Editing Essential amino acid (section)

==Amino acid requirements and the amino acid content of food==
Historically, amino acid requirements were determined by calculating the balance between dietary [[nitrogen]] intake and nitrogen excreted in the liquid and solid wastes, because proteins represent the largest nitrogen content in a body. A positive balance occurs when more nitrogen is consumed than is excreted, which indicates that some of the nitrogen is being used by the body to build proteins. A negative nitrogen balance occurs when more nitrogen is excreted than is consumed, which indicates that there is insufficient intake for the body to maintain its health. Graduate students at the [[University of Illinois]] were fed an artificial diet so that there was a slightly positive nitrogen balance. Then one amino acid was omitted and the nitrogen balance recorded. If a positive balance continued, then that amino acid was deemed not essential. If a negative balance occurred, then that amino acid was slowly restored until a slightly positive nitrogen balance stabilized and the minimum amount recorded.<ref name=RoseIsoleucine/><ref name=McGilvery1979>{{cite book |title=Biochemistry, a Functional Approach |edition=2nd |isbn=0-7216-5912-8 |last1=McGilvery |first1=Robert W. Ph.D. |collaboration=Gerald Goldstein M.D. |date=1979 |orig-date=1970 |publisher=W. B. Saunders Company |chapter=Chapter 41 Nutrition: The Nitrogen Economy |pages=785–796}}</ref>

A similar method was used to determine the protein content of foods. Test subjects were fed a diet containing no protein and the nitrogen losses recorded. During the first week or more there is a rapid loss of [[lability|labile]] proteins. Once the nitrogen losses stabilize, this baseline is determined to be the minimum required for maintenance. Then the test subjects were fed a measured amount of the food being tested. The difference between the nitrogen in that food and the nitrogen losses above baseline was the amount the body retained to rebuild proteins. The amount of nitrogen retained divided by the total nitrogen intake is called [[net protein utilization]]. The amount of nitrogen retained divided by the (nitrogen intake minus nitrogen loss above baseline) is called [[biological value]] and is usually given as a percentage.<ref name=McGilvery1979/>

Modern techniques make use of [[ion exchange chromatography]] to determine the actual amino acid content of foods. The [[USDA]] used this technique in their own labs to determine the content of 7793 foods across 28 categories. The USDA published the final database in 2018 to the public.<ref name="USDAFoodData">{{cite web |title=FoodData Central Standard Reference (SR) Legacy Foods | url=https://fdc.nal.usda.gov/ |publisher=U.S. Department of Agriculture, Agricultural Research Service |date=April 2019}}</ref>

The limiting amino acid depends on the human requirements and there are currently two sets of human requirements from authoritative sources: one published by [[WHO]]<ref name="WHO"/> and the other published by [[USDA]].<ref name="DRItext"/>

{| class=wikitable style=text-align:right
|+ This table displays the number of Items in each Category with the same limiting Essential Amino Acid <ref name="WHO"/><ref name="DRItext"/><ref name="USDAFoodData"/>
!
! colspan=9 | Based on WHO Requirements
!
! colspan=9 | Based on USDA Requirements
|-
! Category
! {{vertical header|Tryptophan}}
! {{vertical header|Threonine}}
! {{vertical header|Isoleucine}}
! {{vertical header|Leucine}}
! {{vertical header|Lysine}}
! {{vertical header|Methionine+Cystine}}
! {{vertical header|Phenylalanine+Tyrosine}}
! {{vertical header|Valine}}
! {{vertical header|Histidine}}
!
! {{vertical header|Tryptophan}}
! {{vertical header|Threonine}}
! {{vertical header|Isoleucine}}
! {{vertical header|Leucine}}
! {{vertical header|Lysine}}
! {{vertical header|Methionine+Cystine}}
! {{vertical header|Phenylalanine+Tyrosine}}
! {{vertical header|Valine}}
! {{vertical header|Histidine}}
|-
| American Indian/Alaska Native Foods
| 4
| 0
| 0
| 10
| 4
| 0
| 0
| 15
| 0
|
| 7
| 2
| 0
| 3
| 6
| 5
| 0
| 0
| 10
|-
| Baby Foods
| 2
| 1
| 0
| 7
| 35
| 8
| 0
| 11
| 1
|
| 5
| 1
| 0
| 5
| 34
| 13
| 0
| 0
| 7
|-
| Baked Products
| 0
| 1
| 0
| 5
| 338
| 1
| 0
| 5
| 1
|
| 0
| 1
| 0
| 0
| 339
| 2
| 0
| 0
| 9
|-
| Beef Products
| 276
| 0
| 6
| 2
| 0
| 0
| 0
| 649
| 2
|
| 289
| 1
| 0
| 176
| 6
| 300
| 0
| 159
| 4
|-
| Beverages
| 0
| 0
| 0
| 2
| 11
| 5
| 0
| 1
| 1
|
| 0
| 0
| 0
| 1
| 12
| 5
| 0
| 0
| 2
|-
| Breakfast Cereals
| 0
| 0
| 1
| 1
| 40
| 1
| 0
| 0
| 0
|
| 0
| 0
| 1
| 1
| 40
| 1
| 0
| 0
| 0
|-
| Cereal Grains and Pasta
| 0
| 0
| 0
| 9
| 143
| 0
| 0
| 3
| 1
|
| 2
| 1
| 0
| 4
| 148
| 0
| 0
| 0
| 1
|-
| Dairy and Egg Products
| 19
| 6
| 4
| 21
| 16
| 122
| 0
| 12
| 3
|
| 19
| 19
| 0
| 0
| 11
| 122
| 0
| 0
| 32
|-
| Fast Foods
| 4
| 3
| 0
| 9
| 39
| 8
| 0
| 62
| 1
|
| 6
| 4
| 0
| 10
| 82
| 15
| 0
| 1
| 8
|-
| Fats and Oils
| 0
| 0
| 0
| 4
| 4
| 4
| 0
| 0
| 0
|
| 0
| 0
| 0
| 0
| 2
| 4
| 0
| 0
| 6
|-
| Finfish and Shellfish Products
| 3
| 3
| 0
| 15
| 0
| 0
| 0
| 228
| 0
|
| 5
| 3
| 0
| 174
| 0
| 0
| 0
| 0
| 67
|-
| Fruits and Fruit Juices
| 15
| 0
| 9
| 54
| 12
| 31
| 3
| 3
| 14
|
| 15
| 1
| 7
| 40
| 11
| 35
| 3
| 1
| 28
|-
| Lamb
| 10
| 0
| 5
| 254
| 3
| 2
| 0
| 155
| 0
|
| 10
| 0
| 2
| 207
| 9
| 112
| 0
| 2
| 87
|-
| Legumes and Legume Products
| 0
| 0
| 0
| 1
| 26
| 154
| 0
| 22
| 0
|
| 0
| 0
| 0
| 1
| 27
| 175
| 0
| 0
| 0
|-
| Meals
| 1
| 0
| 0
| 1
| 15
| 0
| 0
| 14
| 0
|
| 2
| 2
| 0
| 2
| 24
| 0
| 0
| 1
| 0
|-
| Nut and Seed Products
| 0
| 0
| 1
| 24
| 96
| 8
| 0
| 0
| 0
|
| 0
| 0
| 1
| 13
| 103
| 10
| 0
| 0
| 2
|-
| Pork Products
| 11
| 0
| 1
| 54
| 0
| 2
| 0
| 249
| 0
|
| 20
| 0
| 0
| 197
| 0
| 73
| 0
| 15
| 12
|-
| Poultry Products
| 6
| 12
| 6
| 58
| 1
| 0
| 0
| 287
| 0
|
| 36
| 22
| 0
| 167
| 5
| 8
| 0
| 99
| 33
|-
| Restaurant Foods
| 0
| 9
| 3
| 14
| 24
| 3
| 0
| 41
| 1
|
| 1
| 25
| 0
| 9
| 33
| 12
| 0
| 0
| 15
|-
| Sausages and Luncheon Meats
| 5
| 0
| 1
| 31
| 0
| 2
| 0
| 78
| 0
|
| 14
| 11
| 1
| 68
| 1
| 11
| 0
| 4
| 7
|-
| Snacks
| 2
| 0
| 0
| 6
| 83
| 6
| 0
| 4
| 1
|
| 2
| 1
| 0
| 6
| 81
| 9
| 0
| 0
| 3
|-
| Soups
| 0
| 0
| 2
| 7
| 10
| 28
| 0
| 7
| 0
|
| 0
| 0
| 0
| 1
| 21
| 31
| 0
| 0
| 1
|-
| Spices and Herbs
| 3
| 0
| 0
| 6
| 11
| 3
| 0
| 1
| 1
|
| 3
| 2
| 0
| 3
| 12
| 4
| 0
| 0
| 1
|-
| Sweets
| 0
| 1
| 0
| 3
| 17
| 47
| 0
| 1
| 2
|
| 0
| 1
| 0
| 1
| 17
| 47
| 0
| 0
| 5
|-
| Vegetables and Vegetable Products
| 7
| 0
| 8
| 238
| 114
| 199
| 0
| 18
| 19
|
| 13
| 28
| 0
| 112
| 144
| 246
| 0
| 2
| 58
|}

===Protein quality===
{{Main|Protein quality}}

Various attempts have been made to express the "quality" or "value" of various kinds of protein. Measures include the [[biological value]], [[net protein utilization]], [[protein efficiency ratio]], [[protein digestibility corrected amino acid score]] and the [[complete protein]]s concept. These concepts are important in the [[animal husbandry|livestock industry]], because the relative lack of one or more of the essential amino acids in [[fodder|animal feeds]] would have a limiting effect on growth and thus on [[feed conversion ratio]]. Thus, various feedstuffs may be fed in combination to increase net protein utilization, or a [[Dietary supplement|supplement]] of an individual amino acid (methionine, lysine, threonine, or tryptophan) can be added to the feed.

===Protein per calorie===
Protein content in foods is often measured in protein per serving rather than protein per calorie. For instance, the USDA lists 6 grams of protein per large whole egg (a 50-gram serving) rather than 84&nbsp;mg of protein per calorie (71 calories total).<ref>{{cite book |title=SDA National Nutrient Database for Standard Reference |date=2008 |publisher=U.S. Department of Agriculture, Agricultural Research Service |edition=Release 21}}</ref> For comparison, there are 2.8 grams of protein in a serving of raw broccoli (100 grams) or 82&nbsp;mg of protein per calorie (34 calories total), or the [[Daily Value]] of 47.67g of protein after eating 1,690g of raw broccoli a day at 574 cal.<ref name=Broccoli>{{Cite web |url=https://www.nutritionvalue.org/Broccoli%2C_raw_nutritional_value.html |title=Broccoli, raw: nutritional value and analysis |last=Vanovschi |first=Vitalii |website=www.nutritionvalue.org |access-date=4 November 2019}}</ref> An egg contains 12.5g of protein per 100g, but 4&nbsp;mg more protein per calorie, or the protein DV after 381g of egg, which is 545 cal.<ref>{{Cite web |url=https://www.nutritionvalue.org/Egg%2C_poached%2C_cooked%2C_whole_nutritional_value.html |title=Egg, poached, cooked, whole: nutritional value and analysis |last=Vanovschi |first=Vitalii |website=www.nutritionvalue.org |access-date=4 November 2019}}</ref> The ratio of essential amino acids (the quality of protein) is not taken into account, one would actually need to eat more than 3&nbsp;kg of broccoli a day to have a healthy protein profile, and almost 6&nbsp;kg to get enough calories.<ref name=Broccoli/> It is recommended that adult humans obtain between 10–35% of their 2000 calories a day as protein.<ref>{{cite web |url=http://www.webmd.com/food-recipes/protein |title=Web MD Protein: Are You Getting Enough? |publisher=webmd.com |date=2014-09-05 |access-date=2015-03-31}}</ref>