Fasting blood sugar levels: Early and late stages


Fasting has been in our lifestyle through religious traditions and cultures. It has many side-effects alongside the bigger view of positive effects. Your schedule and daily activities determine the benefits of a fast. People started leaning towards fasting when they heard that fasting could reduce weight. It can act as a weight-loss treatment. So, you need to study the physiology of fasting if you are curious to know if fasting for weight loss and fat reduction works. An essential point to remember through every stage of a fast it that fasting
reduces blood sugar levels over time.

We know that carbohydrate acts as the primary energy fuel due to the natural energy production process, the glycolysis. So, how does the body divert to different reserves for energy with reduced carb-intake and, in general, reduced food-intake?

How does the body maintain blood sugar levels in the early stages of fasting?

In the early stages of fasting, the body rapidly uses up all the available blood sugar–glucose (a product of glycolysis)–for energy. It is also a consequence of increased insulin levels. Thus, the liver glycogen (glucose reserve) depletes after the first 6-24 hr of a fast.

The proteins provide 15% of total energy. Since it has various essential functions other than energy production, the body limits protein use in the early stages. Besides, it also makes sure that all the proteins are not burnt to fill the void in prolonged fasting conditions.

Apart from the glucose reserve, i.e., glycogen present in the liver, the body also stores glucose as glycogen in the muscles. The muscles lack Glucose-6-phosphate to convert it directly into glucose to release into the bloodstream.

Hence, the fats now act as energy suppliers. Fats consist of a glycerol head and fatty acid tails. They are present in the form of triglycerides in the adipose tissues– the long term fat stores. These fats can supply the system with 85% of total energy. In other words, a 70kg person can use the fats for basic caloric requirements.

What happens as the blood sugar levels reduce with a fast?

Reduced sugar supply translates to lowered glucose levels. As fasting reduces blood sugar levels, the insulin (regulates sugar metabolism) levels fall, simultaneously increasing the glucagon levels– work to increase glucose levels. Thus, gluconeogenesis (the process of producing glucose) characterizes the early stages. The brain takes up most of the glucose synthesized since it is the best fuel for neural cells. The remainder is sent to the resting muscle cells and other parts.

Liver gluconeogenesis takes up a significant portion while meeting a glucose requirement of 75%. Meanwhile, lactate and amino acids take up 10-15% each for glucose production. Glycerol (a by-product of fat metabolism) contributes up to 1-2%.

Though the human body cannot release glucose directly from fats, it uses the energy from fats’ hydrolysis to produce glucose from lactate and glycerol.

It is also essential to note that alanine contributes a significant portion with 30-40% of amino acid-derived glucose. Meanwhile, the hydrolysis of highly concentrated branched-chain amino acids provides the system with a nitrogen source for transamination for the synthesis of alanine. The alanine ultimately reaches the liver, which uses Alanine most-efficiently to produce glucose.

As alanine is resynthesized into glucose, the amino groups convert into urea and excrete the system via urine.

Feedback inhibition to slow down gluconeogenesis

The body can’t keep creating glucose constantly. Hence, it also needs a feedback inhibition response. The two components are– (i)insulin reduces gluconeogenesis by inhibiting hepatic uptake and (ii)ketosis inhibits gluconeogenesis by decreasing the degradation of branched-amino acids and cut-off the nitrogen source.

The fate of fatty acids

After slowing down gluconeogenesis, the free fatty acids rapidly mobilize to the liver.

In the liver, fats may undergo partial oxidation to ketones, or re-synthesize to triglycerides.

The primary ketones are BHB and acetoacetate. Acetone is also among the products but leaves the system while exhaling, giving a bad breath (Observed on a keto diet).

As fasting continues, the brain uses most of the ketone bodies. Free fatty acids regulate the energy supply to the muscles. Meanwhile, the kidney reabsorbs the ketones, reducing the ammonia, eventually reducing urinary excretion.

Do proteins still act as glucose precursors in a prolonged fast?

The total urinary excretion decreases as fasting progresses. This decrease is evident that the body reduces protein catabolism for gluconeogenesis. Meanwhile, it also makes sure there is enough energy for essential functions–including providing the system with enough precursors for other protein syntheses as well.

With a long fasting schedule, the body enters a state of protein conservation.

Moreover, in hyperketonemia, ketones can oxidize the branched-chain amino acids, cutting-off the nitrogen source for alanine synthesis. This mechanism reduces alanine formation, thereby decreasing alanine dependent gluconeogenesis.

The body soon starts relying more on fats. The basic functions use up minimal energy from fat production. In the long run, this results in weakness.

In other words…

  1. Soon after reduced food intake, the system maintains energy levels by using up all the glucose available. The brain also depends on glucose as the primary fuel. As it progresses, gluconeogenesis characterizes the early stages of fasting, using amino acids as the precursors.
  2. As fasting continues, ketosis increases due to the mobilization of fats to the liver. The oxidation of free fatty acids results in increased ketone levels. With more ketones, the brain starts using it as the primary fuel, reducing the need for gluconeogenesis.
  3. This ketotic stage gradually progresses into protein conservation due to a prolonged fasting schedule.

Side-effects of fasting

  • Headache and dizziness: Headache and lethargy are common side-effects of fasting. It is a consequence of lowered insulin level in the system. As it drops, the body signals the kidney for urinary excretion. Commonly, sodium is a significant part of this excretion. In this process, the body tends to lose out electrolytes resulting in headaches, dizziness, and diarrhea. Make sure you give your system enough electrolytes to maintain the balance.
  • Constipation: Constipation is by far the primary physiological effect of fasting. We tend to have one or two bowel movements a day. During fasting, we don’t give our body food to move the previous intake out of the system. Hence, constipation is common.
  • Acid reflux: The stomach is a reserve of hydrochloric acid, which helps in digestion. Since we don’t give enough products for a breakdown in the stomach, the HCl builds up, resulting in acid reflux. People who have a history of acid reflux have an intense experience at the start. The HCl secretion reduces gradually with the necessary stimuli.
  • Anxiety: Anxiety is a consequence of adrenaline rush at the wrong times during fasting. The rush may occur while you are trying to sleep, worsening the condition. Try maintaining a proper sleep schedule.
  • Bad breath: You may have heard about bad breath when on a keto diet. The same is the consequence of fasting. The by-products of fat metabolism are ketones, among which acetone is exhaled out of the system via the breath. The condition will likely improve once all the fat is metabolized.

Does low fasting blood sugar level translate to weight loss?

In the early stages, the body relies on fat oxidation for energy resulting in weight loss. However, weight loss occurs at a higher rate in the early stages of fasting and reduces gradually.

The system loses sodium in the form of salts of keto acids translating to weight loss. As the situation progresses, the body enters the state of ketoacidosis, opening the way for ammonia production. The system leans towards ammonia to excrete keto acids. This reaction results in a reduced-sodium loss in later stages of fasting, translating to reduced weight loss in the later stages.

Who enters the ketotic stage faster?

Ketosis is the stage where our body synthesizes ketones (the alternate fuel) from fats. It is a product of fat oxidation.

Obese vs. nonobese

In general, fat mobilization increases with low insulin levels. But in obese, the standard level of insulin is generally high, along with high glucagon. Hence, nonobese enter the ketotic stage faster than obese. Also, the growth hormone level is low in nonobese, making fat mobilization difficult.

Although obese are sensitive to insulin, the high levels can affect at any moment. Insulin, with its lipolytic activity, results in enhanced lipolysis.

Men vs. women

Studies showed no difference between obese men and women. But the difference was significant in nonobese men and women. Women have low alanine levels and thus enter the ketotic stage faster.

The early and more exceptional ketosis is a consequence of hormonal variation and fuel stores, along with energy requirements as well.

Can fasting cause psychological effects?

The fasting subjects reported a loss of appetite and well-being in general. Fortunately, none reported cases of total anorexia nervosa. The ketosis, which develops in fasting, is responsible for anorexia. Researchers explain the situation concerning levels of Beta-endorphins– opiates linked to satiety– during fasting.

Additionally, a few subjects experienced euphoria. To explain this condition, scientists postulate the accumulation of aceto-acetic acid, which induces mild intoxication and is responsible for the euphoric psychological response. Others also suggest high levels of isopropyl alcohol in the brain, responsible for the mystical, religious, hallucinatory experiences.

Related: Eating disorder types: Which one can you relate to?

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