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If an average human is 5'10" (1.8m), when scaled to be roughly 50 ft (15 m) how much food would it need to remain healthy?
Let's also assume there are three types of this giant; carnivorous, herbivorous and omnivorous. What would be the dietary requirements of each?
The giant is 8.6 times taller than the human. This means that your giant is roughly 630 times heavier than a human. The square cube law is your biggest problem here. Your giant is simply too big to hold his own weight up. Good news for you, your giant is magical and doesn't care about that particular piece of physics.
Fortunately, 630 times greater mass does not mean 630 times greater food. If this relationship were true, large animals would never be able to find enough food, and mice would eat a kernel of corn per week. The real relationship is less than linear.
Kleiber's Law states that the metabolism of a creature is proportional to its body weight to the 0.75 power. GM = C*GW0.75 In this GM and GW are giant metabolism and weight, and C is a constant number depending on the type of animal.
If we say that the average man weighs 80 kilograms and, requires 2000 calories per day we can write this as: $ 2000 \frac{calories}{day} = C*80 Kg$
We can also say that the giant weighs about 50400 Kg. We can solve both equations for C and equate them, and substitute in GW=50400.
$\frac{GM}{50400^{0.75}}=\frac{2000}{70^{0.75}}$
Solving for giant metabolism gives us 280000 calories per day, 138 times as much as is needed for an average human.
A pound of beef has roughly 1100 calories. It would take 250 pounds of beef per day to feed this giant. If the giant doesn't eat entrails and bones, that comes out to a whole cow every other day. A pound of grain comes out to roughly 1400 calories. It would eat 200 pounds of grain per day.
Note: I made edits to calories in grain. My original figures were incorrect.
Note that these are rough estimates and do not take into account SOOO many other factors. Wikipedia has a link to Kleiber's original chart. Kleiber didn't see any animals who have 10 times higher metabolism than his law predicted, but he did examine a few who were half or double his predictions.
In conclusion: An omnivorous giant would eat a quarter cow and 100 pounds of grain per day or twice that, or half that.
So, basing these calculations on the dietary guidelines released by the US department of health for 2015-2020, and the x630 mass factor given by @Nex Terren in their comment, these are the recommended daily allowances for a giant:
Calories: 1260000 - 1890000
Protein: 28980 - 32760 g
Carb: 81900 g
Fiber: 17640 - 19404 g
Linoleic acid: 7560 - 10080 g
Linolenic acid: 693 - 1008 g
Calcium: 630000 - 819000 mg
Iron : 6930 - 11340 mg
Magnesium: 195300 - 258300 mg
Phosphorus: 441000 - 787500 mg
Potassium: 2961000 mg
Sodium: 1449000 mg (This is an upper limit - not a recommended allowance)
Zinc: 5040 - 6930 mg
Copper: 560700 - 567000 mcg
Manganese : 1134 - 1386 mg
Selenium: 34650 mcg
Vitamin A: 441000 - 567000 mg
Vitamin E: 9450 mg
Vitamin C: 47250 mg
Thiamin: 693 - 756 mg
Riboflavin: 693 - 819 mg
Niacin: 8820 - 10080 mg
B6: 819 mg
B12: 1512 mcg
Choline: 267750 - 346500 g
Vitamin K: 47250 - 56700 mcg
Folate: 252000 mcg
So our giant will be eating a whole lot, to say the least. For reference, an entire cow, properly butchered, yields approximately 513,713 calories. That would be a suitable entrée for our giant, with a side salad of an entire lettuce patch.
A 5'10" man should weight 129-174 lb, and would require 2,433-2,854.5 calories per day. A 50 ft tall giant would weigh 81,236-109,574 lb and require 305,852.4-358,839.1 calories per day.
There are a few different animals that could provide enough calories.
hare - 3,852 roe deer - 13,200 ibex - 45,780 beaver - 48,000 reindeer - 60,000 humans - 81,500 deer - 119,040 musk-ox - 140,400 red deer - 163,680 horse - 200,100 boar - 324,000 cow - 367,200 bear - 600,000
A man for breakfast, a deer for lunch and a deer for dinner every day should do just fine.
For the herbivorous giant, it would have to eat a ridiculous amount of food to stay alive. apples - 520 per kg bananas - 890 per kg sweet potatoes - 1010 per kg brown rice - 1230 per kg avocado - 1600 per kg chickpeas - 1640 per kg bagels - 2500 per kg macadamia nuts -7180 per kg
So unless your giant is eating 500 pounds of avocados a day, it's going to have to be a carnivore, or at least an omnivore.
Let's use formulas that apply to normal humans and scale them up, this won't necessarily be 100% accurate, but neither are actual giants so let's go!
A fifty foot human would need to weigh between 9500 and 12750 lbs to be in a healthy range of BMI (18.5-24.9). That said, other ranges are certainly possible. To answer this question we will use multiple metabolic equations such as the harris-benedict equation and use activity multipliers that we use for real humans to come up with how many calories these giants need to sustain themselves each day.
So what do these numbers mean and what are the implications? These numbers represent your giant's total daily energy expenditure (or TDEE for short), this is how many calories your giants expend on a daily basis. Ranges were provided to suggest how individual differences are a thing, so a light male who is sedentary and burns 70k per day will likely continue to burn 70k per day. Being younger means energy expenditure is higher and being older means that energy expenditure is lower (values were calculated for 30 years of age). If a giant consumes less than his or her TDEE, that giant will lose weight. If a giant consumes more than his or her TDEE, that giant will gain weight.
This leads me to the implications. Your giants are likely going to be sedentary and light individuals unless food is simply incredibly abundant. Additionally, your giants are likely going to live in VERY small groups and be pretty territorial about land since getting sufficient food will likely be difficult. Your giants will likely be carnivores since vegetables tend to be light on calories. A grown cow weighing about 1000 lbs might provide about 650 lbs of meat which would give your giants somewhere in the range of 0.5-0.75 million calories. Enough to feed about 10 or so sedentary giants. Would they only want to eat meat though? Hard to say, it might not sustain them nutritionally and giants may die young due to nutritional deficiencies. Additionally because of the high energy costs of activity, giants would not go out of their way to be active.
Herbivorous giants are out of the question, but carnivorous and omnivorous giants are more plausible. Perhaps they operate as herders. Skyrim's concepts of giants keeping mammoths as company seems entirely logical from this perspective. I do not believe that they would do so well without a constant reliable food source however which circles back to them being territorial, not active unless the need arises, and not particularly large (more thin giants than anything else).
