Why Do We Feel Tired and Weak During a Fast?

Understanding the body's reaction to fasting: fluctuating blood sugar levels, increased acidity, and resulting symptoms like fatigue and headaches.

We are all familiar with the external symptoms of fasting: headaches, fatigue, weakness, heaviness, and sometimes even heartburn and muscle pain. Why does this happen? A few reasons cause our internal systems to enter a state of 'stress':

Firstly, due to the drastic drop in blood sugar levels, our general energy levels plummet, while acidity levels rise to new heights, disrupting the homeostasis (balance, maintaining internal stability over time) of the body. When the body loses its necessary sugar levels to remain balanced, it uses existing stores (primarily carbohydrates). The problem is that during a fast, these stores are already lacking, so to complete the process, the body 'decides' to produce a molecule similar to sugar, which it 'extracts' from fat, muscles, or bones. This fact alone is enough to endanger our body, which is why various weight-loss methods recommend not overdoing fasts.

Additionally, when carbohydrate reserves (known as glycogen) are 'lacking', our muscles receive less water, causing increased hunger and 'false' weight loss since the fat mass in our bodies hasn't really decreased, but the fluid amount has. When the body doesn't receive enough fluids, our blood volume drops, and the metabolic rate slows down.

What does this mean?

To maintain proper function of all systems, the body requires a substantial daily energy supply (which we receive from food and water in the physical aspect, and through Torah study, good news, and doing things we love, for example – in the spiritual aspect).

When our body's energy stores remain static and aren't replenished, the body loses its 'desire' to produce new energy through metabolism and instead uses the little energy left, until even that runs out. In other words, if it doesn't receive food and water, the command to break down food into essential building blocks – is delayed. Regarding food, the body can manage the deficiency, but this isn't the case with water. The body longs for fluids, and when they're lacking, it receives a command from the brain to enter emergency mode and direct the few remaining fluids first to vital organs.

This is a threat in itself because this 'prioritization' can be costly to other organs that also need fluids.