Green leaf lettuce is a mild flavoured leaf vegetable that is often used in sandwiches and salads. It’s leaves are delicate and tender unlike the leaves on iceberg and romaine lettuces which are crisper.
So can guinea pigs green leaf lettuce and if they can how much of it can they eat?
Lets take a look at their nutritional data and find out more.

In particular, their acidic, fat, calcium, sugar, and phosphourus content is especially pertinent to guinea pigs.

Read More http://nutritiondata.self.com/facts/vegetables-and-vegetable-products/2477/2#ixzz2spLaXXUS
As you can see, green leaf lettuce contains a hint of phosphorus, a little calcium and a little acid. But it does contain a good deal of vitamin c which is fantastic for guinea pigs.
With that in mind guinea pigs can eat green leaf lettuce almost on a daily basis as they contain little bad stuff for piggies. Five times a week as standard would suffice, but because of its acidic and phosphorus content, not every day and it does them good to have a break from it.