Back in the day, when I was training and competing in marathons and triathlons, I started to experience certain food sensitivities, poor sleep, diarrhea, and nausea. I put it down to stress, high training volumes, and working long hours as a personal trainer. Although doing a food sensitivity (IgG) test helped me remove particular foods that increased the symptoms (whey, eggs, gluten, cashews, alcohol, etc.), it wasn't enough to fix the underlying cause. I later found out I had a leaky gut, and once I fixed it, all of these symptoms went away.
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Leaky guy played havoc with my sleep, my ability to recover and, therefore, my health and happiness. Today, my gut health is significantly better, and I wish I had addressed the problem sooner.