been inversely related to heart disease in some studies.  Triglycerides do have an inverse relationship to HDL so when triglycerides are high, HDL is low and vice versa.  It is possible that the HDL is just a stable marker for efficient triglyceride catabolism so higher levels of HDL mean there is better catabolism of VLDL; triglyceride values can vary greatly from day to day and HDL does not.

High levels of HDL are primarily genetically determined; only small (5 to 10%) changes can be achieved with diet or lifestyle.  Weight loss that lowers fasting triglycerides can result in a small increase in HDL, due to the inverse relationship discussed above.   The loss of weight could be by change in diet and/or increase in physical activity.  However, it is not correct to say that physical activity will increase HDL to levels found in endurance athletes.

Effect of diet on HDL:

A moderate / high fat diet will increase HDL and a low-fat (less than about 25% of total energy) diet will decrease HDL.  All types of dietary fat, except trans fats will increase HDL, but extra virgin olive oil has been shown to have an independent effect on increasing HDL.