Nutrition and Metabolism

accounting for 15 percent of body heat loss); 3) Radiation (the transfer of heat through

radiation to the environment, accounting for 60 percent of body heat loss); 4) Evaporation (the

transfer of heat through the evaporation of water or sweat, accounting for 20 percent of body

heat lost).

The basal metabolic rate or BMR is the daily energy expenditure by humans at rest in the post-

absorptive state and in a neutrally temperate environment. About 70 percent of all daily energy

expenditure comes in the form of basal metabolic functions, 20 percent comes from physical

activity, and 10 percent comes from body thermoregulatory processes. The BMR goes down

when lean muscle mass is lost and with age.

NUTRITION AND METABOLISM

The energy used and needed by the body involves what’s called the “nutritional calorie.” This is

the amount of heat necessary to raise a kilogram of water by 1 degree Celsius. Approximately

1500-2000 calories per day are necessary to sustain daily activities—but it depends on body

mass, gender, height, age, and activity level. More calories are required in situations of more

activity. An extra 3500 calories per day are required to gain a pound of weight. The reverse is

true when one wants to lose weight. Surprisingly, very little calories are burned in exercise. An

example is jogging. One full mile of jogging will burn just 100 calories.

Carbohydrates require the least amount of energy to process, while the processing of proteins

requires the most energy. The balance of weight depends on what is taken in, how many

calories are taken in, and how much energy is expended. The body mass index or BMI is a

measure of the weight-to-height ratio and is a calculation used to determine a person’s level of obesity. A normal BMI is between 18 and 24.9 kg/m2. Being overweight involves a BMI of between 25 and 29.9 kg/m2, and being obese is a BMI of greater than 30 kg/m2. Being

overweight or obese is associated with an increase in many types of diseases, ranging from type

2 diabetes to heart disease and certain cancers.

Vitamins are a variety of organic molecules needed in biochemical processes but that are

generally not synthesized de novo by the body. When it comes to metabolism, the B vitamins

play the largest role. Some vitamins are directly taken in by the body through food, while others

are made from food-related compounds, such as the synthesis of vitamin A from beta-carotene

in vegetables. There are fat-soluble vitamins and water-soluble vitamins. Most fat-soluble

vitamins are absorbed in the small intestine except that vitamin D is both absorbed and

synthesized via UV light exposure. Water-soluble vitamins (the B vitamins and vitamin C) are

absorbed in food.

Minerals are inorganic compounds in food that cannot be made in the body. The main minerals

in the body include sodium, potassium, phosphorus, calcium, chloride, and magnesium, which

make up four percent of the total body mass. Calcium and phosphorus are the most common

minerals in the body, being part of the human skeleton. Sodium and potassium are ionic

electrolytes that are important in the regulation of all cellular structures. Iron is crucial to

hemoglobin synthesis. There are many trace minerals that have low concentrations but have

important biochemical properties.

Important trace minerals include iron (used in ATP production and oxygen transport), zinc (used

in immunity, blood clotting, and hormone functioning), copper (used in RBC production,

collagen formation, immune function, and as an antioxidant), iodine (used in thyroid function),

sulfur (used to make amino acids), fluoride (used for bone and tooth structure), manganese,

cobalt (a part of vitamin B12), selenium, chromium, and molybdenum.