Digestion, Absorption and Macronutrients


Students will identify specific organs of the digestive system and explain their roles in the digestion and absorption of food and beverages. Additionally, students will define a specific range of macronutrient needs and compare their dietary intake with recommendations set forth by nutrition scientists.

Learning Objectives

  1. Describe how materials used to renew body tissues come from the food we eat in the form of nutrients.
  2. Identify the organs of the digestive tract and accessory organs.
  3. Describe digestion and absorption as processes that make nutrients in foods available for use by the body.
  4. List common digestive disorders and explain how they are related to dietary intake.
  5. List and differentiate between different types of carbohydrates: simple sugars, starches, and dietary fiber.
  6. Compare and contrast simple sugars with complex carbohydrates.
  7. Identify foods containing naturally-occurring sugars, as well as identify added sugars in processed foods using food labels.
  8. Discuss the uses in foods and health implications of sugar alcohols and artificial sweeteners.
  9. Identify foods that are rich sources of complex carbohydrates.
  10. Compare the caloric content of carbohydrates with protein and fat.
  11. Discuss the ways that fiber benefits health, and identify sources of dietary fiber.
  12. Explain how the glycemic index classifies carbohydrates.
  13. List the major functions of protein in the body.
  14. Describe how protein can be used for energy.
  15. Explain the difference between "essential" and "nonessential" amino acids.
  16. Describe protein complementation.
  17. Give examples of amino acid supplements and discuss their safety and efficacy.
  18. Outline the functions of fats in foods and in the body.
  19. Explain how the body converts the various energy nutrients into fat stores.
  20. List the various types of dietary fat, explain their differences, and discuss their positive and negative health effects.
  21. Discuss the available fat substitutes and their place in a healthy diet.
  22. Correctly interpret label information regarding the fat content of foods.
  23. Identify fats that raise blood cholesterol levels more than other types: saturated fats and trans fats.
  24. State current recommendations for fat and cholesterol intake.



Within this section I am asking that you read the text, view some videos and interact with some animations to help you to better understand the digestive system and the macronutrients that provide your body with essential nutrients and energy. After thoroughly reviewing these materials, it is my hope you will have a greater understanding of what foods you should be consuming to meet your body's nutrient needs while not exceeding your energy needs.


Digestion is the process of breaking down the foods we consume and absorption is the process of moving the digested food into the blood stream so the digested food can be repackaged by various organs primarily the liver. The digestive system is a highly complex organ system and the topic would require much more time than we have in this introductory nutrition course to study it; however, an elementary understanding of the major anatomical structures and physiological functions of the digestive system are worthy of some investigation.Digestion Cartoon

Personally, I know I learn complex material when it is presented in a variety of ways, e.g. videos, animations, labeled graphics, etc. Therefore, I think it would best benefit you to review some excellent materials I have procured for you so you can learn some of the basics of the digestive system. There is a somewhat "cheesy" (no pun intended), but potentially effective video on digestion for you to view. The video can be accessed via the Digestion: Eating to Live link. Additionally, there is a well designed animation to help you learn the different parts (anatomy) of the digestive system as well as their functions (physiology). Click on the Digestion link to view the interactive animation.

You are What You EatAfter you gain an introductory understanding of digestion and absorption of food, the next step is to examine the different nutrient molecules your body will digest and absorb. First, we'll start with the energy yielding molecules. Collectively they are known as the macronutrients. In the next section we will look at the micronutrients. The chief difference between the macronutrients and micronutrients is that the macronutrients provide energy. Please take time to view the Nutrients Prezintation I created for you so you can develop a better understanding of the macro and micro nutrients.

The Macronutrients

The macronutrients are the large (macro) molecules in food that provide our body with energy.

The necessary macronutrients: carbohydrate, protein and lipid each yield stored energy when released. The energy stored in food is measured in the form of calories. We will look more specifically at calories and energy in Section 5. But for now, what you need to know is that carbohydrates yield four calories per gram; proteins also yield four calories per gram; and lipids (fats) yield nine calories per gram. AlcoholAlcohol, a non-essential energy yielding molecule, yields seven calories per gram. Alcohol is not essential and the human body eliminates it as the body considers alcohol to be a toxin. There is much controversy about alcohol and diet, but most of the current research indicates that one drink per day for adult females and two drinks per day for adult males is acceptable. However, please remember that the energy in alcohol is quite concentrated and provides almost double the amount of energy as is found in the same weight of carbohydrate and protein. Additionally, alcohol has the potential to be addictive. Interestingly, the idea of sugar, a carbohydrate metabolite, being addictive is quite a popular topic in addiction and food studies. See How to Get Over Your Sugar Addiction.

No less controversial is the discussion centered around high protein, low carbohydrate, or low fat, etc. diets. After years of research it is my belief that the best diet focuses upon whole grains, whole vegetables and whole fruits eaten in moderation. When combined with modest servings (3-4 ounces) of lean protein consisting of wild caught fish, free range chicken and grass fed beef, the human body has its best chance at performing optimally while simultaneously increasing the potential for a longer life. (http://www.health.harvard.edu/healthbeat/healthy-eating-without-the-hassle).

Nutrient DensityMost Registered Dietitians support a ratio of energy coming from: 55-60% carbohydrates, 20-30% fat, and 10-20% protein. There are many variations of diets that propose weight loss using low carbohydrate ratios, as low as 10% of total calories coming from carbohydrates. However, researchers continue to determine that longevity and decreased risk of disease is most frequently achieved in cultures where a Whole Food Plant Based Diet consisting of whole grains, fruits, vegetables, nuts, seeds and legumes that provides 60-80% of total calories is optimal for wellness (Lessons for Living Longer).

Macronutrient Molecules


Carbohydrates are an oft misunderstood nutrient. Because many heavily processed foods, e.g. crackers, chips, cookies, candies, sodas, etc. are loaded with carbohydrates, the term carbohydrate has a negative connotation as these foods are linked to overweight, obesity and their related diseases. One must understand that not all carbohydrates are equal, however. Simple carbohydrates, the carbohydrates made from the single sugar molecules fructose and glucose are much different than the complex carbohydrates found in oligosaccharides and polysaccharides that are found in starches and fiber.
Simple carbohydrates are broken down quickly and rapidly increase blood glucose levels. Conversely, complex carbohydrates break down more slowly and blood glucose levels rise much more gradually. How quickly carbohydrates break down into glucose is important as elevated blood glucose levels require the pancreas to produce insulin to help with assisting the glucose molecules across cell membranes where the glucose molecules are used as a form of cellular energy. When excessive amounts of glucose are present in the blood, the pancreas must work extra hard to produce the amount of insulin necessary to move the glucose across cell membranes. Subsequently, individuals who consume large quantities of processed foods packed with sugar may begin to struggle with adequate insulin production. Additionally, some cells develop insulin resistance when blood glucose levels are consistently elevated thus leaving an individual at risk for developing Type II diabetes.

Dividing carbohydrates into simple and complex, however, does not account for the effect of carbohydrates on blood sugar and chronic diseases. To explain how different kinds of carbohydrate-rich foods directly affect blood sugar, the glycemic index was developed and is considered a better way of categorizing carbohydrates, especially starchy foods. The glycemic index ranks carbohydrates on a scale from 0 to 100 based on how quickly and how much they raise blood sugar levels after eating. Foods with a high glycemic index, like white bread, are rapidly digested and cause substantial fluctuations in blood sugar. Foods with a low glycemic index, like whole oats, are digested more slowly, prompting a more gradual rise in blood sugar. Low-glycemic foods have a rating of 55 or less, and foods rated 70-100 are considered high-glycemic foods. Medium-level foods have a glycemic index of 56-69.

One thing that a food’s glycemic index does not tell us is how much digestible carbohydrate – the total amount of carbohydrates excluding fiber – it delivers. That’s why researchers developed a related way to classify foods that takes into account both the amount of carbohydrate in the food in relation to its impact on blood sugar levels. This measure is called the glycemic load. A food’s glycemic load is determined by multiplying its glycemic index by the amount of carbohydrate the food contains. In general, a glycemic load of 20 or more is high, 11 to 19 is medium, and 10 or under is low. The glycemic load has been used to study whether or not high-glycemic load diets are associated with increased risks for Type II diabetes risk and cardiac events. In a large meta-analysis of 24 prospective cohort studies, researchers concluded that people who consumed lower-glycemic load diets were at a lower risk of developing Type II diabetes than those who ate a diet of higher-glycemic load foods (Harvard School of Public Health).
For more information on the glycemic index visit: Glycemic Index and Glycemic Load of 100+ Foods.

Lipids (Fats)

One of the many nutritional fallacies students face when learning about nutrition is that eating fat makes one fat. This is FALSE! Body fat accumulation is the result of excessive energy intake regardless of the nutrient source. One can gain weight eating all whole foods for example. This is unlikely, but possible. Additionally, one can lose weight eating all junk food as long as the individual creates a calorie deficit. See story about a Kansas State University nutrition professor who lost 27 pounds eating mostly junk food. The challenge with fat is that one gram of fat has nine calories. Conversely, one gram of carbohydrate or one gram of protein has only four calories. Therefore, it is easier to gain weight eating high fat foods because one does not need to ingest as much food because of the energy density of fat.

Fats are an essential component of a healthy diet and they should not be avoided-- they should be consumed daily. Fats provide essential nutrients including fat soluble vitamins and Omega 3 fatty acids. Fat soluble vitamins and Omega 3 fatty acids are found in fatty fish like salmon and tuna. They can also be found in walnuts, canola oil and a host of other foods with fat. Regular consumption of Omega 3 fatty acids has been linked to decreased risks of cardiovascular disease.
Probably of greatest importance is that individuals limit their total fat intake to no more than 35% of their total calories-- although <30% would be preferred by many nutrition experts. Additionally, the type of fats consumed: saturated, mono, poly or trans fats is important. One should completely eliminate all trans fats and many dietitians recommend reducing total saturated fat intake to <7% of total calories consumed. There is some concern, however, that the recommendations for fat restriction, specifically saturated fat restrictions may be unfounded (http://www.hsph.harvard.edu/nutritionsource/fats-full-story/).
The easiest way to reduce saturated fat intake, if desired, is to reduce the consumption of some types of red meat specifically high fat cuts of steak and hamburger. Pork sausage and bacon can also be high in saturated fat as can full fat dairy products, e.g. milk, cheese, ice cream, etc. Additionally tropical oils like coconut and palm kernel oils are high in saturated fat.
There is much debate regarding whether one should use margarine or butter. If margarine is selected, it should not include any hydrogenated oils as these are the same as trans fats. Butter is high in saturated fat. Remember, all trans fats should be completely eliminated. Reducing fast food and processed foods is the easiest way to eliminate trans fat consumption.
Eating lean cuts of meat, preferably fish and poultry, and consuming whole foods like avocado, nuts, and seeds is the preferred way to ingest fat calories according to most dietitians.


The Zone Diet and others have advertised better health by reducing one's calorie consumption of carbohydrates and increasing their consumption of protein. Remember, weight loss and weight gain are the result of total energy intake vs. total energy output. As a result, any combination of carbohydrates, fat and protein will result in weight gain if too many total calories are consumed or weight loss if too few calories are consumed. The issue with protein is that it is difficult to consume absent of fat. For example, if I asked you to list three foods with a lot of protein many of you would likely identify eggs, meat and milk. You would be correct. However, these foods can also be very high in fat. Additionally, T. Colin Campbell found in The China Study that cultures that consumed the least amount of animal protein had the lowest rates of cancer.

Obviously, protein can be found in lower fat food sources specifically beans, grains, vegetables and fruits. However, the American public through years of insidious advertisements from the meat and dairy industry have been led to believe that proteins derived from animal sources are best. There is some truth to this claim as only animal products can deliver complete proteins. However, through complementation one can combine non-animal protein sources, e.g. beans and rice, bread and peanut butter, to ingest complete proteins.

I would recommend a diet of 50-60% of calories from carbohydrates, 20-30% of calories from fat and 20% of calories from protein. As mentioned previously, most Registered Dietitians recommend a diet consisting of 55% carbohydrates, 30% fat and 15% protein.
The 50-60% carbohydrates should come from whole food sources like grains, vegetables and fruits. The 20-30% of calories from fat should come from grass fed beef, dairy products from grass fed cattle, cold water, line caught fish, free range poultry, free range eggs, nuts and seeds. Avocados and some monounsaturated oils like extra virgin olive oil are also recommended. The sources of protein that should be approximately 20% of your food intake should come from free range eggs, beans, nuts, seeds and legumes. Some people may add tofu, tempah and other soybean products to increase their protein intake.
Regardless if you consume 40% carbohydrates, or 40% of your calories from protein or fat, one must make sure their food intake addresses their energy and nutrient needs. It just may be more likely that one is able to meet their nutrient needs with a diet that consists primarily of complex carbohydrates.

Nutrient Density

Not only is it important that one understand the different nutrients, but also of equal importance one should understand the foods that are the most nutrient dense. As an aside, the typical American diet consists of energy rich foods that are not nutrient dense. Snickers Candy BarFor example, a Snickers bar is energy dense, but not nutrient dense. It has a high amount of calories (energy density), but provides very few nutrients. In comparison a carrot would be considered a nutrient dense food as a carrot provides many more nutrients than a Snickers bar, but it contains approximately 10% of the total calories found in a Snickers bar.
To assist its citizens with making good food choices the U.S. government has made a variety of tools available. In the previous section you were exposed to MyPlate. There is a wealth of resources available via the MyPlate web site to assist individuals with proper food choices. Carrots

Despite the wealth of resources available to assist individuals with making nutrient dense food choices, many Americans continue to eat foods that are high in salt, sugar and fat. To view the dietary habits of others around the world, please view What the World Eats.

To get a more detailed perspective on the macronutrients view the All About Nutrients video. All About Nutrients examines the dietary practices of a typical college-aged student in the U.K. and provides insights into the food choices made by young adults. When combined with the Basic Nutrition video from the previous section you should have a good introduction to all of the essential nutrients and some information about the important functions nutrients perform within our bodies.


I strongly believe it is important that one apply what one is learning to develop a deeper and potentially lasting comprehension of the concepts and ideas being presented. Subsequently, I have designed the journal activities to provide you with an opportunity to write about what you are learning.

During the first two journals I asked you to identify the importance of your diet and I asked you to examine some packaged/boxed foods commonly found in your cupboards and/or refrigerator. During this next section I am requesting that you rate your current diet using a standardized rating scale similar to the one used for calculating a student's G.P.A. The other journal is designed to allow you to begin investigating percentages of macronutrients that not only meet your nutrient needs, but also your taste preferences and economic considerations.

In addition to the journal assignments, I have created two specifically designed written assignments to allow you an opportunity to explore food and drink from a variety of applications.

Both of the written assignments, a Diet Diary Analysis and a Dietary Plan, will be submitted using the Assignments feature within Blackboard. Please review the requirements for submitting these assignments located within the Assignments area of Blackboard. The first written assignment, a Diet Diary Analysis, is designed to allow you to research your eating habits to determine whether your energy and macronutrient needs are being addressed. It is my expectation that you will work towards completion of the Diet Diary Analysis during the second unit of this course. It is due prior to the first checkpoint. Please consult the syllabus for the dates of the two checkpoints.

The Diet Diary Analysis will require you to record everything you eat and drink for two days while tallying the macronutrient quantities in each of the food and drink items you consume. You will record the calorie information for each food item and the macronutrient information for each food item as measured in grams (carbohydrates, fat, all four types including total fat, protein and fiber). You will also identify the percentages of need for the different areas within your diary. The description of the Diet Diary Analysis along with the grading rubric detailing how you will be evaluated for this assignment can be found in the Assignments area of Blackboard.

The second written assignment, a Dietary Plan, will allow you to create an eating and drinking plan for yourself based upon a synthesis of what you have learned about nutrition. The Dietary Plan assignment will be due near the end of the class.

>>> Section 3: Micronutrients and Supplements