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Low carb diets are lacking in fiber. Every plant-based food has some fiber. All animal products have no fiber. A lack of fiber increases your risk for cancers of the digestive track (because transit time is lengthened) and cardiovascular disease (because of fibers effect on fat and cholesterol). It also puts you at a higher risk for constipation and other bowel disorders.

Low carb diets lack sufficient quantities of the the many nutrients / phytonutrients / antioxidants found in fruits, vegetables, legumes, and whole grains, necessary for health and aiding in prevention of cancer and heart disease. In fact, you need these nutrients even more so when you're consuming too much fat as is often the case on a low carb high protein diet.

Low carb, high protein diets cause an unhealthy physiological state called ketosis, a type of metabolic acidosis. You may have heard the phrase, "fat burns in the flame of carbohydrate". Excess acetyl CoA cannot enter the Krebs Cycle (you remember the old Krebs Cycle) due to insufficient OAA. In other words, for fat to burn efficiently and without production of excess toxic ketones, sufficient carbohydrate must be available. Ketosis can lead to many health problems and can be very serious at it's extreme.