【Watch Out for Immune Fatigue】Why You Keep Catching Infections, How Constant Eating and Poor Sleep Weaken Immunity, and the Diet and Fasting That Help It Recover

“I feel like I’ve been catching colds more easily lately.” “I can’t shake off my fatigue, and the smallest thing makes me sick.” Have experiences like these been piling up?

There are many reasons immunity can weaken, but one that recent research has been spotlighting is immune fatigue caused by “all-day grazing” (the habit of eating all day long).

When you keep eating something throughout the day, your immune system is overworked with no time to rest, and it can’t perform when it really matters.

That said, the way you eat isn’t the only reason you might catch infections easily.

In this article, I’ll explain the science behind how all-day grazing wears out your immune system, and how fasting and better eating can help you rebuild a body that stands up to infections.

I’ll also walk you through other causes that make infections more likely, such as lack of sleep and stress, along with the warning signs your body gives you.

Why Do You Keep Catching Infections? Signs Your Body Gives When Your Immunity Is Low

Immunity is your body’s “security system,” which finds viruses and bacteria that get inside and drives them out.

Immune cells such as white blood cells and NK cells are like security guards patrolling around the clock.

As long as the guards are in good shape, they’ll catch any virus that gets in and deal with it before symptoms appear.

But if the guards are short on sleep or tied up with other jobs, they’ll let intruders slip past.

That may be what’s really behind the feeling that you “catch infections easily” or “take a long time to recover.”

Immunity Has Two Lines of Defense: “Innate” and “Adaptive”

Did you know your immune security is broadly divided into two teams?

It’s easy to picture innate immunity as “the local police officers who rush to the scene” and adaptive immunity as “a specialist investigation team carrying the culprit’s photo.”

People who catch infections easily are thought to have one or both of these teams not working properly.

And it’s become clear that both teams are strongly affected by diet, sleep, and stress.

Weakened immunity actually shows up as a variety of everyday problems.

Signs that are easy to dismiss as “just being tired” are often important signals from your body.

These are signs that your immune system is “tired.”

Your immunity isn’t fixed from birth; it changes greatly depending on your daily eating habits, sleep, and stress management.

Recent immunology research has shown that “what you eat and when” in particular has a direct effect on immune function.

The main causes that make infections more likely can be sorted into five: “how you eat,” “sleep,” “stress,” “unbalanced nutrition,” and “lack of exercise.”

In this article, we’ll first look at the effects of how you eat (all-day grazing), and then at the effects of other lifestyle habits.

Note that if you have a sudden high fever that won’t go away, trouble breathing, or repeated bouts of pneumonia, there may be an illness unrelated to lifestyle behind it.

In those cases, don’t judge for yourself; see a doctor as soon as possible.

Cause #1 of Frequent Infections: 3 Ways All-Day Grazing Wears Out Your Immune System

Let’s break down why “eating constantly” weakens your immunity into three mechanisms.

① An Overworked Digestive System → Immune Cells Run Out of Energy

About 70% of all immune cells are concentrated in the small and large intestines.

In other words, the gut is like “security headquarters,” where the largest number of guards are stationed.

When you eat all day long, your digestive system works nonstop without a break, and the immune cells gathered there also get pulled into supporting digestion.

This creates a situation in which the energy and immune cells that should be defending against infections are constantly used for digestion instead.

It’s just like how, if security headquarters spent the whole day signing for deliveries, it would have no staff left for patrols.

② Chronic Inflammation → Innate Immunity Gets Overused

Constantly eating refined carbs and processed foods triggers chronic, low-level inflammation in the body.

Chronic inflammation is a state in which small smoldering fires, not big enough to become a blaze, keep smoldering here and there throughout your body.

To deal with this inflammation, innate immunity (the first-response system for inflammation) stays switched on all the time, using up immune resources that should go toward fighting viruses and bacteria.

A study published in the journal Cell (Christ et al., 2018) showed in mice that
a so-called Western-style diet high in fat and sugar rewrites the programming of monocytes (a type of immune cell), so that overreactions to inflammatory triggers continue long after the diet is returned to normal.

This is the idea that a “memory of inflammation” gets etched into the immune system.

It’s worth noting that this suggests the effects of eating habits may not end on the same day.

③ Damaged Gut Flora → Gut Immunity Breaks Down

Constantly consuming processed foods, sugar, and alcohol reduces the good bacteria in your gut and damages the gut barrier.

When the gut wall weakens, “leaky gut” occurs, in which bacterial toxins (LPS) leak into the bloodstream, followed by body-wide inflammation and immune problems.

Picture holes opening up in the walls of security headquarters, letting intruders pour in one after another.

Studies have also reported that when gut flora is out of balance, even vaccines become less effective.

The fourth item, a blood sugar spike, is when your blood sugar shoots up after a meal.

It’s known that when blood sugar stays high, neutrophils, which eat and destroy bacteria, become sluggish.

This mechanism is likely part of the reason people with diabetes are said to catch infections more easily.

Cause #2 of Frequent Infections: Lack of Sleep, Stress, Poor Nutrition, and Lack of Exercise

Alongside how you eat, four other commonly cited causes of frequent infections are sleep, stress, nutrition, and exercise.

Each one weakens immunity on its own, but when they overlap, the impact becomes even greater.

Lack of Sleep: The Guards Lose Their Break Time

During sleep, your body tunes up its immune cells and makes cytokines (the immune system’s messenger substances) to fight viruses.

When you’re short on sleep for a long time, your immune cells work less well, and your body is said to make fewer antibodies after a vaccine.

In a U.S. study in which a cold virus was dropped into the noses of healthy adults, people who slept less than 7 hours had about three times the risk of developing a cold compared with those who slept 8 hours or more.

It’s no surprise that guards who work with no breaks are more likely to miss intruders.

Stress: The Immune Brakes Stay Pressed Down

When you’re under heavy stress, your body releases a hormone called cortisol to get through it.

Cortisol helps calm inflammation, but if it keeps being released, it puts the brakes on immune function itself.

Catching a cold right after exams or a busy season at work is a sign that these brakes were pressed for a long time.

Poor or Unbalanced Nutrition: The Guards Don’t Have Enough Gear

Even if you’re getting enough calories, your immune cells can’t work properly if you’re short on protein, zinc, vitamin D, vitamin C, and other nutrients.

Getting by on sweet pastries and instant noodles is like sending guards out on patrol without any weapons.

Also be careful: if extreme diets leave you undernourished, you can actually become more prone to infections.

Lack of Exercise and Feeling Cold: The Patrol Slows Down

Immune cells patrol your whole body by riding the flow of blood and lymph.

When a lack of exercise slows your circulation, the patrols fall behind and intruders tend to be spotted later.

Moderate exercise like walking supports immunity, but exercise so intense that it wears you out can temporarily lower immunity instead, so moderation is key.

Smoking, Heavy Drinking, and Aging Matter Too

You can’t change your age itself, but you can start reducing the “changeable causes,” such as smoking, drinking, how you eat, and sleep, at any time.

And if you have a chronic illness or use medications that affect immunity, such as steroids or anticancer drugs, don’t try to manage it with lifestyle changes alone; talk to your doctor.

Simple habits that keep viruses out of your body in the first place, such as washing your hands and gargling, are also an important foundation for protecting your immunity.

The Science of How Fasting Helps Restore Your Immunity

If eating constantly is a burden on your immune system, what happens when you do the opposite and create “time without food”?

The effects of fasting on immunity have been scientifically confirmed through several pathways.

Autophagy → Renewing Immune Cells and Clearing Out Waste

Autophagy (the cell’s self-cleaning function), which kicks in after about 16 hours of fasting, breaks down and recycles old or damaged immune cells and encourages them to be replaced with fresh, functional ones.

Autophagy is a process in which cells break down their old internal parts and reuse them, a kind of “cellular recycling plant.”

A study published in Cell Stem Cell in 2014 reported that cycles of prolonged fasting reduced old white blood cells and shifted blood-forming stem cells toward producing new immune cells.

Picture retiring the worn-out guards and hiring new recruits.

However, this was mainly a mouse experiment, and testing in humans is still at a very small-scale stage.

Keep in mind that the same thing doesn’t necessarily happen with everyday 16-hour fasting.

Activating NK Cells (Natural Killer Cells)

Several studies have shown that during fasting, the activity of NK cells (natural killer cells), which directly attack virus-infected cells and cancer cells, goes up.

It’s thought that the ketones that increase during fasting boost NK cell activity and improve your ability to respond quickly to infections.

Ketones are a “backup fuel” made from fat when your body runs low on sugar.

Resetting Your Gut Flora

Giving your gut a rest through fasting helps restore the diversity of your gut bacteria and increases beneficial bacteria important for immune function, such as lactic acid bacteria (Lactobacillus) and bifidobacteria (Bifidobacterium), as well as butyrate-producing bacteria.

When your gut environment is in good shape, gut immunity returns to normal and your line of defense against infections is rebuilt.

If you think of fasting as “time to let your gut rest,” it changes how you see hunger, doesn’t it?

Giving your stomach and intestines a break actually leads to the recovery of your entire immune system.

I explain the link between fasting and the gut in more detail in “【Time Without Food Nurtures Your Gut】Can Fasting Improve Gut Health? How the Reset Works and How to Do 16-Hour Fasting.”

On the other hand, long fasts of several days without food can lead to nutritional deficiencies and may actually lower your immunity.

If your goal is to protect your immunity, building short fasts of about 12 to 16 hours into your daily life is the realistic approach.

Foods That Boost Immunity and Eating Habits to Avoid

Just as important as creating time without food is what you choose when you do eat.

Imagine you’re delivering plenty of gear and food to your guards, and keep the following foods in mind.

Foods That Support Immunity

Protein, the building material for immune cells, is also essential, so include at least one of meat, fish, eggs, or soy products at every meal.

Rather than eating a lot of one particular food, combining small amounts of many different foods helps you get all the nutrients your immune system needs.

A Sample Day of Immune-Supporting Meals

Here’s one example of how to work the foods above into real meals.

The key is to spread fermented foods, protein, vegetables, mushrooms, and seaweed evenly across your day.

The Japanese set-meal style is a combination that naturally tends to bring together immune-supporting foods.

Even on days when you rely on convenience stores, don’t settle for just a rice ball; adding one more item such as a boiled egg, a salad, miso soup, or natto makes a big difference.

Eating Habits That Weaken Immunity

Be especially careful about the habit of constantly sipping sweet drinks and nibbling on snacks.

Not only does your blood sugar rise and fall over and over all day, but your gut also loses its time to rest.

Late-night meals also lower your sleep quality and cut into your immune system’s recovery time, so ideally finish eating as early as you can.

For your immunity, it’s not just “what you eat”; the perspective of “when you don’t eat (whether you create fasting time)” may be an important piece that modern people are especially missing.

FAQ: Causes of Frequent Infections and Immune Fatigue

Here are some questions readers often ask.

Rebuild a Body That Stands Up to Infections With Fasting and Better Eating

Here’s a concrete action plan for building a body that stands up to infections.

Don’t rush; start with just one or two steps.

Once STEP 1 and STEP 2 have become habits, also try to make sure you get at least 7 hours of sleep.

When you have both time without food and time asleep, your immune cells get a solid “break” to recover.

I explain exactly how to get started with 16-hour fasting in “【The Complete Beginner’s Guide】What Is Intermittent Fasting? Types, Benefits, How to Do It, and Why You Might Not Be Losing Weight, Explained Simply.”

On days when you’re not feeling well, don’t force yourself to keep fasting; put fluids and easy-to-digest meals first.

And if you have a chronic illness, take medication, or are pregnant or breastfeeding, talk to your doctor before you start fasting.

Changes in immunity aren’t visible the way weight is, so it takes time before you notice the effects.

I recommend slowly looking back season by season: “Am I catching fewer colds?” “When I do catch one, do I get better faster?”

In most cases, a body that’s vulnerable to infections isn’t something you’re born with; it’s something your eating habits have created.

By stopping all-day grazing, giving your gut a rest, and getting the nutrients you need, your body can recover remarkably well.

If you’d like to make sticking with fasting easier, check out ufeelgreat’s science-based fasting program.

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