During starvation there is many source for glucose such as pyruvate, lactate, amino acids (except leucine and lysine) and glycerol from triglycerides but what is the major source of blood glucose ?
closed as off-topic by MattDMo, anongoodnurse, fileunderwater, AliceD♦, Chris♦ Dec 14 '16 at 22:28
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy." – MattDMo, anongoodnurse, AliceD, Chris
The (very) short answer is gluconeogenesis. When glucagon levels are high in response low glucose or low Insulin (perceived in the case of Insulin resistant DM Type II) key enzymes will be inhibited to prevent glycolysis and other enzymes will be activated to produce glucose from various substrates.
There are a few sources that can feed into the gluconeogensis machine. Two that are commonly used in short term fasting are lactate via the Cori Cycle and Alanine via the aptly named Alanine Cycle. Now when we get into long term fasting (i.e. starvation) things start to change a bit.
In starvation, glucose is conserved as much as possible and many tissues start using ketones (which are derived from Acetyl-CoA which are in turn usually derived from fatty acids) for energy. But even then, there is still a demand for at least a minimal level of glucose. This can be synthesized from a few sources. The backbone for triglycerides, glycerol, can be fed into gluconeogenesis. But that is not the main source for glucose during starvation. The main source is from gluogenic amino acids. These amino acids can be metabolized into pyruvate which can then enter into gluconeogensis.
This is one of the reasons you get muscle wasting during starvation but not in calorie restricted diets. During calorie restriction you have enough glucose to maintain adquete blood sugar between meals, and any energy deficits can be met through β-oxidation of fat. Thus you lose fat. However, when you enter starvation, the glycerol obtained from lipolysis (breaking down fat) isn't sufficient to maintain adequate blood glucose and you have to turn to muscle breakdown. This breakdown of muscle tissue is mediated by high levels of Cortisol, a hallmark of starvation that is not seen in calorie restricted diets. If you want to see the effects of starvation levels of Cortisol in a non-starving person, look into Cushing's Disease. You see similar breakdown in muscle that you would normally see in a starving person.
There are a couple of different levels of starvation, from fasting to malnourishment to long-term starvation.
Let's start from consumption of a carbohydrate-heavy meal. The carbs in this meal are readily converted into glucose, and blood sugar levels will increase. Assuming you're not diabetic, this will lead to increases in insulin production, which will lead to among other things, uptake of glucose by hepatocytes (liver cells) and conversion into glycogen for short-term storage. At some level, excess energy is also converted into fatty acids for more long-term storage. Other cells (like muscle, brain etc) will continue to use glucose for energy.
Say this was an evening meal, and you don't eat again until the morning. Over time, blood sugar levels will drop. This leads to decreased production of insulin and increased production of glucagon. As a result, hepatocytes reverse course, breaking down glycogen, turning it into glucose and secreting it from cells to raise blood sugar.
But let's say you miss the morning meal. Eventually, the liver's stores of glycogen will run low. At this point, the body can turn to stores of fat for energy. But most fatty acids cannot be converted into sugars. As a result, blood sugar levels do not rise. Instead, they are converted to molecules called ketone bodies, which can be released in the blood and used by most cells for energy.
However, some amount of sugar is still required to construct certain biological molecules. Also, certain cell types (particularly red blood cells) cannot use ketone bodies for energy. Even with abundant fat reserves, if no additional carbs are consumed, those sugars have to come from somewhere, and they can't come from fat. Instead, the body will begin to degrade cellular protein - some amino acids can be broken down to generate sugars. During long-term starvation, muscles, including those in the heart, begin to be consumed, and anemia is often an early symptom of starvation (since RBC's don't have sufficient energy).