"At 175 cm and 70 kg, this fat alone could carry me through 80 days!"

"What I was eating was not food — it was greed, attachment, and ignorance.
Autophagy… please digest me… Right now I AM the trash…
I will empty out the garbage and fill my hollow body and mind with bright light."

Jeonghoon (inwardly) "When I cut off desire rather than food, I see the real me… I feel the energy of the universe in my body."

Minho (inwardly) "Since my brother started fasting, his mind has become so calm — I admire him so much…"

"I haven't eaten — yet my mind races as if it could fly!
I cleansed decades of waste through fasting, and I've become a human-version of 'AI'…
With this clear mind and clean body, I will uncover the truths of the universe through science."

When you fast, your protein requirement drops by a factor of ten

Dr. Lee "Today, by understanding the physiological changes during a 40-day fast, we'll come to grasp the effects of fasting at each stage. Last time, Grace, you raised the question of whether fasting beyond a single day might burden the body. Could you remind us?"

Grace "Yes, that's right. My concern is that during a long fast the body's nutrients become depleted, which seems dangerous. Take protein, for example: the daily requirement is roughly 0.8 g per kilogram of body weight. So a person weighing 60 kg needs about 48 g of protein per day. If body proteins begin to break down, the body's structure itself would collapse."

Dr. Lee "A very natural concern, and one that many people share. Let me ask you a question first, Grace. Where in the body is most of the daily protein actually used?"

Grace "I haven't given that deep thought, but according to the textbooks, protein is used to build muscle, to make antibodies for immune cells, and to make digestive enzymes — among many other things."

Dr. Lee "Yes, very good. Scientific studies show that about 90% of the protein the body uses is consumed within the digestive tract."

Grace "Astonishing — 90% in the digestive tract! I knew digestion consumed a great deal of energy, but I had no idea the share was that large."

Zoe "Oh — I remember! You said the esophageal epithelial cells, with a lifespan of just three days, are among the shortest-lived cells in the body. The gastric epithelium lasts about seven days, and the small intestine's lifespan is similarly short. Continuously regenerating those epithelial cells must require a lot of protein."

Dr. Lee "Exactly. On top of that, the digestive tract needs enzymes to break food down into pieces small enough for cells to absorb — and those enzymes are also made of protein. As Figure 4-2 shows, the stomach uses about 43% of the daily protein requirement, the esophagus 30%, and the liver 20%. By contrast, immune cells use about 6% and muscles only about 2%."

Figure 4-1. Estimated protein requirement of 225 adults (Rand et al., 2003).

Figure 4-2. Fractional protein synthesis rate by tissue.

Protein synthesis rate was measured by introducing L-[¹³C]leucine or L-[²H]phenylalanine into tissue protein. The fraction shown represents the protein synthesized each day.

Zoe (surprised) "Muscles use less than I expected! My younger brother drinks protein shakes constantly because he lifts weights — I should show this to him."

The body's daily protein requirement averages 0.6 g per kilogram

Susan "Then how much protein should we actually eat each day? I've heard that protein deficiency causes many problems."

Dr. Lee "Good question. The nutrition field recommends 0.8 g per kilogram of body weight per day. But individual requirements vary widely — from 0.375 g to 1.675 g. On average, 0.6 g per kg is sufficient (Figure 4-1). For example, seoritae black beans contain 21 to 43 g of protein per 100 g. Beef has about 21 g, so beans contain a similar amount — sometimes twice as much. For a 60 kg adult, the requirement is 60 × 0.6 = 36 g per day. Divided across three meals, that's 12 g of protein per meal. If we assume beans contain about 30 g of protein per 100 g, that comes to about 40 g of beans per meal — roughly 200 beans."

The body uses the largest share of protein for digestion

Dr. Lee "Let me ask another question. Is the digestive tract inside the body, or outside?"

Zoe "Well, when we eat, food goes inside us — so the digestive tract feels like it's inside the body."

Grace "But food in the digestive tract hasn't actually entered any cells yet, so I'd say it's still outside the body."

Dr. Lee "Yes — the digestive tract is considered to be outside the body. But although it's outside, food is in very close contact with the body's epithelial cells. The digestive tract averages 8.5 meters in length, and once food enters, it remains in contact with our epithelial cells for 24 to 72 hours or more. That's why the gut-associated lymphoid tissue (GALT) runs along the digestive tract and accounts for about 70% of the body's immune system. At the terminal ileum of the small intestine sit the Peyer's patches; lymph nodes; B cells that produce secretory immunoglobulins; macrophages; dendritic cells; T cells; and many other immune cells (Figure 4-3). They interact with beneficial gut microbes to defend cooperatively against harmful ones."

Figure 4-3. The digestive tract and the gut-associated lymphoid tissue (Peyer's patch).

Zoe "Wow — amazing! So many immune cells in the digestive tract. That must be why people say a healthy gut means a healthy body. And it's fascinating that beneficial microbes support our health — that's why we're told to eat plenty of fiber, the food of these microbes. They must thrive there. I heard microbes can divide from one to two in just 30 minutes."

Dr. Lee "Yes. Microbial growth needs three things — moisture, nutrients, and warmth — and the digestive tract supplies all three abundantly. That's why eating the fiber that beneficial microbes love is so important. They block harmful microbes from invading and also break fiber down into short-chain fatty acids — excellent nutrients delivered back to the body."

Grace "Then from the moment we begin eating, there must already be mechanisms in place to block harmful microbes from invading."

Dr. Lee "Yes. Chewing slowly and thoroughly to disinfect food is one. But the most characteristic mechanism of all is the release of hydrochloric acid in the stomach. The gastric epithelial cells secrete HCl into the stomach to sterilize the contents."

Zoe "Hydrochloric acid is so dangerous. I've seen even one drop fume and dissolve almost any material. If our body produces it, doesn't it harm us?"

Dr. Lee "Good question. That's why our digestive tract secretes mucus along its walls to protect against damage. Not just the stomach — the small intestine and the colon also secrete protective mucus. And the main component of mucus is, again, protein."

Zoe "So if we eat a lot, the body has to make a lot more protective mucus. And that drives up the protein requirement at the same time."

Dr. Lee "Exactly. For the stomach to move well and sterilize properly, it's better to eat less than to overeat. With overeating, the stomach can't churn well and harmful microbes survive more easily. And because the stomach can't sterilize 100% of the microbes, immune cells inside the digestive tract release antibodies into the gut lumen — and antibodies, too, are made of protein."

The body's largest immune organ is the gut-associated lymphoid tissue

Grace "I understood antibodies to be produced by B cells — adaptive, post-priming immune cells. Are you saying B cells reside in the digestive tract?"

Dr. Lee "Yes. From a comparative-zoology perspective, in very primitive animals the immune cells are the digestive cells. The lancelet is a textbook example: its immune cells engulf food, break it down, and use it for nutrition. Evolutionarily, the immune system begins in the digestive tract — and it had to, because the digestive tract is the part of the body in longest contact with foreign material."

Figure 4-4-illust. When the body is hungry, macrophages engulf zombie cells more efficiently.

Zoe "Wow — immune cells in the digestive tract! With 8.5 meters of gut, there must be an enormous number of them."

Dr. Lee "Yes. The gut-associated lymphoid tissue truly is the largest immune organ in the body (Vighi et al., 2008). Immune cells also reside in the respiratory epithelium."

Zoe "Of course — when we breathe, the air carries microbes too, so the respiratory epithelium needs immune cells as well."

Dr. Lee "Yes. The alveoli, in particular, contain macrophages — called type 2 alveolar cells — that engulf harmful microbes. So now we arrive at the conclusion: if we stop eating, the body's protein requirement drops by nearly 90%, and only 10% remains."

Grace "Oh — so that means we can fast for more than a day, even for quite a long time."

Dr. Lee "Yes. We've now seen that even during prolonged fasting we don't have to worry much about protein depletion. Let's take a short break, and next we'll look at how the major energy metabolism shifts during a long fast. Thank you, all."

All "Thank you."