Food and Metabolism

The amount of energy that is needed or ingested per day is measured in calories. The nutritional Calorie (C) is the amount of heat it takes to raise 1 kg (1000 g) of water by 1 °C. This is different from the calorie (c) used in the physical sciences, which is the amount of heat it takes to raise 1 g of water by 1 °C. When we refer to “calorie,” we are referring to the nutritional Calorie.

On average, a person needs 1500 to 2000 calories per day to sustain (or carry out) daily activities. The total number of calories needed by one person is dependent on their body mass, age, height, gender, activity level, and the amount of exercise per day. If exercise is regular part of one’s day, more calories are required. As a rule, people underestimate the number of calories ingested and overestimate the amount they burn through exercise. This can lead to ingestion of too many calories per day. The accumulation of an extra 3500 calories adds one pound of weight. If an excess of 200 calories per day is ingested, one extra pound of body weight will be gained every 18 days. At that rate, an extra 20 pounds can be gained over the course of a year. Of course, this increase in calories could be offset by increased exercise. Running or jogging one mile burns almost 100 calories.

The type of food ingested also affects the body’s metabolic rate. Processing of carbohydrates requires less energy than processing of proteins. In fact, the breakdown of carbohydrates requires the least amount of energy, whereas the processing of proteins demands the most energy. In general, the amount of calories ingested and the amount of calories burned determines the overall weight. To lose weight, the number of calories burned per day must exceed the number ingested. Calories are in almost everything you ingest, so when considering calorie intake, beverages must also be considered.

To help provide guidelines regarding the types and quantities of food that should be eaten every day, Health Canada has published a simplified “Eat Well Plate” graphic to summarize the recommendations found in Canada’s Food Guide (Figure 1). Such representations seek to put the recommended elements of a healthy meal into the context of a place setting of food. The accompanying websites canada.ca/foodguide gives clear recommendations regarding quantity and type of each food that you should consume each day, as well as identifying which foods belong in each category. The guidelines in general suggest you “Make half your plate fruits and vegetables.”  The other half is grains and protein, with a slightly higher quantity of grains than protein. Dairy products are represented by a drink, but the quantity can be applied to other dairy products as well.  All of these foodstuffs contain the energy-containing nutrients carbohydrates, lipids, and proteins in varying amounts, as well as various vitamins, minerals, and essential nutrients.  Specifics vary with particular choices within each group, but in general grain products, vegetables and fruit contain higher amounts of carbohydrates than the other groups, whereas meat and dairy products contain higher amounts of protein and lipids.

In addition to providing chemical energy, ingested foodstuffs must also provide any molecules that cannot be produced fast enough (or in some cases, at all) by the body to meet the body’s needs.  Such molecules are referred to as essential because they must be ingested to allow normal functioning of the human body.

There are two essential fatty acids that humans must ingest: linoleic acid (LA), an omega-6 fatty acid, and linolenic acid (ALA), an omega-3 fatty acid.  These two fatty acids serve as precursor molecules that can be modified by the body, particularly in the liver, to produce other lipid molecules.  However, they cannot be created from other molecules in the human body and so must be provided by consuming an external source.

There are eight essential amino acids that humans must ingest from other sources: tryptophan, methionine, valine, threonine, phenylalanine, leucine, isoleucine, and lysine.  An additional two – histidine and arginine – are essential for infants but not for adults.  Any protein that contains in its primary structure any of these amino acids will not be made at all in their absence.  All of the essential amino acids are found in animal product proteins (e.g. eggs, milk, fish, most meats), but almost no single plant source contains all of the essential amino acids.  However, combinations of plants can be ingested together to provide them; for example, a combination of cereal grains (e.g. corn) and legumes (e.g. beans) can provide all eight essential amino acids.

Although humans do produce it as a byproduct of cellular respiration, water is also an essential nutrient.  We lose far more water through constant evaporation from our breath, mucous membranes, and sweat than is produced.  Thus humans must ingest water regularly.  Plant and animal cells consist largely of water, so a substantial amount of water can be obtained from (non-dehydrated) dietary sources.  Nevertheless, humans living in all but the most comfortable of environments typically require access to a source of additional liquid water in addition to plant and animal sources.  Excessive water loss (dehydration) can be fatal from a combination of an inability to sweat allowing a dangerous rise in body temperature  and a dramatic drop in blood volume and increase in blood viscosity due to water loss from the blood plasma.  Under extreme conditions (e.g. exercising strenuously in a hot environment) the lack of a reliable water sources can prove fatal within a few hours; an adult in comfortable surroundings could survive up to about a week without any water intake before succumbing.  Generally the lack of other dietary nutrients in an otherwise health human would not prove fatal nearly as quickly.

The other essential nutrients are the vitamins and minerals.  Vitamins in general can be ingested directly or produced by modifying precursor molecules that can be ingested instead, but they are required and cannot be produced from other types of nutrients.  Minerals are inorganic ions and as such cannot be ‘produced’ in the human body at all and must be ingested in an appropriate form.

Vitamins

Vitamins are organic compounds found in foods and are a necessary part of the biochemical reactions in the body. They are involved in a number of processes, including mineral and bone metabolism, and cell and tissue growth, and they act as cofactors for energy metabolism. The B vitamins play the largest role of any vitamins in metabolism (Table 3 and Table 4).

You get most of your vitamins through your diet, although some can be formed from the precursors absorbed during digestion. For example, the body synthesizes vitamin A from the β-carotene in orange vegetables like carrots and sweet potatoes. Vitamins are either fat-soluble or water-soluble. Fat-soluble vitamins A, D, E, and K, are absorbed through the intestinal tract with lipids in chylomicrons. Vitamin D is also synthesized in the skin through exposure to sunlight. Because they are carried in lipids, fat-soluble vitamins can accumulate in the lipids stored in the body. If excess vitamins are retained in the lipid stores in the body, hypervitaminosis can result.

Water-soluble vitamins, including the eight B vitamins and vitamin C, are absorbed with water in the gastrointestinal tract. These vitamins move easily through bodily fluids, which are water based, so they are not stored in the body. Excess water-soluble vitamins are excreted in the urine. Therefore, hypervitaminosis of water-soluble vitamins rarely occurs, except with an excess of vitamin supplements.

Minerals

Minerals in food are inorganic ions or compounds that work with other nutrients to ensure the body functions properly. Minerals cannot be made in the body; they come from the diet. The amount of minerals in the body is small—only 4 percent of the total body mass—and most of that consists of the minerals that the body requires in moderate quantities: potassium, sodium, calcium, phosphorus, magnesium, and chloride.

The most common minerals in the body are calcium and phosphorous, both of which are stored in the skeleton and necessary for the hardening of bones. Most minerals are ionized, and their ionic forms are used in physiological processes throughout the body. Sodium and chloride ions are electrolytes in the blood and extracellular tissues, and iron ions are critical to the formation of hemoglobin. There are additional trace minerals that are still important to the body’s functions, but their required quantities are much lower.

Like vitamins, minerals can be consumed in toxic quantities (although it is rare). A healthy diet includes most of the minerals your body requires, so supplements and processed foods can add potentially toxic levels of minerals. Table 5 and Table 6 provide a summary of minerals and their function in the body.