How true is it? I don't know. But even then I don't see why they couldn't take multivitamins.
Practically pure proteins can be provided by turkey breast or chicken breast, adding e.g. around 300 kcal for an adequate daily protein intake, and the rest of the food can be composed of non-starchy non-sweet non-oily vegetables, which provide only about 20 to 50 kcal per 100 grams. One could eat more than 1 kg of various such vegetables without exceeding 800 kcal.
There remain a few things that would have to be taken from supplements, e.g. omega-3 fatty acids, vitamin B12, vitamin D3 and vitamin E (which is abundant only in high-caloric food, like olive oil or almonds). The simplest is just to take one pill per day of those offered by various vendors, which include all the essential vitamins and minerals, to be sure that none of them is insufficient.
A little vegetable oil would be needed for linoleic acid.
It's probably a mistake to connect the two.
Yes, we Americans overeat beef / Hamburgers, but I don't think anyone eating burgers is at huge risk of malnutrition... as much as a risk of heart problems, diabetes, cholesterol, etc. etc.
It turns out that a standard burger with beef, ketchup, lettuce, onion, and bread covers most of your macro, and even micro-nutrients. It also is way overboard on salt and fats, but... that's not the particular problem here.
One of the bigger problems vegetarians have is finding a proper replacement of meat and all of the easy nutrients meat contains. All amino acids, various vitamins, iron, etc. etc. There's a fair amount of nutrients here.
And even the worst McDonalds burger contains a variety of lettuce / onions / ketchup that covers few nutrients that beef is missing (ex: Ketchup has Vitamin A, Potatoes / Fries has Vitamin C).
The amount of lettuce, tomatoes and onions is so minimal I doubt it contributes to your vitamin intake in any useful manner. If you eat a salad you get 50x that amount of those vitamins.
Pretty strongly disagree here. US Beef consumption is about the same as in early 1900's, which was a time where personal health in several metrics was far better. (Weight being the obvious one)
Always very skeptical when people talk about cholesterol, it's a really nuanced topic, but the average person seems to be stuck in the 1970's with "cholesterol bad". Thinking high ldl is always bad is only slightly better.
If you eat only the beef and veggies from a Big Mac three times a day you'll get plenty of vitamins and minerals with minimal calories (~650 calories)
If you eat the three entire Big Mac meals you won't get any more essential nutrients to speak of and you'll consume far too many calories (3960 calories)
The average american has junk to cut from their diets with no nutritional loss whatsoever. Typically fried and sugary foods.