Well, it appears, it's what the Scot hasn't eaten which might have been the key to the cramping episode which almost ended his US Open at the outset.
Certainly, in much cooler conditions on Thursday night, the British No.1 showed no ill effects of that almost catastrophic cramp- filled Monday as he put German Matthias Bachinger to the sword with an ruthless display to set up a meeting today with the dangerous and in-form Russian Andrey Kuznetsov, the world No.93.
Yet, meticulous fitness addict that he is, Murray and his team have been looking deep into what made such a finely-tuned athlete, who purposely spends large amounts of his training blocks in the humid heat of Miami to ensure there are such no concerns, cramp up so badly.
Their discoveries were intriguing - and salty - as were the 27 year-old's revelations that he is constantly weighing himself to make sure his problems are being kept in check.
"I have changed a few things, not so much what I'm eating but what I'm drinking," Murray admitted.
"I'm making sure I have enough fluids down me and take a bit more salt to see if that helps.
"Not salt tablets but there are little one gram sachets of salt in the restaurant I take. The conditions are tough and you need to make sure you are on top of everything before matches so you don't get caught out.
"I don't weigh my food but I weigh myself three times a day - when I get up, normally after practice and before I go to bed. I know exactly what my weight should be in the morning and in the evening. There were no fluctuations earlier in the week in terms of my weight.
"I had drunk enough in terms of quality but maybe something was missing. I will make sure I am on top of it for the rest of the tournament."
Murray will certainly need to have all facets of his game working against Kuznetsov who, in defeating David Ferrer at Wimbledon earlier this summer before dispatching of No.31 seed Fernando Verdasco in five sets this week, has certainly displayed the ability which saw him win the junior title at SW19 in 2009.
"I saw a bit of that match. Those sorts of results happen. That's why you don't take anyone for granted," added Murray. "You would say it is a surprising result but the reality is that will happen at most tournaments. Upsets do happen in all big sporting competitions.
"He played a very aggressive match and went for it. Ferrer was a bit off and that's what can happen. He is lasting well in the five-set matches because Verdasco is a pretty fit guy and matches with him are always physical."
Kuznetsov, although not necessarily owning the kind of firepower which could knock Murray off path, is considered a fine player who is not afraid to counter an opponent with the kind of aggressive, exciting shot-making which was too hot to handle for Verdasco on Thursday evening.
For someone coached by his own father, the 23 year-old cuts a very relaxed figure. "I am ready for this match," he said. "[Murray] is a grand slam champion but for me it's no pressure. I'm sure I will have some chances. To be honest, it seems like a destiny. Second time, very similar situation. First round I beat a local guy and then second round I play with Spanish guy and beat him in five sets. Two slams same situation.
"Before the match with Verdasco my girlfriend told me it's probably going to be like a destiny and you should win in five sets so … I did not think about this during the match but it happened like this."
Even if Kuznetsov comes at Murray hard, the Scot showed against Bachinger that his movement and shot making are no approaching their best.
His serve, too, looked to be back firing. The Scot won over 80% of points on his first serve, a vast improvement of his first round performance against Haase which, interestingly, was not played in anything like the windy conditions encountered on the Arthur Ashe stadium on Thursday. Today's match, though, will be back on Louis Armstrong court in the stifling heat of afternoon, and Kuznetsov aims to take advantage.
"I'll try to play aggressive. That's the way I try to play every game. I will try to show my best tennis, my aggressive tennis and I hope it will help me to get chances and to use them," added Kuznetsov.
"It has been the best year for me, the most successful. It's good when you pass two rounds in two grand slams in a row. Your level is probably rising up so I'm happy about it."
The problem for the Russian is that Murray's game is hotting up too. He surely won't be rubbing salt in the Scot's healing wound.