If and why is McDonald's food considered extremely unhealthy?
"Extremely unhealthy" is what some people say for many foods, but to say that fast food is "considered unhealthy," more evidence is needed. I have no evidence to say that McDonalds is better or worse than others in this regard.
In the experiment about eating in McDonalds shown in the documentary Super Size Me and described in this Wikipedia article, the guy claims he was consuming 5,000 Calories per day:
Spurlock ate at McDonald's restaurants three times per day, eating
every item on the chain's menu at least once. Spurlock consumed an
average of 20.9 megajoules or 5,000 kcal (the equivalent of 9.26 Big
Macs) per day during the experiment. An intake of around 2,500 kcal
within a healthy balanced diet is more generally recommended for a man
to maintain his weight. As a result, the then-32-year-old Spurlock
gained 11.1 kilograms (24 lb), a 13% body mass increase, increased his cholesterol to 230 mg/dL (6.0 mmol/L), and experienced mood
swings, sexual dysfunction, and fat accumulation in his liver.
So, if he had a severely hypercaloric diet for a month, this alone (and not where and which exact foods he was eating) could result in weight gain and consequently in increased cholesterol (CDC). Any double calorie diet at any other restaurant or at home could do the same. So, this experiment does not provide any convincing evidence to say that McD food is unhealthy.
In another experiment, another guy was on a hypocaloric diet by eating carefully at McDonalds for 180 days - he lost 13.6 kg (30 pounds) and his cholesterol levels dropped significantly.
Is there any other evidence to say that fast food is unhealthy?
A typical food pattern in fast food restaurants includes hamburgers and sandwiches (white bread, processed beef or bacon, cheese), chicken nuggets and French fries (with fried oils), salad with dressing (oils), apple pies, chocolate cookies, ice cream and sweetened beverages (sugar) (McD menu). Such food pattern tends to:
- have little fiber, so it can be constipating
- be energy dense and can be, in association with lack of fiber, less satiating, which can make you eat more than you need and gain excessive weight
- have a lot of quickly absorbed carbohydrates (plain starch in white bread and fries, sugar), which is, along with excessive weight, a risk factor for diabetes type 2
- have an excessive amount of trans fats (in French fries, chicken nuggets, pies, biscuits - USDA, Foodwatch), which can increase LDL cholesterol, which can be a risk factor for cardiovascular disease
- contain a lot of preservatives and other food additives, which spoil the taste
To be fair, in many fast food restaurants, including McDs, you can also buy an apple, a salad without dressing, a veggie burger and mineral water.
Are sandwiches themselves considered the worst kinds of junk food and are the those from McD significantly worse than sandwiches in general?
Sandwiches typically contain processed meat. High consumption of processed meat is associated with increased risk of cardiovascular disease and diabetes type 2. I don't have any evidence to say that sandwiches are the worst food and which brand is worse, though.
In conclusion, there is evidence that food patterns (not the food items themselves) promoted by fast food restaurants are associated with increased health risks. The risk increases with frequent eating energy dense, low-fiber foods.