In her 2010 book GAPS Gut and Psychology Syndrome, Dr. Natasha Campbell-McBride offers a very strong criticism of the health impacts of vegetarianism (and to an even greater degree, veganism, hereafter implied).
Highlights include the claims that:
Vegetarian children are more prone to health problems than children who eat meat, particularly to psychomotor impairment and diseases of the blood.
Vegetarians are prone to muscle loss and bone damage. They, on average, have lower muscle strength.
Misguided vegetarianism is rapidly becoming a major cause of mental illness in our young people. What these young people do not know is that plant foods are generally hard to digest and they are low in useful nutrition. Foods from animal origin are easy for the human gut to digest and assimilate and animal foods provide concentrated amounts of all essential nutrients for human physiology. As plant foods are hard to digest and as they contain a list of anti-nutrients, which can damage the gut, digestive problems are the first symptoms which appear in those beginner vegetarians. They usually develop IBS symptoms, such as bloating, indigestion, constipation, diarrhoea and flatulence. If the person already has a weak digestion, moving to a plant-based diet is positively dangerous. As the digestive system becomes more and more damaged, it is less able to nourish the person, and so nutritional deficiencies develop quite soon. Vitamins B12, B6, B1, B2, niacin, essential amino acids, zinc and proteins are the first nutritional deficiencies a beginner vegetarian usually develops.
Apart from causing malnutrition through the damage to the digestive system, plant foods are a very poor source of nutrients to start with. In a laboratory we can use all sorts of methods and chemicals for extracting nutrients from plants: methods which our human digestive system does not possess. Human gut has a very limited ability to digest plants and to extract anything useful from them. Nature has created herbivorous animals (ruminants) to eat plants and in order for them to be able to digest these plants, Nature has equipped them with a very special digestive system: it is very long with several stomachs full of special plant-breaking bacteria. The human digestive system is similar to the gut of predatory animals, such as wolves and lions: our digestive system is fairly short and we have only one stomach with virtually no bacteria in it. In fact, our human stomach is designed to produce acid and pepsin, which are only able to break down meat, fish and eggs. In short, our digestive systems have been designed to cope best with animal foods.
She references the book, The Vegetarian Myth by Lierre Keith. She also goes on later in the book to list additional health concerns resulting from vegetarianism. Needless to say, this contention is extremely controversial. Many people still believe that vegetarianism, and even veganism, are healthy lifestyle choices if executed properly.
My question is, what does the highest level science have to say about vegetarianism/veganism? Presumably individual studies have shown conflicting results, offering firepower for people on both sides of the debate. Have there been any systemic reviews/meta-analyses outlining a high level picture of what we know about this subject from a scientific perspective, which would be more credible than a single study? If there is a healthy protocol for vegetarianism/veganism, how does it address each of Dr. Campbell-McBride's concerns (psychomotor impairment, diseases of the blood, lower muscle strength, prone to bone damage, digestive problems such as IBS, lack of human-usable nutrients)?