- ACRONYMS AND DEFINITIONS
- AASLD - American Association for the Study of Liver Diseases
- ADA - American Diabetes Assoc
- GFR - Glomerular filtration rate
- RCT - Randomized controlled trial
- RDA - Recommended dietary allowance
- USDA - United States Department of Agriculture
- OVERVIEW OF PROTEIN
- Protein is a major structural and functional component of all cells in the body
- Half the protein in the body is present in skeletal muscle
- Protein can be broken down into amino acids which are used to create energy
- USDA RECOMMENDED DIETARY INTAKE
- Percent of total calories
- Adults should consume 10 - 35% of total calories from protein
- Based on body weight
- Adults - 0.80 grams/kilogram of body weight/day
- 14 - 18 years - 0.85 grams/kilogram of body weight/day
- 4 - 13 years - 0.95 grams/kilogram of body weight/day
- 1 - 3 years - 1.05 grams/kilogram of body weight/day [38]
- ENERGY FROM PROTEIN
- Macronutrients
- Macronutrients provide energy for humans
- There are 3 types of macronutrients:
- Proteins
- Carbohydrates
- Fats
- Energy from macronutrients
- Protein - 4 calories/gram
- Carbohydrates - 4 calories/gram
- Fats - 9 calories/gram
- EXERCISE AND PROTEIN
- Protein metabolism
- Almost half of the body's protein stores are found in muscle
- During exercise, the body initially utilizes carbohydrates for energy. Once carbohydrates are depleted, it must use fat or protein.
- Physically fit people do not have a lot of fat stores, so their bodies must use protein. Since protein is a major component of muscle, protein metabolism is viewed as undesirable since it may lead to muscle breakdown. An enlarging muscle will also need protein and amino acids to grow.
- Protein and amino acids supplements are heavily marketed to athletes based on the theory that increasing dietary protein will enhance muscle growth and inhibit its breakdown
- Nitrogen balance
- Nitrogen balance is a measure of protein use by the body
- Nitrogen balance can be approximated by comparing the amount of protein consumed in the diet to the amount of urea excreted in the urine
- A positive nitrogen balance means more protein is consumed than broken down
- A negative nitrogen balance means more protein is being broken down than consumed
- Exercise and nitrogen balance
- A number of studies have looked at nitrogen balance to determine if physically active people require more protein
- In general, physically active people tend to require an increase in protein above the RDA (0.8g/kg/d) to maintain a neutral or positive nitrogen balance
- Depending on the intensity and duration of exercise, protein intake on the order of 1.2 - 1.4 grams per kg of body weight per day may be appropriate for people engaging in regular endurance exercise
- Protein intake of 1.6 - 1.8 grams per kg of body weight per day may be appropriate for people engaging in heavy resistance (strength) training [1, 2, 3, 4]
- Some studies have shown that the nitrogen balance may turn negative when first starting an exercise program (for about 2 weeks), but then returns to neutral even when protein intake remains the same [2]
- Studies have shown that increasing the consumption of carbohydrates may also prevent a negative nitrogen balance [4]
- Exercise studies
- Studies evaluating the effects of high protein intake on exercise endurance and muscle strength have been mixed
- It is unclear if high protein intake improves exercise endurance or muscle strength [1, 5, 6, 7, 8, 9, 10]
- WEIGHT LOSS AND PROTEIN
- Overview
- Dietary macronutrient composition has become a big topic in the discussion on weight loss and dieting. Most of the debate has centered around low-carb vs low-fat diets (see low-carb vs low-fat diets), but a few studies have compared the effects of different dietary protein levels on weight loss.
- The first study detailed below compared an average-protein diet to a high-protein diet. The second study looked at the effects of dietary protein content on energy expenditure.
- A study in the NEJM enrolled 811 overweight adults
Main inclusion criteria
- Age 30 - 70 years
- BMI 25 - 40
Main exclusion criteria
- Diabetes
- Unstable cardiovascular disease
- Use of medications that affect body weight
Baseline characteristics
- Average age 51 years
- Average BMI - 33
- Female sex - 64%
- Average weight - 205 lbs (93 kg)
Randomized treatment groups
- Diet 1 - Low-fat, average-protein (204 patients) - 20% fat, 15% protein, and 65% carbohydrates
- Diet 2 - Low-fat, high-protein (202 patients) - 20% fat, 25% protein, and 55% carbohydrates
- Diet 3 - High-fat, average-protein (204 patients) - 40% fat, 15% protein, and 45% carbohydrates
- Diet 4 - High-fat, high-protein (201 patients) - 40% fat, 25% protein, and 35% carbohydrates
- Participants received continuous dietary counseling via group and individual sessions throughout the study. Daily meal plans were provided.
- Participants were given individual caloric goals that provided a 750 kcal/day deficit
- The study had a 2X2 factorial design that compared the low vs high fat diets and average vs high protein diets
Primary outcome: Weight loss at 2 years
Results
Duration: 2 years | |||||
Outcome | High-protein | Average-protein | High-fat | Low-fat | Comparisons |
---|---|---|---|---|---|
Primary outcome | 7.9 lbs | 6.6 lbs | 7.2 lbs | 7.2 lbs | p=0.22 for protein | p=0.94 for fat |
|
Findings: Reduced-calorie diets result in clinically meaningful weight loss regardless of which macronutrients they emphasize
- A study in JAMA enrolled 28 healthy people into an inpatient metabolic unit
Main inclusion criteria
- Age 18 - 35 years
- BMI 19 - 30
Main exclusion criteria
- Smoker
Baseline characteristics
- Average age - 24 years
- Male sex - 64%
- Black race - 64%
Randomized treatment groups
- During the first phase, subjects were fed a weight-stabilizing (isocaloric) diet for 13 - 25 days
- Subjects then entered a second phase for 8 weeks where they were overfed by about 950 calories a day
- Group 1 - Low-protein diet (10 patients) - 6% of calories from protein, 52% from fat, and 42% from carbohydrates
- Group 2 - Normal-protein diet (9 patients) - 15% of calories from protein, 44% from fat, and 41% from carbohydrates
- Group 3 - High-protein diet (9 patients) - 26% of calories from protein, 33% from fat, and 41% from carbohydrates
Primary outcome: Change in body weight, body composition (measured by dual-energy x-ray absorptiometry), and
calorie expenditure (measured by ventilated hood and doubly labeled water) from baseline to Week 7 - 8 of overfeeding
Results
Duration: 8 weeks | ||||
Outcome | Low-protein | Normal-protein | High-protein | Comparisons |
---|---|---|---|---|
Increase in body weight | 7 lbs | 13.3 lbs | 14.3 lbs | 1 vs 2 and 3 p=0.002 |
Increase in fat mass | 8.1 lbs | 7.6 lbs | 7.6 lbs | 1 vs 2 vs 3 p=0.91 |
Change in lean body mass | -1.54 lbs | +6.3 lbs | +7.0 lbs | 1 vs 2 and 3 p<0.001 |
Change in total energy expenditure | +176 kJ/d | +2186 kJ/d | +1898 kJ/d | 1 vs 2 and 3 p=0.007 |
Change in resting energy expenditure | -86 kJ/d | +669 kJ/d | +949 kJ/d | 1 vs 2 and 3 p<0.001 |
Change in nonresting energy expenditure | +245 kJ/d | +1296 kJ/d | +756 kJ/d | 1 vs 2 vs 3 p=0.24 |
Findings: Among persons living in a controlled setting, calories alone account for the increase in fat; protein affected energy expenditure and storage of lean body mass, but not body fat storage.
- Summary
- High-protein diets have not been shown to improve weight loss
- The NEJM study showed that in general, overall caloric intake is the primary determinant of weight loss, and macronutrient composition is less important
- The JAMA metabolic study showed that overall calorie intake was still the main predictor of fat gain. It also showed that diets higher in protein were more "metabolically efficient" meaning the ratio of calories to weight gain was lower. This may be counterproductive for patients trying to lose weight.
- The normal- and high-protein diets did cause a significantly greater increase in lean body mass when compared to the low-protein diet. This may be important to people who are trying to gain muscle mass (e.g. athletes, bodybuilders, frail and elderly)
- Patients who need to lose weight should focus on overall calorie intake. See weight loss for more.
- KIDNEY DISEASE AND PROTEIN
- Overview
- High protein intake has been shown to induce "hyperfiltration" in the kidney evidenced by an increase in GFR when patients consume high protein diets
- The increase in GFR has raised theoretical concerns that protein may increase intraglomerular pressure and lead to long-term loss of kidney function
- A number of observational studies have evaluated the association of protein intake and kidney function in healthy individuals. In general, these studies have not found evidence that protein intake affects kidney function. [19, 42]
- Diabetic kidney disease
- Protein-restricted diets have been studied as a treatment for diabetic kidney disease
- See protein and diabetic kidney disease for a full review of protein-restricted diets in diabetes
- LIVER DISEASE AND PROTEIN
- Overview
- Patients with advanced liver disease have trouble metabolizing ammonia
- Elevated ammonia levels may lead to a condition called "hepatic encephalopathy." Hepatic encephalopathy is marked by mental status changes and confusion.
- Ammonia is produced by bacteria in the intestine when they break down amino acids from proteins. Ammonia is then absorbed into the bloodstream by the intestine.
- Because of the association between protein and ammonia production in the intestine, it was a widely held belief that restricting protein intake in individuals with hepatic encephalopathy would help improve the condition
- For years, patients with advanced liver disease were often placed on protein-restricted diets even though there are no good trials that support this practice. Furthermore, patients with advanced liver disease are often malnourished, and limiting protein intake may worsen their prognosis. [43,45,46]
- AASLD recommendations
- Protein-restriction is not recommended for patients with hepatic encephalopathy
- Patients with cirrhosis often have relatively higher protein requirements than healthy individuals
- Daily protein intake of 1.2 - 1.5 grams of protein/kg body weight/day is recommended in patients with hepatic encephalopathy and chronic liver disease [50]
- OTHER CONDITIONS AND PROTEIN
- Osteoporosis
- Protein promotes the renal excretion of calcium (the loss of calcium in the urine)
- There has been concern that protein-induced calcium loss could promote osteoporosis
- In general, high protein intake with adequate calcium intake does not appear to negatively affect bone health
- Low protein intake may be detrimental to bone health [1, 11, 12, 13]
- Kidney stones
- Protein promotes the renal excretion of calcium (the loss of calcium in the urine)
- There has been concern that increased calcium in the urine from protein could promote kidney stones
- Results from studies evaluating the effect of dietary protein intake on kidney stones have been mixed
- There is no conclusive evidence that high dietary protein intake affects kidney stone risk [1, 14, 15, 16, 40]
- Heart disease
- Studies evaluating the association of high-protein diets with heart disease have been mixed
- Overall, high amounts of vegetable protein in the diet appear to have a positive effect on heart disease risk
- High amounts of meat protein (particularly red meat) may have a negative effect on heart disease risk [20, 23, 24, 25, 26]
- Lipid parameters
- Effects of high-protein diets on cholesterol levels have been mixed
- Most trials have compared the effects of diets with high carbohydrate content with diets that have high protein content
- In general, higher protein diets lower triglyceride levels when compared to higher carbohydrate diets
- Effects on LDL, Total and HDL cholesterol are inconsistent and inconclusive [26, 27, 28, 29, 30]
- Blood pressure
- Effects of high-protein diets on blood pressure in clinical trials have been mixed
- There is no conclusive evidence that high-protein diets lower blood pressure [27, 28, 34, 35, 38, 39]
- Diabetes
- Effects of high-protein diets on diabetic blood sugar control have been mixed
- Most trials have evaluated the effect of replacing carbohydrates with protein
- There is no conclusive evidence that diets higher in protein improve blood glucose control in diabetics [27, 28, 30, 31, 32, 33, 34, 35]
- Cancer risk
- Some foods high in protein (animal meats) have been thought to account for higher cancer rates in Western diets
- Studies evaluating effects of high-protein diets on cancer risk have been mixed
- There is no conclusive evidence that diets high in protein (regardless of protein source) increase cancer risk [1, 25, 36, 37, 38]
- DIETARY SOURCES OF PROTEIN
- Animal
- Animal meats - beef, fish, poultry
- Eggs
- Dairy products - yogurt, milk, cheese
- Vegetable
- Nuts - almonds, peanuts, walnuts
- Beans - kidney beans, black beans, soybeans
- VEGETARIAN DIET AND PROTEIN
- Since vegetarians do not consume meat, there has been concern that they may not receive enough dietary protein. Also, certain amino acids (e.g. lysine, sulfur amino acids, threonine) are lower in plant proteins.
- Based on the available evidence, it appears that diets that contain a complementary mixture of plant proteins provide an adequate amount of protein for most vegetarians
- Vegetarians should also be careful to consume adequate amounts of calcium, iron, vitamin B12, zinc, and n-3 fatty acids [1, 38]
- RESOURCES FOR FOOD CONTENT
- USDA food composition database - extensive nutritional information about numerous foods including restaurant foods
- myfitnesspal - commercial website that has extensive database of food calorie content
- Lose it! - commercial website that has extensive database of food calorie content
- SATIETY AND PROTEIN
- Satiety is defined as the sense of fullness or satisfaction a person feels after eating food
- Because of the obesity epidemic, there has been interest in the satiety of macronutrients (protein, carbohydrates, and fat) as a means to control weight gain
- A number of studies of varying designs have been performed to measure the effect of different macronutrients on satiety
- In short, there is no conclusive evidence that one macronutrient is superior to another in providing satiety that leads to reduced calorie intake [1,58]
- BIBLIOGRAPHY
- 1 - Dietary Reference Intake, Institute of Medicine, 2005
- 2 - PMID 10919959
- 3 - PMID 15640513
- 4 - PMID 11023001
- 5 - PMID 18347652
- 6 - PMID 20508536
- 7 - PMID 18156659
- 8 - PMID 16917926
- 9 - PMID 19106243
- 10 - PMID 18467544
- 11 - PMID 12612168
- 12 - PMID 12612169
- 13 - PMID 12936953
- 14 - PMID 8659482
- 15 - PMID 9550636
- 16 - PMID 15579526
- 17 - PMID 1531426
- 18 - PMID 11096144
- 19 - PMID 12639078
- 20 - PMID 19443643
- 21 - PMID 19588328
- 22 - PMID 17943769
- 23 - PMID 10426698
- 24 - PMID 20881068
- 25 - PMID 15671256
- 26 - PMID 16002829
- 27 - PMID 20150293
- 28 - PMID 18469290
- 29 - PMID 11451718
- 30 - PMID 12816768
- 31 - PMID 15941879
- 32 - PMID 14522731
- 33 - PMID 15507157
- 34 - PMID 15800559
- 35 - PMID 21246185
- 36 - PMID 18444162
- 37 - PMID 20661817
- 38 - USDA 2010 Dietary recommendations for protein
- 39 - PMID 20216558
- 40 - PMID 11784873
- 41 - PMID 18175733
- 42 - PMID 20338292
- 43 - PMID 11467622
- 44 - PMID 15246205
- 45 - PMID 21447768
- 46 - PMID 16707194
- 47 - PMID 12499328
- 48 - PMID 19246357
- 49 - PMID 22215165
- 50 - PMID 25015420