After taking food, the three main components of food i.e. carbohydrate, protein and fat break down into glucose, amino acid and fatty acid respectively those are absorbed into the blood (plasma) from the intestine. An optimum level of these nutrients in the blood is always required for health. In absence of food, plasma levels of glucose, amino acid and triglycerols fall, triggering a decline in insulin (A hormone that lowers blood glucose level) and an increase in glucagon (A hormone that increases blood glucose level) release. The decreased insulin to glucagon ratio and the decreased availability of substrates make the period of deprivation from the nutrients a catabolic period characterised by degradation of tryacylglycerol, glycogen and protein. This sets into motion an exchange of substrates among liver, fatty tissue, muscle and brain and is guided by two priorities

1. The need to maintain adequate plasma levels of glucose (which is normally 80-120 mg/dl in fasting state) to sustain energy need of brain and other glucose requiring tissues; and

2. The need to mobilise fatty acids from fatty tissue and ketone bodies from liver to supply energy to all other tissues.

Normally energy is stored in our body in three substances named fat, glycogen and proteins. In a normal 70 kg man the stored energy distribution is

Fat (15 kg) = 1,35,000 Kcal

Glycogen (0.2 kg) = 800 Kcal

Protein (6 kg) = 24,000 Kcal

During Ramadan we fast for 12-14 hours according to seasonal variation which is preceded by a well-fed shehri and followed by iftar. Even though body fat is an enormous source of energy. During this short period of starvation the main bulk of energy is provided by the glycogen stored in the liver. Fat utilisation starts slowly over the later part of the day and protein utilisation occurs usually during chronic starvation, i.e. starvation for days long.

The liver is the prime store organ of energy of the body and it first uses glycogen degradation, then gluconeogenesis (synthesis of glucose from non-glucose substance) to maintain glucose levels to sustain energy metabolism of brain and other glucose requiring tissues.

During the absorptive period after sehri, glucose from the diet is a major source of blood sugar. Several hours after the meal blood glucose levels have declined sufficiently to cause an increased secretion of glucagon and decreased release of insulin. The increased glucagon to insulin ratio causes rapid mobilisation of liver glycogen (which contains about 100 gm of glycogen after well-fed sehri). Although the liver glycogen is nearly exhausted after 10 to 18 hours of fasting, it can provide the main bulk of blood glucose during the brief period of starvation of 12-14 hours during Ramadan by the process of glycolysis.

The synthesis of glucose and its subsequent release into the blood circulation are vital function of the liver during the later part of the starving day. The carbon skeletons for gluconeogenesis are derived primarily from glycerol, amino acids and lactate. Gluconeogenesis begins 4-6 hours after sehri meal. It becomes fully active only when liver stores of glycogen are wholly depleted i.e. after 10-18 hours (which varies from person to person). As we are taking iftar meal within 12-14 hours, gluconeogenesis rarely reaches its full blown state. But it plays an essential role during both overnight and prolonged fasting for days. During glucose synthesis by gluconeogenesis fat is preffered over protein as a substrate and thus body conserves its essential protein storage.

The oxidation of fatty acids derived from fatty tissue is the major source of energy in hepatic tissue in the post-absorptive state. On the other hand, glucose transport into the adipocytes (fat cells) and its subsequent metabolism are depressed owing to low levels of circulating insulin. This leads to a decrease in fatty acid and triglycerol synthesis in the adipose tissue.

The activation of hormone sensitive lipase enzyme and subsequent hydrolysis of stored triacylglecerol are enhanced by the circulating catecholamines in blood. Adrenaline, particularly Noradrenaline release from the sympathetic nerve endings in adipose tissue are also physiologically important activators of hormone sensitive lipase.

Fatty acids obtained from the hydrolysis of stored triacyleglycerol are released into blood. Bound to the plasma proteins, albumin, they are transported to the variety of tissues as fuel. The glycerol produced following triacylglecerol degradation is used as a gluconeogenic procedure by the liver. Fatty acids are also converted to Acetyl Co-A which can enter the TCA cycle, thus providing energy for adipocytes.

During the brief period of starvation glucose transport into the skeletal muscle cell via insulin dependent glucose transport protein in the plasma membrane and subsequent glucose metabolism are depressed because of low levels of circulating insulin.

Thus we see that during the fasting state of Ramadan our normal body metabolism changes appropriately to maintain the blood glucose level as to sustain the brain activity. The prime role is played by the liver via its stored glycogen and the adipose tissue and muscle also change their metabolism in a way to prevent excessive breakdown of muscle protein which would have occurred if we were to starve for days after days consecutively.