NASA’S NUTRITION BIOCHEMISTRY LABORATORY

The Effect of Dietary Protein on Bone

To the General Public and Space Flight Personnel

Katherine K. Olansen

7/20/2012

Human bones are constantly being formed and broken down through the function of osteoblasts and osteoclasts. A variety of contributing factors determine the health of bone including nutritional intake accompanied by exercise. Protein is a macronutrient of large interest concerning the upkeep of bone mineral density (BMD) as people face prolonged lives and harsh environmental conditions, including space flight. The effect that either increased or decreased dietary protein has on bone is a long researched debate without an absolute answer. Dynamic circumstances of the body and of the environment continue to keep this question alive. Ambiguity is present in many studies including animal protein versus vegetable protein, long term versus short term results, the cause of hypercalciuria, the effects of protein on acid-base balance, the contribution of other nutrients to the bone, different populations studied (i.e., males versus pre- and post- menopausal women), and various downfalls in research methods. The findings on these contributing factors leads to the conclusion that balance in the diet is absolutely key. Too much of any one thing can be harmful, such as increased protein spiking acidity, and likewise, too little of something such as decreased vitamin D effecting calcium absorption will hinder the body’s ability to function properly.

1 The Effect of Dietary Protein on Bone

The bones serve the body in multiple ways including support, structure, protection, and mineral storage. Therefore, it is essential that this material made up of osteoblasts, osteoclasts, and osteocytes, is well maintained. A variety of factors contribute to keeping the bones strong and healthy, but for the sake of this research we will focus on the nutritional components; specifically protein. Bone structure is composed of collagen and minerals. The epiphyseal plate (commonly referred to as the growth plate), as seen in the diagram to the right, is the site of bone growth from infancy to adulthood. The mostly collagenous bones at birth ossify into compact bone as the baby matures until the diaphysis, or shaft, of the bone is completely formed. This process is influenced by complete nutrition and bone care which includes proper consummation of protein, calcium, and phosphate. Although bones solidify completely at adulthood, they are continuously being broken down and rebuilt all throughout the lifespan. It is therefore extremely important for adequate nutrients to be present during all of the wear and tear that the body experiences over the years to avoid loss of bone density.

It is inevitable that some bone will be lost with age, but as knowledge increases on ways to either slow or prevent this truth, it is debatable whether or not increased dietary protein can have a positive effect on bone mineral density (BMD) under diverse conditions. Many studies have been performed without any coming to an absolute conclusion. Ambiguity is present in the studies over animal protein versus vegetable protein, long term versus short term results, the cause of hypercalciuria, the effects of protein on acid-base balance, the contribution of other nutrients to the bone, different populations studied (i.e., males versus pre- and post-menopausal women), and various downfalls in research methods. The previous meta-analysis studies done on this topic have begun to crack the seal towards an answer, but the numerable countermeasures that oppose every attempted claim continue to cause uncertainty.

Many health issues today are influenced specifically by the health of bone in the body. Osteoporosis is one of the most prevalent and frequently diagnosed bone diseases in today’s society. In the United States today, 10 million people have osteoporosis while 34 million others have low bone mineral density (BMD), called osteopenia (1).

The question of whether or not dietary protein will protect or negatively affect BMD is applicable to not only those with osteoporosis, but also those in other circumstances such as prolonged bed-rest or space flight where the environment itself accelerates bone loss. Studies have been performed by NASA’s Nutritional Biochemistry Laboratory on bed-rest patients at University of Texas Medical Branch. Not enough applicable data has been recovered from space flight yet to make an absolute statement because of the concurrent use of the ARED exercise device. For the purposes of this specific review, research is through the PubMed database and other resources provided via Johnson Space Center. Dietary protein clearly has a role in bone health and it is a major inquiry of ours to better understand exactly what role that is and how it can benefit our public society as well as future personnel in space exploration.

2 The Effect of Dietary Protein on Bone

Animal and plant foods will have different effects on bone, but it is likely because of many other nutrients associated with those types of foods, not just the protein content (2).The proteins that come from meats and dairy products versus those that are consumed through nuts, legumes, and leafy greens are associated with foods that provide the body with different outlying nutrients that could potentially affect bone health. These extra nutrients that are also known to affect bone health include calcium, potassium, sulfur, phosphorus, etc. It is well recognized that an increase in animal protein intake is associated with an increase in urinary calcium excretion (2). However, exceptions can be made to these findings, and there is question as to whether the increase in urinary calcium is due to an increase in calcium absorption or from an increase in bone breakdown. The amount of calcium in the diet is also an important factor in the response of dietary protein on bone. For example, the calcium content found in milk makes up for the urinary calcium losses generated by the milk protein just like the high potassium levels in plant foods, including legumes and grains, will decrease urinary calcium (2). Six different studies can be taken into consideration that examined the effect of total protein, plant protein, animal protein, and calcium intakes on bone health in elderly men and women. The mean age of participants, both men and women, was 66 years. Over the course of six years these studies were performed by different researchers. The study by Promislow et al., with 572 women over the course of four years at the mean age of 71, had a mean protein intake of 71 g/d with a mean calcium intake of 985 mg/d, finding that the increase in animal protein increased BMD while the increase in plant protein decreased BMD. Munger et al. had 32,006 women at the mean age of 61 over three years with a mean protein intake of 78 g/d and a mean calcium intake of 1150 mg/d and found that increased animal protein decreased fracture and increased total protein decreased fracture. Hannan et al. assessed BMD and fracture from 391 women and 224 men over four years at the mean age of 75 with a mean protein intake of 68 g/d and 810 mg/d mean calcium intake to find that increased animal protein decreased fracture and increased total protein increased BMD. Sellmeyer et al. had the lowest intake of protein at 50 g/d for the mean and 853 mg/d mean calcium intake with 1,035 women over seven years at the mean age of 73 finding that with both animal and plant protein increase that hip fracture increased as well. The final two studies were very similar in test procedure and results with Dawson-Hughes et al. having 79 g/d mean protein level and 1346 mg/d mean calcium level on 138 men and women over three years with a mean age of 70, and Feskanich et al. having 79.6 g/d mean protein level and 718 mg/d mean calcium level on 85,900 women over 12 years with a mean age of 46. They both found that with total protein increase, the BMD increased but the fracture rate also increased. From this data, the puzzle continues. It is obvious that both animal and plant proteins have effect on BMD and bone fracture risks, but further research is necessary to pin down how exactly it is either beneficial or detrimental and in what levels, for which people.

Duration Number of Subjects Mean Age of Subjects Protein Intake (g/day) Calcium Intake (mg/day) Findings4 yrs. 572 W 71 71 985 AP increase/BMD increase; PP increase/BMD decrease3 yrs. 32,006 W 61 78 1150 AP increase/FX decrease; P increase/FX decrease4 yrs. 391 W/224 M 75 68 810 AP increase/FX decrease; P increase/BMD increase3 yrs. 138 M&W 70 79 1346 P increase/BMD increase7 yrs. 1035 W 73 50 853 AP & PP increase/Hip FX increase12 yrs. 85,900 W 46 79.6 718 P increase/FX increase

Animal vs. Plant Protein Effects on BMD and FX

P=total protein, AP=animal protein, PP=plant protein, BMD=change in bone mineral density, FX=fracture

3 The Effect of Dietary Protein on Bone

The most definite statement established concerning this topic of research is that increased dietary protein increases urinary calcium which can pose various issues including an introduction to hypercalciuria (3). Hypercalciuria is elevated urinary excretion of calcium and can be dangerous due to the potential increased risk for renal stones. However, although high-protein diets cause hypercalciuria, some studies show no changes in bone resorption and increase intestinal calcium absorption (#16). Most calcium is absorbed in the small intestine and moderately in the large intestine through the paracellular pathway which is thought to be predominant when dietary calcium is plentiful. In other situations, when dietary calcium is not abundant, the vitamin D-dependent transcellular pathway takes over for calcium absorption (1). Therefore, vitamin D is also a crucial component in the diet and through sunlight as it aids in the absorption of calcium in the intestine. The deficiency of vitamin D is directly linked to bone health and is branded by inadequate mineralization, or demineralization, of the skeleton (1). In children, vitamin D deficiencies can lead to rickets which is characterized by inadequate mineralization whereas in adults, it can lead to osteomalacia which is a mineralization defect in the skeleton (1). Although the absorption of calcium is crucial, it cannot be absorbed if it is not present in the system to begin with. Calcium is vital for normal growth and development of bones and peak bone mass will not be reached without it. Due to this understanding, it is obvious why hypercalciuria poses an issue, but it can also be observed that if calcium is showing up in the urine, it has been absorbed, made it into the blood, and the kidneys filtered it. Or, the calcium could come from bone breakdown. When calcium shows up in the feces, then it wasn’t absorbed and other intestinal problems should be given attention. Without protein, a great deal of calcium intake would be depleted. Therefore, further research is necessary to determine the actual level of protein intake that will not yet cause elevated urinary calcium, but will provide enough calcium for appropriate absorption and use.

The studies that have been performed on this topic are difficult to completely accept because of the possible downfalls in research. Specifically in this case, illegitimate measurements could be at stake. A great deal of the population tested may be deficient in protein to begin with, so the increase in dietary protein simply brings the participants up to par with their necessary protein intake rather than actually measuring the effects of a high protein diet. Simultaneously, higher protein intakes are affecting calcium retention most when the calcium intakes are also inadequate (2). All participants need to be thoroughly examined in regards to both their past and present standards of dietary health prior to experimentation. Without these preliminary observations, results may not be reliable. It is also understood that not all results of dietary changes to the body are immediate. Some of the tests performed on this subject matter could be a set up for lifestyle change with effects seen later on, rather than instant answers. For this reason, the bed-rest model and space flight are of large interest because under these conditions bone loss is accelerated in healthy people, so dietary effects can be observed sooner than they would be in a healthy ambulatory population.

4 The Effect of Dietary Protein on Bone

Dietary proteins do not only directly affect bone, but rather affect many other factors that then relay to bone. As mentioned previously, the bone is formed by three cell types; osteoblasts which are responsible for the mineralization and formation of bone, osteoclasts which are responsible for the shaping and remodeling of bone, and osteocytes which are the most abundant cells in the adult skeleton (4). One less frequently referenced study found connection between polyphenols, found in fruits, vegetables, and grains, and osteoblast metabolism. Although this is not an effect of dietary protein, it is an example of the many other possible factors that contribute to the upkeep of BMD. Another example is found in the IFG-1, a factor that exerts positive activity on skeletal development and bone formation that is enhanced by dietary protein (5). The Insulin-like growth factor 1 acts as a mediator between bone formation and resorption, or breakdown, because although it is not a direct effect of protein, its function is amplified in the presence of protein to promote mineralization (3). These cases show positive effects on bone through factors that are not as commonly voiced.

One factor that is quite commonly coined as negative is the acid-base balance in bone that high protein in the diet often times alters. Epidemiology backs the assumption that a high acid load in the diet adversely influences bone (3). The maintenance of acid-base homeostasis is crucial for all living organisms and is generally based mainly on the diet (15). Net endogenous acid production (NEAP) is the amount of acid production from foods consumed in the diet which is of our concern because extraneous acid production in the body accelerates bone demineralization. The substances that humans eat both generate and consume protons and in turn the average adult on a Western diet will generate 1 mEq per kg body weight of acid each day (17). As a result of more or less acid being consumed in the diet, systemic acidity will be greater or less respectively. Increased intake of dietary protein will of course boost the level of acidity. Just as throughout life the bones will decrease in strength and size, the overall renal function also decreases with age and along with that goes the inability to excrete acid to the full potential and therefore the older a person gets, the more acidic their system will be (17). As we understand, osteoporosis has a more typical onset in the elderly population where this increased acidity of their system will be working against the maintenance of BMD already without any added protein in the diet to increase acidity further. Unfortunately, only a very small amount of dietary acidity will have a large impact over time (17). The following diagram depicts the regulation of acid-base homeostasis which is the basis of issues with acid-base balance. When protein in the diet is increased and the acid-ash levels are then increased, this regulation is easily sent off track.

Characteristic No Hip Fracture Hip FractureAge (y) 61.4 63.2

Weight (kg) 69.3 63.9Height (cm) 162.8 164.3Smoker (%) 14.5 13.6

Estrogen User (%) 12 5Physical Activity (%) 28 23

Nutrient Intake No Hip Fracture Hip FractureTotal Energy (MJ) 7.53 7.34

Protein (g/MJ) 10.85 10.11Animal Protein (g/MJ) 7.96 6.96

Vegetable Protein (g/MJ) 2.92 3.15Carbohydrate (g/MJ) 28.85 30.54

Dietary Calcium (mg/MJ) 111.93 105.93Dietary Vitamin D (IU/MJ) 34.06 33.53

Phosphorus (mg/MJ) 174.33 167.83Alcohol (g/MJ) 0.55 0.45

The Effects of Various Characteristics and Protein Intake on Hip Fracture

5 The Effect of Dietary Protein on Bone

Regulations of the body can all be easily interrupted, but the type of body observed, such as the elderly or post- versus pre-menopausal women, will also create a great difference in the ways they react to changes. As stated previously, the downward slope of bone demineralization gains speed with aging. Therefore, studies on a younger population that is not already experiencing bone loss before increasing dietary protein will exemplify different results than an older population with already decreasing BMD. Pre- versus post- menopausal women also exhibit unequal results regarding bone loss. Post-menopausal women are of an older age already, but also the rapid decrease in estrogen causes an increase in bone loss. It has been recognized that long-term estrogen supplementation reduces bone loss and fracture risk for the 7-10 years following menopause, but this result may have little effect on women over the age of 75 (16). It must also be taken into consideration that not all women participate in estrogen supplementation through or following menopause. Multiple studies done on post-menopausal women are analyzing the contributions protein in the diet make to the likelihood of hip fracture. Munger et al. performed a survey-conducted study termed “The Iowa Women’s Health Study” in which women ages 55-69 were selected randomly and asked to participate. The data collection consisted of a food-

frequency questionnaire, reproductive characteristic assessments, height and weight, measurements of the wrist, waist, and hip, as well as the women’s smoking histories and level of physical activity (16). With all of these factors considered, it was found that women with consecutive hip fractures had a lower intake of high-protein foods and fractures also increased with greater intake of carbohydrates (16). The numerical data can be found in the following table.

6 The Effect of Dietary Protein on Bone

Conclusions found were that women with the subsequent fractures were indeed older than those without them. Women who weighed less, were taller, and had leaner characteristics had a higher recording of fractures, and those who were less physically active and used estrogen less experienced hip fractures (16). Based on this gathered information, the bones greatly benefit from increased protein in the diet after menopause. Although many other repercussions appear after increasing protein consumption which could be negative in other systems of the body, such as increased urinary calcium and/or throwing off the acid-base balance, a higher protein intake is necessary for post-menopausal women for the highest benefit of BMD. It will not necessarily increase BMD, but it will aid in the slowing down of resorption, especially after the estrogen levels plummet.

Just as increased dietary protein is found to be beneficial to bone in many circumstances; it can also be harmful in others. Due to this accumulated knowledge it can be concluded that more specific research is necessary on the topic. It is possible to come to an absolute answer regarding one setting at a time, but finding a yes or no answer to whether or not increased dietary protein will positively affect bone density in general is too broad of a question. The various situations previously discussed explain the immense amount of other contributing factors to the upkeep of bone than solely protein. Concerning space flight increased dietary protein is most likely favorable when it is correctly balanced with all other necessary nutrients. Calcium, vitamin D, phosphorus, and all other macronutrients must also be in line so that increased protein levels do not override other systems of the body. The ARED exercise device also makes a great contribution to keeping BMD up during flight. However, all credit cannot go to either the ARED or nutritional consummation. The most constructive way to protect the bone from excessive demineralization in harsh environments is to find an agreeable balance between diet and exercise. Likewise for the general public, keeping bones healthy does require proper nutrition, but must also be accompanied by moderate exercise. The body operates at its’ full potential when getting the correct amount and types of fuel. Protein in the diet is absolutely essential. Increased protein to the point that it dominates all other nutrients is most likely not the best alteration, however more research is imperative. When considering how much to increase protein levels, all of the preceding information given must be taken into consideration including one’s environment, previous health conditions, current health conditions, age, pre- versus post-menopausal, and many more. For the sake of this research, it is understood that increased dietary protein does indeed have an effect on bone, but whether that effect is positive or negative will fluctuate. Therefore, the main necessity of the body for optimal bone health is a harmony of all macronutrients including protein, carbohydrates, and lipids, as well as all micronutrients including vitamins and minerals. This balance can and should be altered according to individuals’ needs, but overall this conclusion is suitable.

Acknowledgements

Sara Zwart, Scott Smith, NASA’s Nutrition Biochemistry Laboratory, PubMed resources

References

1. Cashman KD. Diet, Nutrition, and Bone Health. J Nutr. 2007 Nov;137(11Suppl):2507S-2512S. Review. PubMed PMID: 17951494.

7 The Effect of Dietary Protein on Bone

2. Massey LK. Dietary Animal and Plant Protein and Human Bone Health: A Whole

Foods Approach. J Nutr. 2003 Mar;133(3):862S-865S. Review. PubMed PMID: 12612170.

3. Thorpe MP, Evans EM. Dietary Protein and Bone Health: Harmonizing Conflicting

Theories. Nutr Rev. 2011 Apr;69(4):215-30. doi: 10.1111/j.1753-4887.2011.00379.x.

Review. PubMed PMID: 21457266.

4. Trzeciakiewicz A, Habauzit V, Horcajada MN. When Nutrition Interacts with

Osteoblast Function: Molecular Mechanisms of Polyphenols. Nutr Res Rev. 2009

Jun;22(1):68-81. Epub 2009 Feb 26. Review. PubMed PMID: 19243669.

5. Bonjour JP. Dietary Protein: An Essential Nutrient for Bone Health. J Am Coll

Nutr. 2005 Dec;24(6 Suppl):526S-36S. Review. PubMed PMID: 16373952.

6. Kerstetter JE, O'Brien KO, Insogna KL. Low Protein Intake: The Impact on

Calcium and Bone Homeostasis in Humans. J Nutr. 2003 Mar;133(3):855S-861S.

Review. PubMed PMID: 12612169.

7. Heaney RP, Layman DK. Amount and Type of Protein Influences Bone Health. Am J

Clin Nutr. 2008 May;87(5):1567S-1570S. Review. PubMed PMID: 18469289.

8. Dawson-Hughes B. Interaction of Dietary Calcium and Protein in Bone Health in

Humans. J Nutr. 2003 Mar;133(3):852S-854S. Review. PubMed PMID: 12612168.

9. Jesudason D, Clifton P. The Interaction Between Dietary Protein and Bone

Health. J Bone Miner Metab. 2011 Jan;29(1):1-14. Epub 2010 Oct 26. Review. PubMed

PMID: 20972896.

10. Bonjour JP. Protein Intake and Bone Health. Int J Vitam Nutr Res. 2011

Mar;81(2-3):134-42. Review. PubMed PMID: 22139564.

11. Ilich JZ, Kerstetter JE. Nutrition in Bone Health Revisited: A Story Beyond

Calcium. J Am Coll Nutr. 2000 Nov-Dec;19(6):715-37. Review. PubMed PMID:

11194525.

12. Ginty F. Dietary Protein and Bone Health. Proc Nutr Soc. 2003

Nov;62(4):867-76. Review. PubMed PMID: 15018487.

13. Darling AL, Millward DJ, Torgerson DJ, Hewitt CE, Lanham-New SA. Dietary

8 The Effect of Dietary Protein on Bone

Protein and Bone Health: A Systematic Review and Meta-Analysis. Am J Clin Nutr.

2009 Dec;90(6):1674-92. Epub 2009 Nov 4. Review. PubMed PMID: 19889822.

14. Martínez-Ramírez MJ, Delgado-Martínez AD, Ruiz-Bailén M, Fuente Cde L,

Martínez-González MÁ, Delgado-Rodríguez M. Protein Intake and Fracture Risk in

Elderly People: A case-control study. Clin Nutr. 2012 Jun;31(3):391-5. Epub 2011

Dec 17. PubMed PMID: 22182947.

15. Poupin N, Calvez J, Lassale C, Chesneau C, Tomé D. Impact of the Diet on Net

Endogenous Acid Production and Acid-Base Balance. Clin Nutr. 2012

Jun;31(3):313-21. Epub 2012 Feb 18. PubMed PMID: 22342140.

16. Munger RG, Cerhan JR, Chiu BC. Prospective Study of Dietary Protein Intake and

Risk of Hip Fracture in Postmenopausal women. Am J Clin Nutr. 1999

Jan;69(1):147-52. PubMed PMID: 9925137.

17. New A, Susan. Acid-Base Homeostasis and the Skeleton: Is there a Fruit and Vegetable

Link to Bone Health?. Centre for Nutrition and Food Safety, School of Biomedical and

Molecular Sciences, University of Surrey, Guildford, Surrey GU2 7XH, UK. 291-306.