Wait, wait, wait!! What's this thing about Ham gaining weight in his 30s? Did it really happen?
Yes, he really did gain some weight in the 1790s. He was always fairly slender, so I think it was a difference of maybe fifteen pounds, but it was enough to be noticeable–and to make international news. Angelica Church wrote a letter to Eliza Hamilton that she’d heard reports of Hamilton gaining weight in London:
Colonel Beckwith tells me that our dear Hamilton writes too much and takes no exercise, and grows too fat. I hate both the word and the thing, and I desire you will take care of his health and his good looks, why I should find him on my return a dull, heavy fellow! (Intimate Life, p. 73).
Angelica wasn’t the only one concerned about him during this period. Baron Von Steuben wrote to Hamilton on 16 December 1790 about his own concerns for Hamilton’s health. The letter is in french, but to paraphrase, he expresses concern for Hamilton’s recent indispositions, and is convinced that a lack of exercise is the cause. He recommends Hamilton ride eight or ten miles every morning, as his daily walks aren’t enough, and a diet of few meats and copious root vegetables. Enclosing a copy of Clément Joseph Tissot’s pamphlet from which he’d derived the diet, Von Steuben entrusted it’s execution to Eliza.
Hamilton also corresponded with Henry Lee in 1791 regarding a horse to help him get out and exercise more. He’d requested a gentle horse, though, and Lee took pains to find one that would meet with Hamilton’s needs. On 12 August 1791, Lee wrote to Hamilton:
Mr Cox1 was about taking to you my riding horse, but my apprehension of yr. necessary hurry & my wish to compare him with a horse I have sent for, concluded a procrastination of my execution of your request & my ardent desire.2 No other consideration could have induced me to postpone a measure you reckon essential to your health. Nor shall time be lost in presenting you with this trivial testimony of the zeal with which I engage in any matter which goes to your comfort.
The change was visible even in art from this period. In the 1792 portrait by John Trumbull, for example, you can see the hint of extra weight in his face: