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I have read that rabbits should not be given iceberg lettuce. Why is that?

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In short, iceberg lettuce is mostly water with little to no nutrient value. A rabbit can fill up on it (because of the bulk), but still need food because they didn't get enough calories, vitamins, minerals, etc.

If you look at the USDA's webpage on iceberg lettuce. You'll notice that water makes up just about 95% of the nutrients, leaving 5% to be divided up between every other nutrient. Protein is a little less than 1%, Carbohydrates less than 3%, and sugars less than 2%.

Most importantly, you want to pay attention to the vitamin and calorie content. Iceberg lettuce has only 14 calories per 100 grams (2 calories or less in each leaf), and 18mg of calcium.

Other effects to consider, is when fed straight from the fridge, lettuce will be cold and wet, both are conditions that can upset a pet's stomach.

In addition, since most rabbit breeders do not feed lettuce to their baby rabbits, it is an unfamiliar food and if the rabbit owner does not add lettuce to the diet gradually, it will cause stomach upset.

Finally, it is often believed amoung rabbit breeders that

Rabbits should not eat some lettuces (such as iceberg) as they contain a substance called laudanum which can be harmful in large quantities.

RSPCA: Rabbit Diet Myths

But this is disputed because

Lettuce does not contain laudanum. Edible lettuce does contain a chemical called lactucin which is a component of lactucarian. Lactucarian is the milky fluid found in some species of lettuce and occurs in much larger amounts in wild lettuce, Lactuca virosa.

Lactucarium has sedative and analgesic effects. It creates a sense of mild euphoria. The milky exudate resembles that of opium and it can also be reduced to a thick substance which can be smoked in the same way as opium. As a result of which it acquired the name ‘lettuce opium’.

Edible lettuce, Lactuca sativa, does not even contain lactucarium although it does contain lactucin which is in lactucarium.

Lettuce for Rabbits and Guinea Pigs (note that rabbits and guinea pigs have different digestive systems, this link is included for the information on lettuce composition).

It is not known if lactucin turns into lactucarium during the digestive process in the rabbit, there are some accounts of rabbits appearing sedated after ingesting large quantities of lettuce. Regardless of the laudanum/lactucin status, there's no reason to feed iceberg because of the low nutrient value.

The House Rabbit Society's website has even more details on what foods are good for rabbits and why.

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Zaralynda has a fantastic answer, that runs counter to my in belief the "badness" of iceberg lettuce for bunnies. In an attempt to support my beliefs I did lots of research, the following is what I learned.

We all agree that Iceberg Lettuce is not a good source of nutrition, but is it actually bad for rabbits?

There are several mentions online and in print of iceberg lettuce, containing laudanum and being related to Opium producing plant Papaver somniferum, in fact iceberg is no closer to this than any other lettuce Lactuca sativa. Plants are scientifically classified in the Kingdom Plantae (this is the division between plants and animals). The nearest classification they have in common is Eudicots which includes many flowering plants including apples and maple trees. This pretty much destroys the relationship myth.

So where did the myth come from? It seems that many plants including lettuce have a bitter latex like substance that has been used for a substitute in producing the soporific effects of opium Flora of Great Britain and Ireland, Volume 4 By Peter Sell, Gina Murrell, page 117. Lettuce appears to have been a popular source since the time of the anciet Egyptian, this was generally collected when the plant had bolted (flowered). The drug is called Lactucarium, it does not appear to hold it's effects well in commercial productions, but does appear to be less disruptive to the digestive system then opium.

I did not find reliable references on the bloat and gas concerns related to iceberg lettuce

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