It's a bit of a hot topic: Vegans who have decided to incorporate small amounts of "healthy" animal products into their diet.
First of all, the idea of "healthy" animal products is pretty ridiculous considering the numerous studies linking animal products with most first-world diseases. Doesn't matter whether its grass-fed, organic, free-range, unpasteurised, hormone-free, gold-plated, or sun-dried. The bottom line is that animal products are acidic, inflammatory, and not easily digested by humans. So the idea of adding toxic foodstuff to the diet in the name of "health" is absurd.
So. Why would a vegan introduce animal foods into their diet? Why can one person thrive on a vegan diet, and another person suffer? I believe that there must have been something fundamentally wrong with the individual or with the diet. I'll start with the latter.
I question the nature of the ex-vegan's diet, because no two vegan diets are the same. There are a few reasons why people "fall off" a vegan diet:
1. Not eating enough calories. Plant foods are less calorie dense than animal foods, and contain more fibre - so vegans need to eat a greater volume in order to get calories. (This is a great thing - we get to eat more!!)
2. Eating highly processed vegan foods. There is nothing wrong with a faux beef burger, but not every day. Faux meat is generally made from soy and/or seitan - neither of which are particularly healthy or nutritious.
3. Not eating enough fresh fruits and vegetables. There's a reason the raw food crew are taking over the vegan world - because fresh, whole, raw fruits and vegetables are full of nutrition. Not just vitamins and minerals, but also photochemicals and a host of undiscovered compounds that are crucial to health. These chemicals are in abundance, and in the quantities that most benefit human health, in raw fruits and vegetables.
4. Ignoring other aspects of good health, including exercise, sunshine, sleep, hydration, stress management, etc. as much as we vegans enjoy talking about food, being a healthy vegan is SO much more than diet.
I also question the motives of the ex-vegan in adopting a plant-based diet in the first place. For many of us, being vegan goes beyond diet. Some people "go vegan" to lose weight. Others adopt a vegan diet as part of a cruelty- free lifestyle. What draws a person to a vegan diet may be very different to what keeps them there.
Ethically, we have no need to consume animal products. Factory farming is appalling, yes, but even so-called "humane" meat is no different. Ultimately, an animal's life has to be taken, and I'm just not ok with that. Watch the documentary "Earthlings" - it changed my view about the entire animal cruelty industry, from meat to fur to vivisection.
There is also the environmental aspect of Veganism. Raising animals for food is a highly inefficient method of food production. The amount of grain and water required for one cow could feed many more people than the cow itself. Also consider the environmental impact of methane emissions, destruction of rainforest to produce more land for crops or animals, and the huge monocultures of crops such as corn and soy.
Onto my second point. What if there is something physiologically different about the individual, such that he or she cannot survive on a vegan diet? I've personally never heard of it, but it is possible. There are rare metabolic disorders where individuals cannot break down certain types or sugars (or fats, or proteins) and therefore are required to remain on a strict regime where specific foods must be avoided. The average person is not likely to have this problem because most of these disorders are detected early in life and are often associated with other medical and developmental problems - and unlikely to be discovered simply after adopting a vegan diet.
The people who think that they are biologically different and therefore NEED to eat animal products? They are sadly mistaken, and are unaware of (or unwilling to accept) their mistake.
For most of us, we may have compromised digestion due to past abuses - for example, limiting our ability to produce it absorb vitamin B12. This is where supplementation is important (and, there are many types of vegan supplements available). There is no danger of deficiency for most vitamins and minerals in a vegan diet, provided that plenty of raw fruits and vegetables are consumed. It is prudent for us to carefully monitor vitamins D an B12, as we don't spend enough time in the sun (D) and adequate B12 relies on intact digestive processes, which, as mentioned earlier, may be compromised due to past abuses.
This brings up my final question: If a person has damaged their body (via past abuses) and is unable to thrive on a vegan diet, should animals have to suffer?