At some point all of us go through the day without eating for hours. This can happen on days when we sleep through breakfast, work through lunch or are so tired by the time we get home from work that we want nothing but to hit the bed. So it’s important to understand what really happens to our bodies when we skip meals.

Within the first 6 hours of going without food, glycogen, which stores energy in the body, breaks down into glucose that cells use as fuel as a replacement for food. About 25% of this is used by the brain alone for energy, while the rest is used by muscle tissue and red blood cells. After 6 hours, glycogen production reduces significantly and energy levels plummet—this leaves us feeling hungry and tired. This is when the body enters a state called ketosis, where it begins to fast or starve.

While the way your body reacts to skipped meals depends on your age, health and diet, this is more or less what happens to most of us. 

Let’s understand what happens in ketosis. Since there is less glucose in the bloodstream, your body breaks down fat for energy. Fat is available easily, more so in overweight people. This fat in turn breaks down into fatty acids. However, most of these are what are known as long-chain fatty acids, which the brain can’t use. Now that there’s no more glucose to use and these fats are too large to cross the blood-brain barrier (which is a highly selective permeability barrier that separates the circulating blood from the brain extra-cellular fluid in the central nervous system), the brain changes mode and begins to use ketone bodies for energy, for these short-chain derivatives of fatty acids can cross the blood-brain barrier. This works temporarily because just 75% of the brain’s energy can come from ketones. It still needs glucose, which the body doesn’t have. This is the stage at which cognitive functioning starts getting impaired.

After 72 hours, not only do mood and energy levels plummet, but the body starts breaking down its own proteins. These proteins release amino acids which can be converted to glucose. This is good news for your brain but bad for the body, because it means your body is destroying its own muscle mass . The “weight loss" thus obtained is muscle loss, not fat loss.

In women, skipping meals shuts down the menstrual cycle—this is the body’s way of conserving energy expenditure. It also reduces bone density; women who skip too many meals tend to experience a loss of libido as a side effect. 

If you miss meals consistently for weeks, your immunity will drop significantly.

Leah E. Cahill, who has a PhD from the Harvard School of Public Health in the US, found in 2013 that women who skip breakfast had a 20% higher risk of developing type 2 diabetes. She says in an email interview, “As we sleep all night we are fasting, and so if we regularly do not ‘break fast’ in the morning, it puts a strain on our bodies that over time can lead to insulin resistance, type 2 diabetes, and blood pressure problems."

Long intense workouts, too, result in spikes in the stress hormone cortisol, which temporarily suppresses the immune system. Having a meal, especially with carbs, right after a workout helps prevent this immunosuppression. If you skip a meal at this time, in the hope of losing more weight, you fail to fuel your body, so you’re at greater risk of headaches, fatigue, hunger and infection.

The bottom line: Skipping meals will make you lose muscle, not fat. So think again before you miss a meal for any reason other than the fact that you’re not hungry.

Vishakha Shivdasani is a Mumbai-based medical doctor with a fellowship in nutrition. She specializes in controlling diabetes, cholesterol and obesity.