Low-protein diet

A low-protein diet is a diet in which people reduce their intake of protein. A low-protein diet is prescribed for those with inherited metabolic disorders, such as Phenylketonuria and Homocystinuria and reduced protein levels have been used by people with kidney or liver disease. Low protein consumption appears to alter the risk of bone breakage, presumably through changes in calcium homeostasis.^[1] Consequently, there is no uniform definition of what constitutes low-protein, because the amount and composition of protein for an individual suffering from phenylketonuria would differ substantially from one suffering homocystinuria. The amount used by those with liver disease would still result in individuals being in nitrogen balance.

Amino acids that are excess to requirement cannot be stored, but must be modified by deamination (removal of the amine group). As this occurs in the liver and kidneys, some individuals with damaged livers or kidneys may be advised to eat less protein. Due to the sulphur content of the amino acids methionine and cysteine, excess of these amino acids leads to the production of acid through sulphate ions. These sulphate ions may be neutralized by calcium ions from bone, which may lead to net urinary loss of calcium. This might lead to reduction in bone mineral density over time. Individuals suffering from phenylketonuria lack the enzyme to convert phenylalanine to tyrosine so low levels of this amino acid need to be provided in the diet. Homocystinuria is an inherited disorder involving the metabolism of the amino acid methionine leading to the accumulation of homocysteine. Treatment includes providing low levels of methionine and high levels of vitamin B6 in the diet.

Low-protein diets are in vogue among some members of the general public because of the impact of protein intake on Insulin/Insulin-like growth factor 1 Signalling (IIS) and the direct sensing of amino acid availability by mammalian target of rapamycin (mTOR), two systems that are implicated in longevity and cancer proliferation.^[2][3][4][5] Apart from low protein intake, such as in the 80:10:10 diet, other attempts to modulate IIS are through intermittent fasting and the 5:2 diet.

History

By studying the composition of food in the local population in Germany, Carl von Voit established a standard of 118 grams of protein per day. Russell Henry Chittenden showed that less than half that amount was needed to maintain good health.^[6] Horace Fletcher was an early populariser of low-protein diets, which he advocated along with chewing.

Protein requirement

The daily requirement for humans to remain in nitrogen balance is relatively small. The median human adult requirement for good quality protein is approximately 0.65 gram per kilogram body weight per day and the 97.5 percentile is 0.83 grams per kilogram body weight per day.^[7] Children require more protein, depending on the growth phase. A 70 kg adult human who was in the middle of the range would require approximately 45 grams of protein per day to be in nitrogen balance. This would represent less than 10% of kilocalories in a notional 2,200 kilocalorie ration. William Cumming Rose and his team studied the essential amino acids, helping to define minimum amounts needed for normal health. For adults, the recommended minimum amounts of each essential amino acid varies from 4 to 39 milligrams per kilogram of body weight per day. To be of good quality, protein only needs to come from a wide variety of foods; there is neither a need to mix animal and plant food together nor a need to complement specific plant foods, such as rice and beans.^[8] The notion that such specific combinations of plant protein need to be made to give good quality protein stems from the book Diet for a Small Planet. Plant protein is often described as incomplete, suggesting that they lack one or more of the essential amino acids. Apart from rare examples, such as Taro,^[8][9] each plant provides an amount of all the essential amino acids. However, the relative abundance of the essential amino acids is more variable in plants than that found in animals, which tend to be very similar in essential amino acid abundance, and this has led to the misconception that plant proteins are deficient in some way. However, many plants have low concentrations of protein and so growing individuals may not be able to consume enough plant material to get all the protein that they need.

Low-protein vs calorie restriction

Calorie restriction has been demonstrated to increase the life span and decrease the age-associated morbidity of many experimental animals. Increases in longevity or reductions in age-associated morbidity have also been shown for model systems where protein or specific amino acids have been reduced. In particular, experiments in model systems in rats, mice, and Drosophila fruit flies have shown increases in life-span with reduced protein intake comparable to that for calorie restriction. The effect appears to be entirely due to restriction of the amino acid methionine, which is required to initiate protein synthesis.^{[10][11][12][13][14][15]}

The diets of humans living in some of the Blue Zones, regions of enhanced numbers of centenarians and reduced age-associated morbidity, contain less than 10% of energy from protein,^[16] although reports on all the Blue Zones are not available. None of the diets in these regions is completely based on plants, but plants form the bulk of the food eaten.^[17] Although it has been speculated that some of these populations are under calorie restriction, this is contentious as their smaller size is consistent with the lower food consumption.^[18]

Low-protein and liver disease

In the past a standard dietary treatment for those suffering from liver disease or damage was a low protein, high carbohydrate, moderate fat and low salt diet. However, more recent research suggests that a high protein diet is required of 1.2–2 g of protein per kg. Levels of up to 2 g/kg body weight/day have been demonstrated to not worsen encephalopathy. In addition, vitamin supplements especially vitamin B group should be taken. Salt should be restricted to below 5 mg per day.^[19][20]

Low-protein and kidney disease

Low-protein diets to treat kidney disease include the Rice diet, which was started by Walter Kempner at Duke University in 1939. This diet was a daily ration of 2,000 Calories consisting of moderate amounts of boiled rice, sucrose and dextrose, and a restricted range of fruit, supplemented with vitamins. Sodium and chloride where restricted to 150 mg and 200 mg respectively. It showed remarkable effects on control of edema and hypertension.^[21][22] Although the Rice diet was designed to treat kidney and vascular disease, the large weight loss associated with the diet led to a vogue in its use for weight loss which lasted for more than 70 years. The Rice Diet program closed in 2013.^[23] Other low-protein starch-based diets like John A. McDougall's program continue to be offered for kidney disease and hypertension.

Low-protein and osteoporosis

The effect of protein on osteoporosis and risk of bone fracture is complex. Calcium loss from bone occurs at protein intake below requirement when individuals are in negative protein balance, suggesting that too little protein is dangerous for bone health.^[24] IGF-1, which contributes to muscle growth, also contributes to bone growth, and IGF-1 is modulated by protein intake.^[5] However, at high protein levels, a net loss of calcium may occur through the urine in neutralizing the acid formed from the deamination and subsequent metabolism of methionine and cysteine. Large prospective cohort studies have shown a slight increase in risk of bone fracture when the quintile of highest protein consumption is compared to the quintile of lowest protein consumption.^[1] In these studies, the trend is also seen for animal protein but not plant protein, but the individuals differ substantially in animal protein intake and very little in plant protein intake. As protein consumption increases, calcium uptake from the gut is enhanced.^[1][24] Normal increases in calcium uptake occur with increased protein in the range 0.8 grams to 1.5 grams of protein per kilogram body weight per day. However, calcium uptake from the gut does not compensate for calcium loss in the urine at protein consumption of 2 grams of protein per kilogram of body weight. Calcium is not the only ion that neutralizes the sulphate from protein metabolism, and overall buffering and renal acid load also includes anions such as bicarbonate, organic ions, phosphorus and chloride as well as cations such as ammonium, titrateable acid, magnesium, potassium and sodium.^[25] The study of Potential Renal Acid Load (PRAL) suggests that increased consumption of fruits, vegetables and cooked legumes increases the ability of the body to buffer acid from protein metabolism, because they contribute to a base forming potential in the body due to their relative concentrations of proteins and ions. However, not all plant material is base forming, for example, nuts, grains and grain products add to the acid load.^[24][25][26]

Protein composition of foods

Food of different classes differ substantially in the amount of protein and the contribution of protein to Caloric content of the food. This table shows the grams of protein per 100 grams of a representative group of foods, the Calories in that food from protein, fat, and carbohydrate, and the proportion of the Calories due to protein. Refined sugars and oils or fats have not been included because the protein content in those are negligible or zero. Refined protein powders such as isolated soy or whey protein have also been excluded for the opposite reason.

Type	Food	Protein (g/100 g)	Calories (per 100 g)	Percent Calories from Protein
Animal	Atlantic salmon	19.8	136	59.4
Animal	Chicken egg	12.2	142	35.1
Animal	Lamb 1/8 inch fat trim	17.8	222	32.8
Animal	Lean Chicken	23.1	105	90.0
Animal	Lean beef sirlion	21.3	136	63.9
Animal	Pork belly	9.3	504	7.6
Dairy	Camembert	16.4	297	22.6
Dairy	Cheddar cheese	24.8	411	24.7
Dairy	Cottage cheese	11.3	104	44.3
Dairy	Cow's milk	3.3	64	20.9
Dairy	Cream cheese	7.4	339	8.9
Dairy	Low fat yoghurt	5.3	55	39.5
Dairy	Parmesan cheese	34.7	364	39.0
Dairy	Plain yoghurt	5.4	81	27.1
Dairy	Triple Brie	12.2	446	11.2
Dairy	Vanilla ice cream	1.9	178	4.4
Dry fruit	Pitted dates	1.9	338	2.3
Dry fruit	Sultanas	3.1	304	4.2
Fruit	Apple	0.3	59	2.1
Fruit	Avocado	2.0	172	4.8
Fruit	Banana	1.1	100	4.5
Fruit	Black pitted olives	2.2	182	4.9
Fruit	Orange	0.9	52	7.0
Fruit	Plum	0.7	52	5.5
Fruit	Strawberry	0.7	37	7.7
Grain	Jasmine white rice	7.2	361	8.2
Grain	Medium grain whole rice	7.6	363	8.6
Grain	Pearl barley	8.0	304	10.8
Grain	Polenta	8.6	368	9.6
Grain	Rolled oats	13.4	360	15.2
Grain	Wholemeal flour	11.2	324	14.1
Legume	Chickpeas dry	19.3	309	25.6
Legume	Split red lentils	24.2	260	38.1
Legume	Split yellow peas	23.0	296	31.8
Legume	French lentils	23.7	258	37.5
Nuts	Almonds	23.2	565	16.8
Nuts	Cashews	18.2	584	12.8
Nuts	Peanuts	24.7	552	18.3
Nuts	Pecan	9.8	692	5.8
Nuts	Pine nuts	13.0	687	7.7
Nuts	Walnuts	15.2	692	9.0
Processed	Bacon	16.0	294	22.3
Processed	Beef sausages	12.8	224	23.3
Processed	Chicken liver pate	9.1	333	11.2
Processed	Chorizo	18.4	287	26.2
Processed	Commercial Birchir muesli	12.1	410	12.1
Processed	Commercial Hommus	5.4	323	6.8
Processed	Commercial jam	0.3	268	0.5
Processed	Commercial mustard	7.0	150	19.1
Processed	Commercial peanut butter	30.2	597	20.7
Processed	Commercial pepperoni	25.3	393	26.4
Processed	Commercial salsa	1.6	38	17.0
Processed	Commercial sauerkraut	1.3	21	24.4
Processed	Corn flakes	7.8	373	8.6
Processed	Custard powder	0.5	360	0.6
Processed	Dark chocolate	5.0	521	3.9
Processed	Desiccated coconut	6.6	645	4.2
Processed	Dill pickle	0.8	9	33.1
Processed	Fresh pasta sheet	10.9	286	15.6
Processed	Gingernut biscuit	4.9	446	4.5
Processed	Hazelnut spread	5.4	323	6.8
Processed	Italian pasta	12.5	358	14.3
Processed	Rice vermicelli	9.6	353	11.1
Processed	Salted corn chips	8.2	498	6.7
Processed	Salted potato chips	7.6	500	6.2
Processed	Tomato sauce	1.8	141	5.2
Processed	Vitaweat original	11.2	371	12.4
Processed	Wheat bix	12.4	337	15.0
Processed	White chocolate	5.0	571	3.6
Processed	White flour	10.1	344	12.0
Processed	Wholemeal bread	8.3	218	15.5
Seeds	Golden linseed	32.4	288	46.0
Seeds	Pumpkin kernels (peppitas)	24.4	557	17.9
Seeds	Sunflower kernels	22.7	550	16.9
Seeds	White quinoa	13.5	358	15.4
Vegetable	Cauliflower	2.0	30	26.6
Vegetable	Cos lettuce	1.2	21	23.3
Vegetable	Cucumber	0.7	18	15.5
Vegetable	Potato	2.0	84	9.7
Vegetable	Sweet potato	1.6	89	7.3
Vegetable	Tomato	0.9	21	17.2

Values taken from labels on commercial items and from a nutritional database.^[9]

Methionine and Cysteine content of foods

It is often claimed that methionine and cysteine are more likely to be encountered in animal foods than in plant foods. This is often found in arguments for selective consumption of plant foods to combat osteoporosis, and in arguments to choose plant foods in diets restricting methionine. However, this is not strictly true, as the following table shows. Animal protein shows a range of approximately 3% to 4% methionine plus cysteine for meat as well as for milk and dairy. Eggs have higher values in a 4% to 7% range. While many fruit and vegetables have values below 3%, values for grains, seeds, and nuts fall in the 3% to 4% range, and many exceed 4%. Dry or mature legumes have values in the range 2% to 3%, but sprouted legumes exceed 4%. The highest value is for Brazil nuts. This table shows that unless large classes of plant food are avoided, a plant-based diet is unlikely to be significantly lower in methionine and cysteine than an omnivorous diet if the same level of protein is consumed.

Type	Item	Methionine (g/100g)	Cysteine (g/100g)	Protein (g/100g)	Percent Methionine plus Cysteine in Protein
Animal	Abalone, mixed	0.386	0.244	17.1	3.7
Animal	Chicken, breast	0.563	0.274	20.8	4.0
Animal	Lamb 1/8 inch fat trim	0.457	0.214	17.8	3.8
Animal	Lobster, northern	0.529	0.211	18.8	3.9
Animal	Pork, mixed cuts	0.347	0.169	13.9	3.7
Animal	Salmon, Atlantic	0.626	0.219	20.4	4.1
Animal	Sea bass, mixed	0.546	0.198	18.4	4.0
Dairy	Camembert	0.565	0.109	19.8	3.4
Dairy	Goat, soft	0.494	0.084	18.5	3.1
Dairy	Parmesan	0.958	0.235	35.8	3.3
Dairy	Ricotta	0.281	0.099	11.3	3.4
Dairy	Roquefort	0.558	0.126	21.5	3.2
Dairy	Yoghurt, plain, whole	0.102	0.032	3.5	3.8
Milk	Buffalo, Indian	0.097	0.048	3.7	3.9
Milk	Cow	0.082	0.030	3.3	3.4
Milk	Goat	0.080	0.046	3.6	3.5
Milk	Human	0.021	0.019	1.0	4.0
Milk	Sheep	0.155	0.035	6.0	3.2
Egg	Caviar	0.646	0.449	24.6	4.5
Egg	Chicken	0.380	0.272	12.6	5.2
Egg	Duck	0.576	0.285	12.8	6.7
Egg	Goose	0.624	0.309	13.9	6.7
Fruit	Apple	0.001	0.001	0.3	0.7
Fruit	Avocado	0.038	0.027	2.0	3.2
Fruit	Banana	0.008	0.009	1.1	1.5
Fruit	Mango	0.005	0.000	0.5	1.0
Fruit	Orange	0.020	0.010	0.9	3.3
Fruit	Peach	0.010	0.012	0.9	2.4
Fruit	Pear	0.002	0.002	0.4	1.0
Fruit	Pineapple	0.012	0.014	0.5	5.2
Grain	Barley, hulled	0.240	0.276	12.5	4.1
Grain	Brown long-grain rice	0.179	0.096	7.9	3.5
Grain	Durum wheat	0.221	0.286	13.7	3.7
Grain	Maize, white	0.197	0.170	9.4	3.9
Grain	Oats	0.312	0.408	16.9	4.3
Grain	Rye	0.248	0.329	14.8	3.9
Legume	Black beans	0.325	0.235	21.6	2.6
Legume	Chickpeas	0.253	0.259	19.3	2.7
Legume	Fava beans	0.213	0.334	26.1	2.1
Legume	Kidney beans	0.355	0.256	23.6	2.6
Legume	Lima beans	0.271	0.237	21.5	2.4
Legume	Mung beans	0.286	0.210	23.9	2.1
Legume	Pink lentils	0.212	0.327	24.9	2.2
Legume	Soybeans	0.534	0.638	39.6	3.0
Legume	Sprouted lentils	0.105	0.334	9.0	4.9
Nut	Almonds	0.196	0.293	22.1	2.2
Nut	Brazilnuts	1.008	0.367	14.3	9.6
Nut	Cashews	0.362	0.393	18.2	4.1
Nut	Hazelnuts	0.221	0.277	15.0	3.3
Nut	Pecans	0.183	0.152	9.2	3.6
Nut	Pistachio	0.338	0.357	20.6	3.4
Nut	Walnuts	0.236	0.208	15.2	2.9
Seed	Chia	0.090	0.361	15.6	2.9
Seed	Flaxseed	0.370	0.340	18.3	3.9
Seed	Pumpkin, pepitas	0.551	0.301	24.5	3.5
Seed	Sesame	0.586	0.358	17.7	5.3
Seed	Sunflower	0.494	0.451	20.8	4.5
Seed	Watermelon	0.834	0.438	28.3	4.5
Soy	Milk, unfortified	0.027	0.000	3.3	0.8
Soy	Miso	0.129	0.000	11.7	1.1
Soy	Tempeh	0.175	0.193	18.5	2.0
Soy	Tofu, raw, firm	0.202	0.218	15.8	2.7
Vegetable	Breadfruit	0.010	0.009	1.1	1.7
Vegetable	Cabbage	0.012	0.011	1.3	1.8
Vegetable	Capsicum, Hungarian	0.010	0.015	0.8	3.1
Vegetable	Carrot, baby	0.006	0.007	0.6	2.2
Vegetable	Cauliflower	0.028	0.023	2.0	2.6
Vegetable	Celery	0.005	0.004	0.7	1.3
Vegetable	Cucumber	0.006	0.004	0.7	1.4
Vegetable	Lettuce, Cos	0.015	0.006	1.2	1.7
Vegetable	Potato	0.033	0.026	2.0	2.9
Vegetable	Sweet potato	0.029	0.022	1.6	3.2
Vegetable	Taro	0.020	0.032	1.5	3.5
Vegetable	Tomato	0.006	0.009	0.9	1.7
Vegetable	Yam	0.021	0.019	1.5	2.7

The values in the table show only weak correlations between pairs of methionine, cysteine and total protein content. Methionine and cysteine are treated together because they are interconverted via homocysteine and because both amino acids contain sulphur, and so both contribute to the acid load when they are broken down. All values for methionine, cysteine, and protein content of foods were taken from a nutritional database.^[9]

Countries

In the United Kingdom, low-protein products and substitutes are prescribed through the health service.

Claims

Low protein, vegetarian diets have been hypothesized to be linked to longer life.^[27]

See also

• Essential amino acid
• High protein diet

References

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