Food And Energy - Energy And Fatigue: Chronic Fatigue

Content provided by the Faculty of the Harvard Medical School

Food and energy

Like a rechargeable battery, the body stores its own energy and recharges itself. The source of energy is food; its energy potential is measured in calories. Calorie needs vary from person to person, but in general, adults need about 1,500–2,500 calories a day. That's enough to fuel normal daily activities, such as eating, working, thinking, and exercising, as well as to maintain energy reserves in the body's cells.

How many calories should you eat? Your daily calorie goal should be the number of calories it takes to maintain a healthy body mass index (BMI), as shown in Table 1. A healthy BMI for most people is 19–24. If your BMI is higher, you'll need to consume fewer calories, increase your physical activity, or do both in order to bring your weight into a healthy range.

Most organs in the body store sugar for use as energy. The exception is the brain. Because the brain does not store its own supply of sugar, it relies on a relatively constant supply of sugar from the blood. Blood sugar levels rise following a meal and decline thereafter. Relatively low blood sugar levels are one of several signals to the brain that cause it to create the sensation of hunger — encouraging eating, and thereby raising blood sugar levels. In some people, a feeling of fatigue and sometimes lightheadedness and headache accompany the sensation of hunger. (If this describes you, it's best to have more frequent, smaller meals so the peaks and valleys in your blood sugar level are less pronounced.) Other people feel neither hunger nor fatigue, even when their blood sugar level is relatively low.

Consuming more calories than you need won't give you extra energy. In fact, it can have the opposite effect. Excess calories are stored as excess pounds, and the heavier you are, the more effort you'll need to expend in walking, pumping blood, and so forth. The more effort you exert, the more energy you need to burn. The more energy you burn, the more fatigued you'll feel.

Nevertheless, food is the raw material for energy (see Figure 1). Food gets chopped up in the stomach and intestines into millions of microscopically small nutrient and waste molecules. The nutrients are absorbed into the bloodstream and transported along with oxygen to every cell in the body. Inside the cells, small structures called mitochondria use oxygen to convert the nutrients into adenosine triphosphate (ATP) — a chemical that cells use as an energy source.

Figure 1: How food and oxygen become energy Digested in your stomach and absorbed into your bloodstream, the food you eat is delivered to cells throughout the body. Oxygen from your lungs also travels to your cells, where tiny structures called mitochondria use it to convert the food nutrients into a chemical called adenosine triphosphate (ATP), which provides energy for everything from walking to thinking.

Because ATP provides the energy for all cells, it's tempting to ask if abnormalities in making ATP could cause something that a human being would sense as a "lack of energy." There are a few rare diseases that involve the mitochondria and production of ATP, which can sap the muscles of strength and cause mental fatigue and impaired thinking. However, in most conditions and illnesses that cause fatigue, the role of ATP is more often presumed than proved.

Given that food is fuel, it's not surprising that what you eat — and when — can affect your energy level. It's only natural to wonder if certain types of food deliver energy more efficiently than others. Are there energy-boosting foods?

Although the body's energy potential is measured in calories, it takes more than calories to fuel the body's needs. It also takes adequate amounts of specific nutrients: fluids, vitamins, and minerals, as well as a mix of carbohydrates, proteins, and fats.

Are you deficient?

Deficiencies in certain vitamins and minerals can produce fatigue by causing anemia, a shortage of red blood cells. Anemia impairs the delivery of oxygen to the body's tissues. Because oxygen is needed to create ATP, doctors assume that the fatigue that many people with anemia experience comes from reduced production of ATP in the cells.

Several vitamin and mineral deficiencies can cause anemia. The most common is iron deficiency, which prevents the bone marrow from making enough hemoglobin. Red blood cells need hemoglobin in order to carry oxygen throughout the body. Anemia is most common in menstruating women, who lose blood (and, hence, iron) with every period. When this blood loss causes iron-deficiency anemia, iron-rich foods (such as liver, eggs, rice, and beans) and iron supplements (pills or liquid) can help restore the body's iron levels and correct the anemia.

At the same time, there's no evidence that taking iron supplements will boost your energy if you don't suffer from iron deficiency. Indeed, this practice can lead to iron overload — particularly in people born with a genetic vulnerability to becoming overloaded with iron (a condition called hemochromatosis). Such iron overloads can damage multiple organs in the body, including the heart, which can lead to fatigue.

Deficiencies in vitamin B₁₂ and folic acid can also cause anemia. Without enough of these vitamins, the body can't produce enough red blood cells. Vitamin B₁₂ is found only in meat and dairy products. Folic acid is readily available in liver, leafy green vegetables, and whole grains, but cooking can easily destroy it. Many packaged foods, including breads and flour, are fortified with folic acid. Even if your diet contains adequate amounts of vitamin B₁₂ and folic acid, illnesses such as inflammatory bowel disease can interfere with their absorption. Some people take vitamin B₁₂ shots for energy, but this probably won't increase your energy unless you are B₁₂ deficient.

What about carbohydrates?

Beyond the importance of eating a balanced diet, there's relatively little scientific information about the effects of specific foods on a person's energy level. However, certain foods can give you more energy under specific circumstances. Because muscles burn carbohydrates first, tests of physical endurance require a steady supply. Sugar is a simple carbohydrate, which means that it's digested and absorbed into the blood quickly. When you eat a candy bar or some other sugary food, your blood sugar shoots up. Your pancreas responds by flooding your bloodstream with large amounts of insulin, the hormone that carries blood sugar (glucose) into the cells. You feel more alert and physically energized. But the effect is short-lived, lasting only about half an hour, because the insulin rapidly lowers your blood sugar. Afterward, you may actually feel less energetic than you did before eating the sweet snack, probably because of the rapid drop in blood sugar.

To keep your blood sugar and energy levels on an even keel, pick your carbohydrates wisely. Try to avoid highly refined carbohydrates, such as white bread, white rice, processed pasta, and white potatoes, which are digested and absorbed almost as quickly as refined sugar, providing the same short-lived surge of energy and subsequent energy drop. It's better to choose complex carbohydrates, such as high-fiber whole-grain bread, brown rice, whole-grain pasta, and most vegetables. These take longer to break down, so your blood sugar levels rise and fall more gradually.

Using stimulants

Certain substances in food give you an energy boost because they contain stimulants that speed up the chemical reactions inside cells. Caffeine, the best-known stimulant, occurs naturally in coffee, tea, cocoa, and chocolate. It is also added to some soft drinks and used as an ingredient in some medications.

People who consume caffeine on a regular basis develop a tolerance for it.

Caffeine has many effects. When you consume a food or drink that contains caffeine, your brain cells fire messages more quickly, improving concentration and reaction time. The heart pumps blood faster and more forcefully, increasing your physical energy. Some people are more sensitive than others to caffeine. For them, a cup of coffee or a can of cola is all it takes to have some or all of the stimulating effects. But people who consume caffeine on a regular basis develop a tolerance for it, which means that they need to increase their intake to get the same effect (see "Use caffeine to advantage").

Nicotine in tobacco is also a stimulant, but because of the detrimental effects of smoking, it should not be used as an energy booster. Indeed, smoking can cause insomnia, which can then lead to daytime sleepiness and fatigue (see "Avoid smoking").

Last updated:	January 23, 2007