Quite similar to romaine lettuce, red leaf lettuce has leaves that are coloured red and are popular in salads as well as in other foods.
So can guinea pigs eat red leaf lettuce and if they can how much of it can be eaten?
Lets take a look at its nutritional data and find out a bit more about it.
In particular, its calcium, sugar, fat, phosphorus and acidic are most of interest as far as guinea pigs are concerned.
Red leaf lettuce, raw
Nutritional value per 100 g (3.5 oz)
Energyt68 kJ (16 kcal)
Carbohydratest2.26 g
– Sugarst0.48 g
– Dietary fibert0.9 g
Fatt0.22 g
Proteint1.33 g
Vitamin A equiv.t375 μg (47%)
– beta-carotenet4495 μg (42%)
– lutein and zeaxanthint1724 μg
Thiamine (vit. B1)t0.064 mg (6%)
Riboflavin (vit. B2)t0.077 mg (6%)
Niacin (vit. B3)t0.321 mg (2%)
Pantothenic acid (B5)t0.144 mg (3%)
Vitamin B6t0.1 mg (8%)
Folate (vit. B9)t36 μg (9%)
Vitamin Ct3.7 mg (4%)
Vitamin Et0.15 mg (1%)
Vitamin Kt140.3 μg (134%)
Calciumt33 mg (3%)
Iront1.2 mg (9%)
Magnesiumt12 mg (3%)
Manganeset0.203 mg (10%)
Phosphorust28 mg (4%)
Potassiumt187 mg (4%)
Sodiumt25 mg (2%)
Zinct0.2 mg (2%)

food info and image (source: Wikipedia)
As you can see red leaf lettuce contains a little phosphorus and calcium, and is a little acidic, it also contains a hint of fat and sugar,
With that in mind, guinea pigs can eat red leaf lettuce but only two to three times a week because of its acidic content.