Hi moonbeam
Obviously I don't know the source of your information, so it's difficult to comment - unless it's from a published scientific paper with references then I'd be a bit sceptical. Also what is classed as "lower oestrogen levels"? I understand that oestrogen in HRT is at a much lower level than the oestrogen we produced naturally when younger. So if this theory about oestrogen and cortisol levels is correct then wouldn't every single middle aged woman on the planet suffer with heartburn/digestive problems? - but I don't think that's the case. Oestrogen is just one factor amongst many others.
I think both oestrogen and progesterone can affect digestion but progesterone is the main culprit in relaxing muscles which in turn can make heartburn worse. (However I think you said in a previous thread that you had a hysterectomy so I assume you are not taking progesterone). Many GPs including mine, seem to discredit the link between heartburn and HRT......not sure myself, although I have acid reflux issues from time to time and take oral HRT without any problems (i.e. the reflux is no worse or better than before I started taking HRT).
I know there are many factors that can cause heartburn such as increasing age, being overweight, sedentary lifestyle, anxiety, dietary issues, hiatus hernia...so it's impossible to say what your individual triggers may be - would need an expert in gastrointestinal medicine to do that I think...... but I'm not sure that oestrogen would be fully to "blame".....sorry if that's not the reassurance you are looking for....ER.