Intestinal permeability, commonly called leaky gut, describes increased passage of luminal material across the intestinal barrier when tight junction regulation, epithelial integrity, mucus layer function, microbial balance, and immune signaling become disrupted. The intestinal lining normally acts as a selective barrier, allowing digestion products, water, electrolytes, and nutrients to pass while limiting excessive movement of bacterial fragments, food antigens, toxins, and inflammatory compounds. When barrier function is weakened, immune cells in the gut-associated lymphoid tissue may be exposed to more luminal material, increasing inflammatory signaling and oxidative stress. This condition is associated with microbiome imbalance, low dietary fiber intake, reduced short-chain fatty acid production, high intake of refined sugars and ultra-processed foods, alcohol exposure, emulsifiers, chronic stress, poor sleep, metabolic dysfunction, and environmental toxin exposure. The gut barrier depends on mucus production, tight junction proteins, epithelial renewal, mitochondrial energy metabolism, antioxidant defense, and normal immune tolerance. Reduced microbial diversity may lower production of butyrate and other short-chain fatty acids that support colonocyte energy metabolism and epithelial repair. Increased oxidative stress and NF-κB signaling may further weaken barrier regulation. A whole-food plant-based pattern supports barrier function by supplying fermentable fibers, resistant starches, polyphenols, minerals, amino acids, and phytochemicals that interact with the gut microbiome and epithelial cells. Legumes, whole grains, vegetables, berries, seeds, mushrooms, herbs, and spices provide substrates for beneficial microbial fermentation. Broccoli, kale, cabbage-green, cauliflower, and watercress provide glucosinolate-derived compounds associated with Nrf2 antioxidant response and detoxification signaling. Brown-lentils, chickpeas, black-beans, oats-cooked, brown-rice-cooked, and quinoa-cooked provide fiber and plant protein patterns that support microbial diversity and steady metabolic signaling. Barrier support also depends on hydration, potassium, magnesium, vitamin C, vitamin A carotenoid precursors, vitamin B9, zinc, and amino acids involved in epithelial turnover and collagen structure. Sweet-potato-orange, carrot, spinach, kale, red-bell-pepper, blueberry, blackberry, strawberry, pomegranate, flax-seeds-whole-raw, chia-seeds-whole-dried, shiitake-raw, and maitake-raw provide nutrients and phytochemicals associated with oxidative balance, epithelial maintenance, and immune regulation. P53 Nutrition emphasizes no oils, no meat, no dairy, no toxins, and a 100% whole-food plant-based pattern because this approach reduces exposure to processed-food additives while increasing fiber diversity, antioxidant density, and microbiome-supportive plant chemistry. The biological goal is improved epithelial resilience, balanced immune signaling, microbial diversity, short-chain fatty acid production, and reduced inflammatory load across the digestive barrier.