Talg, your concern is genuine and respected but the fact remains even if you tell these( muslim ) labourers to give up fasting for work, I am 100% sure they are not gonna give it up. The only good the wise beings can do is to be kind to them and provide a healthy environment for work during the period.
True islamic teachings prohibit a muslim putiing his life in danger or damaging his health. If a muslim is advised not to fast by doctor he should not fast otherwise he is committing sin.
It is advised to all muslims to keep a check on health and decide on fasting.