Dr. Lee, the Healthy Professor

Protein Intake and Your Health

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Dietary intake of protein currently dominates the scientific and popular nutrition literature. The following review of recently published research is designed to help the consumer make informed choices. The first two reviews studied the effects of protein on bone mass density. The third study is a report on a “meta” analysis of the effect of protein on metabolic rate and appetite control. The fourth study reports on the relationship between protein intake and growth rate among children.

Protein Consumption and Bone Mineral Density

California Study

The role of dietary protein in osteoporosis has been controversial with previous studies having suggested both protection and harm. A California epidemiological study investigated the associations of total, animal, and vegetable protein with bone mineral density (BMD) and the association with calcium intake.

The diets of a group of 572 women and 388 men aged 55–92 years were studied between 1988 and 1992. Statistical review of the data showed a positive association between animal protein consumption, assessed by food frequency questionnaires and BMD. This association was statistically significant in women. For every 15-g/day increase in animal protein intake, BMD increased by 0.016 g/cm2 at the hip, 0.012 g/cm2 at the femoral neck (top of leg bone), 0.015 g/cm2 at the spine and 0.010 g/cm2 for the total body.

Conversely, a negative association between vegetable protein (although the food frequency questionnaire could not assess the quality of the vegetable protein) and BMD was observed in both sexes. Some suggestion of effect modification by calcium was seen in women, with increasing protein consumption appearing to be more beneficial for women with lower calcium intakes. The authors concluded that the study supports a protective role for dietary animal protein in the skeletal health of elderly women.

Promislow, Joanne H. E et al. Protein Consumption and Bone Mineral Density in the Elderly : The Rancho Bernardo Study. American Journal of Epidemioogy. 2002 155: 636-644.

Nebraska Study

A 2003 study at Creighton University investigated the associations of dietary protein intake with baseline bone mineral density (BMD) and the rate of bone loss over 3 yrs in postmenopausal elderly women. A group of 489 women aged 65–77 years were enrolled in an osteoporosis intervention trial. The associations of protein intake (as a percentage of energy) with baseline BMD and the rate of bone loss in 96 women from a placebo group were studied. The authors also examined the effect of the interaction of dietary calcium intake with protein intake on BMD.

The results showed that a higher intake of protein was associated with higher BMD. BMD was significantly higher in the spine (7%), midradius (forearm bone)(6%), and total body (5%) in subjects in the highest quartile of protein intake than in those in the lower 2 quartiles. This positive association was seen in women with calcium intakes > 408 mg/d. There was no significant effect of protein intake on hip BMD. In the longitudinal study of the placebo group, there was no association between protein intake and the rate of bone loss.

The conclusions drawn were that the highest quartile of protein intake (72 g/d) was associated with higher BMD in elderly women at baseline only when the calcium intake exceeded 408 mg/d. This level of calcium intake is well below the RNI so could not be considered a supplemented level. In the longitudinal study, no association was seen between protein intake and the rate of bone loss.

Rapuri, Prema B, Protein intake: effects on bone mineral density and the rate of bone loss in elderly women . American Journal of Clinical Nutrition, Vol. 77, No. 6, 1517-1525, June 2003

Protein Diets on Thermogenesis, Satiety and Weight Loss

A Harvard School of Public Health conducted a “meta” analysis of clinical, randomized studies on the effects of high protein diets on dietary thermogenesis (rate of metabolism), satiety, bodyweight and fat loss. They concluded there is convincing evidence that a higher protein intake increases thermogenesis and satiety compared to diets of lower protein content.

The weight of evidence also suggested that high protein meals lead to a reduced subsequent energy intake (or in other words “controls appetite”). Some evidence suggests that diets higher in protein result in an increased weight loss and fat loss as compared to diets lower in protein, but findings have not been consistent. The authors suggested that in dietary practice, it may be beneficial to partially replace refined carbohydrate with protein sources that are low in saturated fat. They also cautioned that although recent evidence supports potential benefit, rigorous longer-term studies are needed to investigate the effects of high protein diets on weight loss and weight maintenance.

Halton, Thomas L. The Effects of High Protein Diets on Thermogenesis, Satiety and Weight Loss: A Critical Review. Journal of the American College of Nutrition 2004 Oct;23(5):373-85.

Animal Protein and Growth

A recent Danish study investigated the associations between protein intake, serum insulin-like growth factor I (sIGF-I) concentrations, and height in 90 healthy 2.5 year-old children. (54 boys, 36 girls).

They found that the 10th, 50th, and 90th percentiles of protein intake were 2.4, 2.9, and 4.0 g / kg/ day, respectively and 63% was animal protein. Height (cm) was positively associated with intakes of animal protein and milk but not with those of vegetable protein. (quality of vegetable protein was not analyzed). The sIGF-I concentration was significantly associated with intakes of animal protein and milk but not with those of vegetable protein and the sIGF-I concentrations were positively associated with height. An increase in milk intake from 200 to 600 mL/d corresponded to a 30% increase in circulating IGF-I. This suggests that milk compounds have a stimulating effect on sIGF-I concentrations and, thereby, on growth.

Hoppe, Camilla. Animal protein intake, serum insulin-like growth factor I, and growth in healthy 2.5-y-old Danish children. American Journal of Clinical Nutrition 2004 Aug;80(2):447-52.

Further Reading

Daily Protein Requirements
Protein is the Answer
Benefits of Soy Protein
Soy Protein and Your Health: An Update on the Benefits
Soy Protein and Your Health: Dispelling the Myths
Protein Supplement - low-fat, non-GMO (non-genetically modified organism), biologically complete protein contains all 20 amino acids used in human metabolism, including the nine essential ones.