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Is it theoretically possible to eat fast food from McDonald's or various other fast food restaurants every single day and still maintain or even lose weight.

What I am saying is, if you eat fast food everyday say at lunch time and if you count your calories every day, made sure you were not going over your calorie limit by eating something small for breakfast and dinner, like oats in the morning, and getting your servings of fruit and vegetables for dinner, and get regular exercise, going to the gym, not sitting down for too much, drinking plenty of water. Will you still get fat?

My thinking is from what I know is weight lost is dependent on how many calories you put into yourselves and how many you burn off. So if your energy input is high and output is low, then you gain weight and vise versa for losing weight, regardless of carbs/protein/fat content, although fat contains higher energy per gram compared to carbs and protein so it is easier to go over your limit by eating fat.

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    Super Size Me - a documentary, where somebody tried eating only in McDonald's for 30 days. – Martin Jun 18 '15 at 10:11
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    mcdonalds isn't a food, it is a restaurant that has a menu that varies based on location and time of day. That said I remember a news story about a man who ate a Big Mac every day for 30 years; he wasn't obese. – jiggunjer Jun 18 '15 at 18:36
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    Here they sell chicken soup wtih pasta and vegetables. The coffee is ok too. – jiggunjer Jun 18 '15 at 18:40
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    Super size me is irrelevant to the question. It was undertaken by somebody who, on purpose, restricted his diet to the menu sizes and upsell suggestions created by McDonald's. Here the question is about a very different thing - a calorie restricted diet which includes food bought at McDonald's. – rumtscho Jun 20 '15 at 7:56
  • Diet is a matter greater than weight maintenance. Eating fast food daily is one of the worst things you can do to your health. That was my experience and that's what nutritionists say, they advise eating it once a year at most. Nutrition is key to your health, and fast food has very little nutritional value, so it's an inefficient use of caloric intake. Not to mention the effects it has on your arteries, bowels, plus all the chemical additives, etc. I wouldn't make weight the most important factor in determining a proper diet. – sss4r Jun 21 '15 at 13:15
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I have eaten McDonalds for lunch every (week) day for the past 2+ years, I can tell you it has nothing to do with weight gain or loss.

For the vast majority of people, losing and gaining weight is all about calories; nothing else. Genetics plays a role, but it is insignificant in the grand scheme of things.

There are many problems with the quality of food from McDonalds, but I will focus on answering your question within regards to weight gain/loss only.

The foods at McDonalds are very calorie-dense, and non-satiating. One big mac has ~563 calories... add on the large fires (~480) and large coke (~310) with that and it equals over 1300 calories.

1300+ calories is an INSANE amount for one meal, which won't even keep you satiated (full) for very long.

Therefore, it's really a poor choice when it comes to weight loss.. because if you are trying to lose weight, your daily caloric intake wouldn't be too much higher than that (unless you're a bodybuilder or athlete). Some short women wouldn't even have 1300 calories total in their daily intake... that's how much calories that is.

That being said, if you're on some kind of diet such as intermittent fasting, and you don't eat much else other than that single meal a day.. you can still successfully lose weight even if you eat this meal each day.

As long as the calories you consume each day is lower than your TDEE (Total Daily Energy Expenditure), you will lose weight.

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While it seems logical that you only need to consider the deficit or surplus on the energy balance to see if you'll gain or lose weight, this is not going to work because the body will regulate the metabolic rate to keep a certain amount of energy reserves. How much fat reserves your body decides to keep will depend on your physical fitness and physical activity levels, the intake of minerals that are essential for maintaining physical fitness, how much sleep you get etc. etc. In general, when you live an unhealthy lifestyle, your body's regulation of its metabolic rate will tend to lead to larger fat reserves.

From a theoretical point of view, this is quite easy to understand. Whatever the precise biochemical mechanisms are that are involved in regulating metabolism (not everything is known), it remains the case that all these mechanisms have evolved in order to maximize survival probability of animals in Nature who obviously don't do calorie counting.

One of the problems evolution had to solve was how to make sure you don't starve to death due to a small shortage in the energy balance that you cannot make up for. Suppose you eat one sandwich a day worth 100 Kcal a day less and walk a bit more so that you expend 100 Kcal more per day. While this could lead to some weight loss, it cannot be the case that you'll continue to lose weight without limit. However, naive the calorie counting hypothesis suggests that a 200 Kcal deficit per day would lead to a long term weight loss trend of 1 kg of fat per 40 days. So, in a little over 2 years you would lose 20 kg of weight, which is clearly nonsense.

Animals living in the wild may find themselves having to deal with a bit less food that is also a bit harder to find. If they were to lose weight because the metabolic rate is cannot be actively regulated, it would only increase due to physical exertion, the animal would be doomed. This doesn't make sense for warm blooded animals that we know have mechanisms to regulate the metabolic rate, and who have metabolic rates that are ten times higher than what they need to just barely survive.

Instead, it makes far more sense to make the metabolic rate dependent on the degree to which the fat cells are filled. So, if there is a shortage on the energy balance, the animal will initially lose weight, but then the metabolic rate will be down regulated, correcting the energy balance, a slight surplus will be created, allowing the fat cells to be filled.

While the biochemical mechanisms the body uses for this are not well understood, but recently it has been found that fat cells produce leptin, the more filled a fat cell is the more leptin is produced and besides regulating the appetite, leptin will let the hypothalamus produce more TRH, and TRH will let the pituitary gland produce more TSH and TSH will let the thyroid gland produce more thyroid hormone.

Then the body will likely also make the set point for the fat reserves dependent on factors such as the amount of food intake, stress levels, sleep etc. The whole point of the fat reserves is to maximize survival probability, so the probability of a food emergency, the time it can survive without food etc. will all influence the set point for its fat reserves. It then makes sense that the outcome of evolution would be that the set point would be set higher when the animal has less to eat, has more stress doesn't get the optimal; amount of sleep. In that case, a food emergency is more likely and when it happens it is less likely to survive on some given amount of fat reserves. So, the smart thing to do is to save more energy under these circumstances.

In contrast, when you sleep better, eat more and exercise more, the body will think that the prospects of a food emergency are smaller, and if that were to happen you would be in stronger position to take measures to reverse the situation. So, you'll not keep as much fat reserves, because doing so does come at the cost of having to carry all that fat ballast with you all the time.

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It is theoretically possible. However, if your goal is to lose or even maintain weight, eating fast food every day will make something already difficult even more difficult. Calories are not the whole picture. Achieving lasting weight loss requires changing one's relationship to food.

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    This is more of an opinion than an answer that meets health SE standards and requirements for references. It may be a correct opinion and I see that you wanted to make a point, but a slightly more detailed explanation with evidence would make a more suitable answer for this site :-) – Lucky Sep 25 '15 at 21:45
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Yes, it is possible. If you eat 0.0001 grams of McDonald's fast food every day, and nothing else, you will lose weight. Guaranteed.

The point of this rather silly answer is that it's the quantity of calories you eat (and burn), not anything magic about the source of the calories.

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    This is more of an opinion than an answer that meets health SE standards and requirements for references. It may be a correct opinion and I see that you wanted to make a point, but a slightly more detailed explanation with evidence would make a more suitable answer for this site :-). – Lucky Sep 25 '15 at 21:45
  • It's not an opinion. It is a reductio ad absurdum, a 2000+ year old technique of natural philosophy. – Iron Pillow Oct 2 '15 at 4:47
  • There's nothing wrong with the logic of this answer, but it is rather imprecise. " [...] if you eat fast food everyday say at lunch time" was a part of the question - so this answer doesn't really address the main concern here and it basically says the same thing as JohnP's answer in a different way... This answer would be great for a different stackexchange site (IMO) but health SE does have strict policies on corroborating your answer with proven facts. – Lucky Oct 2 '15 at 5:54

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