Glucose as a form of energy
Glucose and ketones form the main substrates to provide energy for bodily funtions. The energy released is captured by ADP (adenosine diphosphate ) which is converted to  Adenosine triphosphate (ATP) and this takes place in the mitochondria. The reverse conversion releases energy at the appropriate site ( eg Actin myosin filaments in the muscle)

Normal adult range of blood sugars range between 4.0 mmol/l to 7.0 mmol/l. This level can be elevated in times of stress and other pathologic states ( illness). However the levels are only just above threshold and will fall back to normal in the non-stressed state. Blood sugars may also be below this range in fasting and in very insulin sensitive individuals. However a blood sugar lower than 4 mmol/l in a non diabetic  individual is not associated with any symptoms.

Where does glucose come from?

Life on earth has learnt to become extremely energy efficient. you can get enough energy from a slice of bread ( 20 grams of carbohydrates) to walk 1 mile.

a) Absorption from gut  Glucose is absorbed from carbohydrates we eat. Poly saccharides ( Starch) is the most common stable form of carbohydrates found in nature. This is broken down into disaccharides and then into monosaccharides which is absorbed.

b) Glycogenolysis Glucose  once absorbed is stored as glycogen in the liver and muscle. This provides rapid release of glucose when we need it ( eg running away from predators, chasing a rabbit)

b) gluconeogenesis Glucose however can also be manufactured from other food substances like aminoacids.  Glucogenic aminoacids include glycine, aspartic acid, glutamic acid. This happens during periods of starvation or carbohydrate deprivation.

Carbohydrates

  • Poly saccharides 

These are glucose molecules bound together but do not taste sweet.. they consist of several thousands of glucose molecules bound together in a stable structure commonly called starch. In our intestine this is gradually hydrolyzed into disacchharides. the enzymes that break these complex starch molecules start in our saliva ( Ptyalin) and later in the small intestine. Try chewing a crisp (potato chip) in your mouth for 5 minutes. It will taste sweet due to glucose being released.

  • Disachharides

There are three main types of disaccharides. These are the easiest to break down and taste sweet

    • Maltose – This is a reducing sugar with two molecules of glucose stuck together. -. This was first discovered in germinating malt – and hence the name. It is broken down to glucose by maltase..
    • Sucrose (Not to be confused with Sucralose – this is a sweetener) –  This  is the most common form of sugar (Cane sugar, Beet sugar) and this consists of one molecule of fructose and one molecule of glucose.
    • Lactose Milk sugar- This is found in mild and is formed by the combination of glucose with galactose. This is broken down by lactase in the intestine to galactose and glucose which is absorbed.
  • Monosaccharides

These are single molecules of energy dense carbohydrates which can be readily absorbed from the intestinal mucosa. These are transported from the intestine via the superior mesentric vein to the liver. On the way,the Superior mesentric vein joins the splenic vein to form the portal vein. ( with high levels of insulin – secreted by the pancretic islets.). In the liver, insulin acts to transport glucose into liver cells where it is converted into glycogen ( or fat) and the hepatic vein has only normal levels of glucose exiting the liver.

Lactose intolerance is common in adults and can result in abdominal bloating flatulence  and diarrhoea in people who lack the enzyme lactase. This means that lactose is not absorbed and passes into the colon where hungry bacteria devour the lactose and convert it into GAS..

 

 

Sucralose is a calorie free sweetener made from disaccharide by adding chlorine. This is not absorbed by the body in sufficient quantities to cause weight gain. It is much sweeter than sugar and does not contribute to weight gain
Elevated blood sugars

Blood sugars higher than 7 mmol when persistent may indicate Diabetes Mellitus. This reading needs to be verified by repeated values in the non-stressed state. Elevated blood sugars may be transient (after eating, stress, exercise). Glucose is an aldehyde and is by nature a toxic molecule. However it also forms the most powerful easily broken down endergy source. ( A necessary evil).  Glucose is kept in balance by several powerful hormones. Insulin on one side is a very powerful anabolic hormone ( Building up). Insulin converts glucose into glycogen or triglyceride

Hormones that lower Glucose

Insulin ( converts glucose to glycogen or fat)

Hormones that increase glucose

Glucagon ( breaks down glycogen to release glucose)

Cortisol ( promotes breakdown of protien to aminoacids)

Growth hormone ( causes body to become resistant to insuiln)

Adrenaline ( rapid breakdown of glycogen and fats)

Insulin action

  • Insulin sensitivity

This is the body’s responsiveness to insulin. This varies from person to person and can be determined by several factors listed below. This can be assessed by the HOMA test which measures the amount of infused glucose required to keep blood sugars steady in the face of a known quantity of insulin.

  • Insulin resistance

Insulin resistance is a condition where the body has resistance to the action of insulin. Hence the affected person requires higher amount of insulin for Glucose disposal (Removal of glucose from the blood). This can be due to various reasons

  • a) Genetic factors Certain genes are associated with insulin resistance . Certain races, particularly south Asians tend to be more insulin resistant
  • b) Stress – Illnesses can make the person more insulin resistant
  • c) Physiologic states – Puberty and pregnancy
  • d) Obesity – Increased fat deposits means that more insulin is required to store glucose. In particular central abdominal obesity leads to insulin resistance
  • e) Age – Increasing age associated with Insulin resistance
  • f) Reduced Muscle to fat ratio. (Increased fat proportion is associated with insulin resistance.
It is estimated that at the end of the last ice age, there were less than 10,000 homosapiens on the planet. Those who survived were those who were able to store and use efficiently
Sleep is an important factor that resets our hormone levels. high levels of cortisol ( for eg) causes insulin resistance and may contribute to weight gain. An average of 7 -8 hours of sleep is essential to reset circadian rhythm.

 

Questions for research

  1. Why is pregnancy and puberty associated with insulin resistance?
  2. Why are insulin resistant genes not becoming extinct with evolution and survival of the fittest?
  3. Why did insulin resistance develop in humans?