Although there are packages that can calculate confidence intervals, the standard installation of R does not.  However they are easily calculated using your own code. For example:

CI <- 0.975 #95% CI

mean <- mean(data$bmi, na.rm=T)

stdev <- sd(data$bmi, na.rm=T)

n <- length(data$bmi)

error <- qt(0.975,df=n-1)*stdev/sqrt(n)

lowerCI <- mean-error

upperCI <- mean+error

mean

stdev

lowerCI

upperCI