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Physiology of Fasting
E. Norbert Smith, Ph.D.


For you created my inmost being; you knit me together in my mother’s womb.
I praise you because I am fearfully and wonderfully made. Your works are wonderful, I know that full well.
(Psalms 139:13-14, NIV)

Introduction. Physiology is the portion of natural science that explains how our bodies operate under normal healthy conditions as well as during illness, stress and injury. To the human body, fasting is starvation and starvation is stressful. For this discussion only the natural physiologic factors of starvation on the physical body will be considered. Needless to say fasting also brings profound spiritual and emotional changes. Supernatural intervention may play a supplemental role under prolonged fasting, but is beyond the scope of scientific discovery. It is assumed sufficient quantities of water will be ingested throughout the fast. Water deprivation was included in Biblical fasting only for matters of life and death (See Esther 4:16.) and involves extreme health risks. Many Bible scholars believe supernatural intervention was involved during both times Moses fasted 40 days and nights without food or water. (Exodus 34:28, Deuteronomy 9:9 and again in Deuteronomy 9:18, 25). Elijah was given two supernatural meals that gave him strength for his forty days (1 Kings 19:6-8) without food or water. Jesus drank water during his 40 days of fasting in the desert (Matthew 4:1-2 and Luke 4:1) in preparation for his earthly ministry and so should we.

Why we eat. Metabolism includes the myriad of biochemical reactions that occur simultaneously throughout our bodies as long as we live. Cellular metabolism is regulated by countless complex protein enzymes. It is estimated that each second over one hundred thousand different enzymatically regulated biochemical reactions are occurring in every living cell in the human body. Metabolism is influenced by a complex interplay of diet, hormones, environmental temperature, emotion and physical activity. We require food for two distinct purposes other than pleasure. Food provides energy for general body maintenance, the regulation of body temperature and for physical and mental activity. In a strict sense it does not matter what we eat for energy. Our bodies were wondrously designed and can obtain the needed energy from each of the three basic food types: carbohydrate, fat and protein. Carbohydrates provide the most rapid source of energy. Fats are a very high energy source, but require longer to digest, assimilate and convert to sugar. Protein can also provide energy, but takes a much larger input of energy and time to digest and process, which is one reason the ever popular Atkins diet is so effective. The large time and energy investments also explain why a large high protein meal must be avoided when breaking a long fast. It is best to break a fast with simple carbohydrates such as juice or cereal. Allow the body time and resources to store the necessary reserves before indulging in that large grilled steak!

We also must eat to provide raw materials for body repair and replacement of worn parts. For example cells that line portions of the gut only live 3 days and must continually be replaced. For this reason it is very important what is eaten. (I have always seen this as an excellent against a vegetarian diet for even without ingesting any animal protein we still digest several grams of animal (human) protein each day!) Healthy bodies require scores of vitamins, essential amino acids, minerals and water. Some of these vital materials are manufactured within our body and are not required in the diet; some are stored for months; others are depleted a few days. A knowledge of these requirements is helpful in approaching any fast. Expect fasting of more than a few days to result in varying degrees of mineral and vitamin deficiency symptoms. For this and other reasons anyone contemplating a fast of more than a couple of days should seek the advice of their health care professional.

Water metabolism. Life as we know it is an aqueous event; all essential biochemical reactions must occur in a water solution. Approximately 57% of the adult body weight is water. Water regulation is complex and involves many parts of the body and is influenced by several important hormones. Water balance is regulated by both the amount of water taken in and the amount of water leaving the body. Dehydration activates the thirst center and we seek liquid drink. About two thirds of our water intake is from drinking fluids. Another third is preformed water in various foods. A small amount (about a cup each day) is made from the oxidation of hydrogen in metabolism. (God created certain desert animals, like our native kangaroo rat so they can live exclusively on this metabolic water! They can live, reproduce and nurse their young without even a drop of water and with only dry food!) Under average conditions a person requires 2-3 quarts of water each day. Much more is needed in hot dry environments and with exercise. From 1 to over 2 quarts of water are lost as urine each day depending on diet, exercise and environmental conditions. As much as 3 to 4 quarts of water per hour may be lost due to sweating with heavy exercise and hot conditions. Each day the body loses about 1.5 pints of water through the skin and by breathing. This is called insensible water loss and we have no control over it. Dehydration also reduces urine production and kidney loss of water and can lead to kidney failure. The human body can withstand only about 10% dehydration. Obese adults, children and infants can not tolerate even this amount. Lack of water rapidly leads to extreme thirst, mineral imbalance, kidney failure and death. Authorities agree most people can not live without water more than a few days. Let’s drink lots of water as we seek God in fasting.

Vitamins. It has been well established the human body requires at least 12 essential vitamins for proper health, growth, repair and energy. More will be discovered. Only vitamin K is manufactured in the human body, all others are required from the food we eat. Small amounts of vitamins are stored in every cell in the body. Large qualities of some vitamins are stored in the liver. For example a healthy person stores as much as a ten month supply of vitamin A and up to 4 months supply of vitamin D in the liver. Storage is slight for vitamin K and most of the water soluble vitamins including C and all the B vitamins except B-12 for which the liver stores a year’s supply. Such storage reserves may be much less for people not getting proper vitamin nutrition. Some studies have shown as many as 30% of faculty children at a major university had vitamin deficiency symptoms. It seems people are no longer taught which foods contain which vitamins and which vitamins can be stored and which are needed each day. This is a sad commentary on our public education. My grandmother knew and practiced these basic nutritional principles...50 years ago.

Fasting will result in vitamin deficiency and can cause clinical symptoms in about a week. This is especially true for vitamin C and most of the B vitamins. Although not life threatening, the absence of vitamin intake will contribute to a loss of energy and stamina. Healing from injury and growth will also be retarded from vitamin deficiency. Many people continue taking supplemental vitamins during a prolonged fast. Again it is wise to consult with your health care professional before starting a prolonged fast.

Energy. Energy metabolism is perhaps one of the most beautifully complex of all physiologic functions and is drastically altered by fasting. Each of us are built with 75 trillion living cells, all of which are fully capable of survival outside the body if given the proper supporting environment and nutritional requirements. Virtually all intracellular activity requires the energy rich compound Adenosine Triphosphate or ATP. ATP is often referred to by physiologists as, “the currency of the cell” for it powers all cellular activity. Thought and problem solving in our brains and all body movement require ATP. Manufacture and secretion of glandular products and even the maintenance of our body temperature require ATP. Cells can not absorb ATP from outside the cell so each cell must manufacture its own supply. This requires a constant supply of the simple sugar glucose. The complexities of intracellular metabolism are beyond this brief introduction. Instead we will briefly consider how the body regulates glucose during fasting. As ATP is the currency of the cell, so glucose is the currency of the body. Complex carbohydrates and other sugars are quickly converted to glucose. Glucose can also be obtained, albeit with more difficulty from dietary fat and protein. Now lets consider the effects of fasting on glucose.

Hunger. Many of our eating habits and portions of our eating pleasure are cultural. We tend to eat more out of habit and social pressure than hunger. It is hard to resist when a good friend comes by and asks us to go out and eat with them...especially if they are buying! Our being or not being hungry at the moment is irrelevant. During fasting hunger becomes our constant companion. It is real and is triggered by several factors. At first true hunger is easily confused with habit. The body “knows” it is time to eat. The family is eating. The aroma of your favorite dish fills the air and your mouth fills with saliva in anticipation of food. This is largely habit and cultural. Deal with it for It will pass. Hunger returns again and again like an unwelcome guest as blood sugar drops. This is a normal and essential response. The animal or human that does not eat will die. Yet, in spite of what your mind and body are shouting, you will not die from a few days of fasting. You body seems unaware of this knowledge and complains loudly.

After just a few hours of fasting you stomach and upper gut are largely empty...for some of us this may be the first time since birth. True hunger pangs occur; the stomach rolls and roars in complaint. As glycogen stores are depleted (see below) hunger intensifies and at first seems unrelenting. For most people hunger begins to subside as the body switches from glucose metabolism to fat metabolism. This usually occurs at about day 3 or 4 of a prolonged fast. Expect hunger to be your constant companion and learn how to use it to draw nearer to God. Be honest and tell God you are hungry and really want that t-bone steak or hamburger, but then in all honesty tell Him you desire Him even more. This is the heart of fasting...hunger reminds you of your need and love for God and you demonstrate it by not appeasing the fleshly desire for food. You will find peace if not satiety in this.

Regulation of blood sugar. Remember every cell in our body needs glucose, a simple sugar. The regulation of blood glucose is a beautifully complex example of the wisdom of our Creator in designing our “inmost being.” Normal glucose regulation involves such diverse organs as the liver, pancreas, adrenal glands, digestive organs, brain, body fat and thyroid gland to mention a few. Unless a person is diabetic, normal blood glucose values drop to approximately 90 milligrams/deciliter 3-4 hours after a meal and rise to perhaps 140 mg/dl after a carbohydrate-rich meal. Blood sugar is highly variable and its regulation requires the interplay of numerous body organs and several important regulatory hormones. The primary short term energy storage organ the liver. Here excess glucose is converted to glycogen and stored until needed. There are smaller, but important glycogen stores in muscle cells. Stored muscle glycogen can be converted to ATP and even used as an emergency energy source during intense exercise when sufficient oxygen is unavailable, but it can not be converted back into glucose for use in other parts of the body. Other cells store only very small amounts of glucose. The two most important hormones regulating blood glucose are insulin and glucagon from the pancreas. Insulin is released as blood sugar rises after a meal. Normally approximately 60% of the glucose rise following a meal is converted to glycogen in the liver. The effect of insulin is to lower blood sugar by facilitating its transport across cell membranes away from the blood and the conversion if glucose to glycogen in liver and muscle and fat storage. The companion hormone glucagon has the opposite effect an and is released between meals. It is perhaps the most important hormone during fasting. Glucagon is your friend.

Fasting, phase one. During the first few hours of a fast expect the normal meal to meal variations of blood sugar, energy level, emotional state and hunger. Here the body is relying largely on blood sugar and it drops to normal pre-meal values as indicated above. It seems the very thought of starting a fast induces feelings ranging from fear to despair and intense hunger. This seems to be exacerbated by the Enemy and the temptation to eat is strong. This should not surprise us for Jesus was first tempted by Satan with the option to eat. Here and throughout your fast recall a fasting passage such as, The time will come when the bridegroom will be taken from them; then they will fast. (Matthew 9:15b, NIV) Use this and other verses as a weapon against the Enemy. Again Jesus is our example. As with other temptations “Resist the Devil and he will flee.” Hunger here is more habit than need. You WILL survive. Mild exercise may help you get food of your mind. So will prayer and Bible study. So will mental discipline and humility.

Fasting, phase two. For the next several hours to the third day your body is using stored glycogen reserves from the liver. Blood sugar will remain slightly low. Very little insulin is released and glucagon becomes your closest friend. As your stomach and upper gut become empty hunger becomes intense. Your stomach will start to complain loudly at times. An interesting event occurs in this phase. You begin to lose a significant amount of weight. Do not get your hopes up yet as most of this is merely water loss. For each pound of glycogen your liver converts to glucose you lose about three pounds of water. This is a normal consequence of using your stored glycogen. As the glycogen reserve is depleted blood sugar drops again. A new level of weakness and lack of energy is normal. Do NOT despair for you will not die...your body is now switching to the next phase of fasting. Here again Satan will remind you of your need for food. Resist. Trust God...for He is your Provider. Some people experience mild to severe headaches, but these too will pass. Sleep may be broken and shallow. Dreams and fantasies of food are common.

Mild to strenuous exercise is helpful as it will cause the release of epinephrine from your adrenal glands and the sympathetic portion of your nervous system.. There are many helpful effects of epinephrine as part of the normal fight or flight response. Blood pressure will increase as will blood flow especially to muscles. Feeling of weakness will largely disappear. Perhaps the most beneficial effect during fasting is exercise will dramatically increase your blood sugar. Epinephrine along with several other stress hormones increases blood sugar by mobilizing fat and protein and converting it into glucose. Unfortunately the rise in blood sugar is only temporary, but it can help you get through some tough times during this phase of fasting. Use the hunger and weakness as reminders of your need and love for your Creator. Blood sugar may come and go...but He will always stay near.

Fasting, phase three. This is the final fasting valley and will last several weeks until fasting is ended. As glycogen stores are depleted the body now switches over to fat metabolism. Glucagon is largely responsible for this change, aided in part by thyroxin from the thyroid gland. Blood sugar will remain constant, but low. The intensity of hunger diminishes for most people as the gut becomes mostly empty. Another interesting event occurs as fat is broken down as the primary source of glucose some glycogen is again stored in the liver and with it water is retained. Some people actually notice a slight weight gain at this point even though they have not eaten in several days. This too will pass for as fat is used as an energy source body weight will begin to decline.

Another important and noticeable physiological adjustment appears at about this time. The body senses starvation and an emergency option comes into play. Your overall metabolism is turned down. It is like your body’s thermostat is readjusted to save the dwindling supply of energy and prolong life. There are two ways you will notice this event. You will feel less energetic, especially when moving. You will tend to walk and move slowly...saving energy. The second obvious factor is you will feel cold and wear additional clothes or a sweater when you would normally do so. You may require extra covers at night. Many believers that frequently fast like to do so in the heat of summer. When their fellow workers or colleagues are sweating they are cool and comfortable. This can be a tangible reward for prolonged fasting. Enjoy and take comfort in it. It is but another way our bodies were skillfully designed.

In this stage you must develop a constant steady approach as the fast continues. Sleep should return to normal. Many people require more sleep during prolonged fasting. Bible study, prayer rest and listening to praise music should become routine. Most find mild exercise helpful. A quite meditative nature walk is refreshing. During fasting many people realize the inordinate amount of time normally spent in food preparation and eating. Use this bonus time wisely. Consider it a valuable gift from God. There are actually many animal studies that suggest regular short fasts actually increase longevity. Eating less prolongs life. One need not be a scientist to realize many of us in America are literally eating ourselves to death. Savor the moment.

Another word of caution is in order here. For most American phase three of fasting can continue for several weeks, because most of us have ample fat reserves. Not all body fat can be metabolized in this phase. When the available fat is gone the body then switches over to protein metabolism and this rapidly becomes life threatening. Let me state it differently. Once fat stores are mostly gone the next material used to provide life giving energy is blood protein. In important part of that blood protein is the portion that provides disease immunity. A person metabolizing protein not only is rapidly losing muscle mass, but soon has no more disease resistance than a terminally ill AIDS patient. This is indeed life threatening and the fast broken long before this occurs. A person metabolizing protein has a characteristic chemical taste and “paint-thinner” or acetone breath. Medically this condition is called ketosis and should be taken as a serious warning to abort the fast. Again, remember not all body fat is available for metabolism. Behind the eye are fat pads that are inaccessible. For most women some fat on the thighs and hips is resistant to breakdown for glucose not matter how great the need. Yes, God calls us to fast, but He also gave us a mind and sound judgement to use.

As with our Savior, Satan will use this time of weakness to tempt us not only with food, but with other fleshly desires. Be on guard and resist him at all times. In the mental twilight separating wakefulness and sleep it seems we are most vulnerable. At this critical time praise music or an audio CD or tape Bible reading are most helpful. Take every vain imagination, lustful and irrational thought captive. Cover yourself with the blood of Jesus. Remember the words of the Psalmist: I lift up my eyes to the hills-- where does my help come from? My help comes from the LORD, the Maker of heaven and earth. He will not let your foot slip-- he who watches over you will not slumber. (Psalms 121:1-3a, NIV) Rest in Him. God will bless you and keep you as you seek His face through fasting, prayer and meditation on His Words. Go for it!

Breaking the fast. Once again caution is advised. I like to break my fasts at sundown with a private Lord’s Supper including prayer, self-examination, rededication and of course the sacramental elements. After this time of worship I drink a small glass of fruit juice and perhaps have a small bowl of cereal or hot oatmeal and go to bed early. Sleep is very sweet and refreshing after a fast. Even the next day avoid overeating, especially protein rich large meals. Save that t-bone steak or juicy hamburger meal for the second or third day after ending your fast. Remember it takes a great deal of energy investment to digest a heavy protein meal. Remain focused on God. May the God of Creation bless and strengthen you as you seek His face by fasting.