As I was stuffing my face today, I wondered if the Universe cared. The short answer is no. The slightly longer and more depressing answer is: my existence is more marginal than a speck of stray DNA on a grain of sand staring at vast oceans (that's literally true, oh the irony...). Clearly, there's no point to existence except amusement. So, here's some:
On average, each of us human beings from birth till death consume about (2000 per day x 365 days x 70 years) calories. That is a pretty big number (51,100,000 calories).Big, of course, is a relative term. The big calories translates to about 0.00002 milligrams of matter. In the scheme of things--compared to, say, the amount of matter Sun converts to pure energy per second--, the amout of matter we manage to process in 70 years is stupefyingly underwhelming. Sun converts about 4,000,000,000 kilograms of mass to pure energy every second compared to our biological knickers-in-knots process*. Still, we are here and we can point a resounding finger at the Sun. That's quite something, isn't it? Life is an extraordinarily strange and fragile business whichever way you look at it (the strangeness includes the looking-at-it part too). Perhaps, in a thousand years, we may climb up the energy ladder, sit alongside stars and have a proper material breakfast of a few hundred tons of hydrogen. It would be way more amusing than what we do with the less-than-nothing we consume today. Of course, we've got to survive to do that.
*The comparison is sort of fudged. Sun does atom crushing, we don't do that. Sun literally converts the mass to energy. OTOH, we do a lot of very very minute electrochemical energy extraction. The comparison aims to show the scale of energies involved, which differ by orders of magnitude. Physics savvy readers please pitch in and clarify my muddle if needed.
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I care.
I care too.
Not me.
While we don't engage in fusion, the energy released from metabolizing food ultimately comes from mass. It's just that the mass difference in a chemical reaction is so small as to be unnoticeable/unmeasureable.
e.g. Burning 12 g of carbon with 32 g of oxygen produces 44 g of CO2. The energy released is about 400 kJ. Using E = mc2, this is equivalent to about 4 billionths of a gram.
Your calculation is off by a factor of 1000. You need to use kilocalories (aka Calories / dietary calories), so your result should be 2.4 mg relativistic mass.
A much more intuitive way to do this calculation is to convert to Watts. 2000 Calories/day converts to 100 W.
If you have incandescent light bulbs in your home you probably have 65 W lights above your head. Leave two lights on and you are using more energy to light the room than it takes to power your body.
A desktop computer uses up ~400 W (not counting the monitor). Your computer is using 4 times the amount of energy it takes to run your body.