It's nutritional profile is significantly less than an average vegetable. I advice switching iceberg lettuce for darker leafy greens. Assuming you eat the lettuce in salads this can be done without much discomfort by slowing introducing darker greens in your salad, and adding more as your body is accustom to them.

You aren't hurting yourself by eating lettuce, but it's a missed opportunity to do so much more for your diet. See the difference for yourself.

https://www.nutritionvalue.org/Lettuce%2C_raw%2C_iceberg_%28...

https://www.nutritionvalue.org/Spinach%2C_raw_nutritional_va...

It's nutritionally equivalent to an unfiltered glass of water

It's not bad of itself but it's nutritionally deficient. My ex raised herps lizards and turtles. Both die of malnutrition if fed too much iceberg lettuce.

I think he's just referring to it having next to zero nutritional value.