Let’s look at this question mathematically. While it’s easy to vilify McDonald’s by employing hyperbole, such as “It’s not real food” or “That stuff will kill you,” in the end, I think the numbers will provide the answer you need. So let’s break this question down into its two component parts:
What will happen if you eat nothing? At first, zero. After a few days of not eating, you will start to lose weight. (That may be good or bad.) After anywhere from a week to three weeks, your body systems will start shutting down or at least failing to perform their tasks properly. You will also feel very weak and have very little energy. And depending on a wide variety of factors, including your starting weight, your overall health, and your body fat, you will die somewhere between one and 10 weeks after you starting eating nothing.
What will happen if you eat McDonald’s? Just like if you eat nothing, the first day or so will be unremarkable. After a few days of eating McDonald’s, you’re still going to be fine, because you’re eating food. Maybe you’ll burp if you eat really fast. After one to 12 weeks, what will happen? Nothing, because you’re eating food, which is a necessity for survival and nutrition. At this point, it’s probably worth lengthening the time periods that we’re checking in with you. After a year of eating assorted McDonald’s food, you’ll in all likelihood still be fine. Just try not to get hit by a bus. Now it becomes a matter of what the most likely thing is to kill you first, as well as your relative health, and your age when you started this silly experiment. But assuming you are relatively healthy in other regards to start with, you will in all likelihood live until you are 70 to 90 years old, barring any unforeseen accident, because this is how long people tend to live. So basically, assuming you started this in your 20s, you will probably die in 50 to 70 years, just as you would anyway.
The thing about dietary factors and their impact on health is that they are statistical. Even if a general dietary pattern is thought to shorten life expectancy, that doesn’t mean it will for an individual, nor does it mean that the difference will be significant. There are millions of people who eat much more than the recommended amount of saturated fat and sugar and still live way beyond the statistical mean. There are also millions of people who eat lean, healthy diets and die early of various causes, from cancer to heart disease, and countless others. So even if a particular diet were found to be dangerously unhealthy and reduce life expectancy by a few percent (which is not fully established yet), you’d still in all likelihood be living till approximately the same normal age range. While we all like the idea that we can control our health outcomes, the vast majority of important health factors are genetic. We may improve our statistical likelihood slightly through diet optimization, portion control, exercise, and other factors, but it’s really a blip on the graph for any particular user, and also relates to the health of the person’s overall lifestyle. On the other hand, the outcome for people who eat nothing is remarkably consistent and well established: rapid death.
Furthermore, it’s worth noting that McDonald’s is not a single food—it’s a whole restaurant with more than 100 items on its menu, each of which has a different nutritional profile. McDonald’s doesn’t serve some evil substance from outer space (or inner chemistry lab). It serves food. Those food items vary from having just 15 calories and 0 grams of fat (side salad or apple slices) to 780 calories and 45 grams of fat (double quarter pounder with cheese), with most others somewhere in between. Some items are heavily processed, and others are not. And there is a wide enough variety that you could meet pretty much all of your body’s needs with their offerings, with a bit of forethought and planning. So just like eating anywhere else in the world, if you make decent choices and consume as varied a diet as possible, eating McDonald’s would be very unlikely to have any impact on your life expectancy or health at all.
In anticipation of people citing Supersize Me as evidence that McDonald’s can have a rapid and deleterious effect on your health, it should be noted that Morgan’s health effects would be totally attributable to a precipitous increase in overall caloric intake and saturated fat, both of which were voluntary and totally avoidable even if eating only at McDonald’s. This doesn’t even address the movie’s profoundly unscientific approach, and lack of blinding, sample size, control, etc. There is also a healthy dose of the “naturalistic fallacy” at work. If you were to suddenly start consuming exclusively cheeseburgers and fries by cooking them at home using the finest grass-fed free-range beef and hand cut organic potatoes, your health outcome would be no better.