fit1 <- lm(bmd ~ weight + age, data=mydf)
mydf[,"a"] <-fit1$fitted.values

fit1 <- lm(bp ~ weight + age, data=mydf)
    mydf[,"bp"] <-fit1$fitted.values

fit1 <- lm(choles ~ weight + age, data=mydf)
    mydf[,"choles"] <-fit1$fitted.values

bmd  bp   choles    weight  age
1    2    3         22.3     12
2    1    2         33.2     13
3    2    5         44.5     16

apply(mydf[ ,-c(4:5)], 2, function(x) lm(x ~ mydf$weight + mydf$age)$fitted.values)

# get names of all dependent variables
dependents <- names(df[, -which(names(df) %in% c("weight", "age"))]

# build a little function
myFit <- function(depName) {
  fit1 <- lm(as.formula(paste(depName, "~ weight + age")), data=mydf)
  return(fit1$fitted.values)
}

# sapply them
fittedValues <- sapply(dependents, myFit)