On each of the web pages about the organic molecules needed in our diets, I have tried to include some fact that reveals how much of each is needed in one day. How are these levels of intake that we need measured? On what are they based?
Well, let's start off by considering the biggest requirement of intake of nutrients-- production of ATP. We use carbohydrates to make monosaccharides to produce ATP through cellular respiration. We use proteins and nucleic acids and lipids, if we do not have enough carbohydrates, to convert into materials for the production of ATP. We have to make lots of energy every day in order to keep living.
How do we measure energy? To understand this, you will need to remember a basic principle (of thermodynamics) that you have learned somewhere before in your educational career: energy cannot be created or destroyed, but it can be converted from one form into another. Energy has many forms... one form is chemical energy, which is the type stored within chemical bonds in molecules like glucose and ATP. Another type of energy is heat. You have already seen, back in the muscle chapter, that when we use lots of ATP in our muscles, like during a long workout, some of the energy from the ATP is lost as heat, and so our muscles heat up. That is an example of energy changing forms. Therefore, energy can be measured as chemical energy or heat energy, or even other types of energy. But the easiest measurement to make is in terms of heat-- all you need is a thermometer! So, energy is typically measured in heat units called calories.
Calories relate back to the temperature you would measure with a thermometer in the following way. If 1000 calories of heat are released in water, the temperature of the water would increase by 1 șC. Water is specified, because different materials accept heat (conduct heat) differently. Nutritionists need to be able to talk about a lot of calories, so it is more useful for nutritionists to talk about kilocalories (which are also called Calories with a capital "C," or "large calories"). A kilocalorie, abbreviated kcal or just Cal, is the amount of heat required to raise the temperature of water 1 șC, and is equal to 1000 calories.
One can relate the amount of each organic molecule consumed back to its caloric equivalence. So, 1 gram of either carbohydrates or proteins yields 4.1 calories of energy. And 1 gram of fats ingested yields 9.5 calories (more than double the amount carbohydrates and proteins yield).
Determining Energy Needs
The amount of energy that we need can be described by our rate of energy consumption. Let's consider our rate of energy consumption at times when we are resting. This rate is called the basal metabolic rate, or BMR. If you think about it, the amount of energy that we consume should be proportional to the amount of energy that we make. And the amount of energy that we make stems from our rate of cellular respiration. The rate of cellular respiration is mainly dependent on our aerobic respiration, which requires oxygen (and glucose or its breakdown products) to occur. Therefore, if you followed me through all those steps, you will see that our BMR is proportional to the rate of our oxygen consumption. So, one's BMR can be evaluated somewhat by one's rate and volume of breathing at rest. Note that when you are active, you breathe faster, reflecting the increased demands of higher energy consumption. (Your book also notes that a clinical method for evaluating BMR is to test thyroid function; you may remember that thyroid hormones T3 and T4 function to increase metabolic rate.)
On average, a person's BMR is one kcal/hr for each kilogram (kg) of body weight. Since 1 kg = 2.7 lb, a person with a body weight of 150 lb can also be described as having a body weight of 55.6 kg. This person would then use at least 55.6 kcal every hour of every day. Over 24 hours this person would use 1362 kcal. This number of 1362 kcal would require that our average person be laying down and not doing anything for all 24 hours. Even just sitting up would increase energy consumption. Your book has a table, Table 18.4, where they show you how many more calories (NOT kcal, even though they capitalize it!) per hour a person would consume by various activities. For example, just walking around at a slow, meandering pace consumes 200 extra calories per hour, or (200 cal * 12 hr =) 2400 cal, or 2.4 kcal extra if you walk around for half the day.
Your energy needs are equal to the amount of energy you will be using in the day. You need to consume a little more than just your energy needs in order to replenish your essential-nutrients-stock. However, to maintain your weight, you should not eat much more than what you will use in a day. For example, if you eat an additional 3500 calories in a day, over the amount you will use, you will gain one pound!
Please note that in all the descriptions above, I have discussed a typical person weighing 150 pounds. Each person has slightly different energy needs. They depend upon the sex, age, reproductive status, and current weight of that person.
© 2011 STCC Foundation Press