Now, why wouldn't this undermine an objective morality? Well think of how we talk about food: I would never be tempted to argue to you that there must be one right food to eat. There is clearly a range of materials that constitute healthy food. But there is nevertheless a clear distinction between food and poison. The fact that there are many right answers to the question, "What is food?" does not tempt us to say that there are no truths to be known about human nutrition. Many people worry that that a universal morality would require moral precepts that admit of no exceptions.
I don't want to nitpick but i think this is a terrible example.
There is no very clear distinction between food and poison.
They are not even opposed.
There is poisonous stuff (non lethal tho) that is food and there is non-poisonous stuff that is not food.
But think of this:
Most substances have an ammount at which it becomes poisonous to our body.
You can O.D. on water, for Petes sake.
Yes, you can take in so much water that your body stops functioning properly (poisoning) and you die.
And that goes for most substances.
So besides the chemical make up of the ingested stuff you will need to know the ammount to figure out how nutricious and/or poisonous it was.
Part of the reason people become increasingly more broken with age is because processing food by the body is breaking the machinery that does it.
Meanwhile radiation from space and the earth (and all manmade radiating things) break the body even more.
It's not a pretty picture, but it is how it is and we have to deal with it.
Ask your doctor if you don't believe me.
Here is the wikipedia article that touches upon the subject with a nice list of more or less common stuff and it's lethality:
http://en.wikipedia.org/wiki/Median_lethal_dose