PT on the Net Research

Protein 101


The truth is, dietary protein is one of our most essential nutrients. The reason is because proteins contribute to key body functions like blood clotting, fluid balance, production of hormones and enzymes, vision, immune responses and cell repair. Protein is an important building block in bone, muscles, cartilage, skin and blood. Along with fats and carbohydrates, proteins are “macronutients,” meaning the body needs relatively large amounts of them. But unlike fat and carbohydrate, the body does not store protein and therefore has no reservoir to draw on when it needs a new supply. The good news is that most of the food we eat contains some amount of protein.

What is protein? Proteins are similar to carbohydrates and lipids in that each molecule contains atoms of carbon, oxygen and hydrogen. The major difference is that protein also contains nitrogen. The four elements carbon, oxygen, hydrogen and nitrogen are combined into a number of different structures called amino acids. Each amino acid possesses an amino group (NH2) and an acid group (COOH), with the remainder being different combinations of carbon, hydrogen and oxygen and in some cases sulfur. Join two amino acids together with a peptide bond and you are on your way to producing a protein. There are nine amino acids the body needs and must get from the food we eat. They are called essential amino acids. The other amino acids found in protein are called non-essential amino acids because the body is able to make them. The body is constantly breaking down proteins we eat into amino acids and then reforming the amino acids back together to form new proteins. The process is called protein synthesis. Fortunately, proteins are generally easy to digest because peptide bonds are broken when water is added back to the protein molecule. Proteins are the most structurally complex molecules known and in some cases include over a hundred amino acids. Each type of protein has its own unique structure and function.

Proteins are found in both animal and plant food. While all plant and animal products contain protein, the amount varies. Animal proteins are generally thought to be higher quality proteins because they contain all nine essential amino acids to support growth and life and because they are in much larger amounts. Most plants have a deficiency of one or more of the nine essential amino acids and are classified as incomplete proteins. Plant proteins can be combined and eaten together in proper combinations to provide complete proteins. The soybean is an exception. Soy is a complete protein and is comparable to animal protein.

Animal protein and plant protein have the same effect on health. It’s the protein package that’s likely to make a difference. A 6-ounce broiled Porterhouse steak is a great source of complete protein – 38 grams worth. But it also delivers 44 grams of fat, 16 of them saturated. The same amount of salmon gives you 34 grams of protein and 18 grams of fat, 4 of them saturated. A cup of lentils has 18 grams of protein, but less than 1 gram of fat. It is important to pay attention to what comes along with the protein in food choices. Animal sources of protein include poultry, meat, eggs, milk, cheese and dairy products. Breads, cereals, grains, vegetables, beans, nuts and fruits are examples of non-animal proteins. The general guidelines for protein intake are to get a good mix of proteins and balance carbohydrate and protein intake. A good mix of proteins will allow you to meet your protein goals without exposing the body to excessive amounts of saturated fats. A balance of protein and carbohydrates will keep you out of ketosis (using fats and protein for energy not carbs) and reduce excessive urinary loss of calcium and water.

How much protein does the body need? There is relatively little solid evidence on the ideal amount of protein in a diet. The Institute of Medicine recommends that adults consume 10-35% of total calories from protein. The RDA for protein is 0.8 grams per kilogram of body weight per day. This seems like a very low recommendation. The body loses about 23 grams of protein a day normally. Most medical professionals believe at least 1 gram of protein per kilogram of body weight is required daily for healthy adults. We have also established guidelines for increasing protein intake. Protein requirements are greatest during growth periods in children, adolescents and pregnancy. During these times, protein intake can be increased to 1.2 grams of protein per kilogram of body weight per day. For patients undergoing general surgery, it is recommended that protein levels be 1.5 grams per kilogram of body weight per day. For cases of multiple trauma, levels are between 1.5 and 2.0 grams per kilogram of body weight per day. Finally for burns and sepsis, protein levels are recommended at 2.5 grams per kilogram of body weight per day.

For years, athletes have ignored protein guidelines and consumed much greater amounts. Some quality research has demonstrated that RDA protein levels for those involved with athletics are inadequate and may impede recovery or limit muscle growth. Are protein requirements in athletes increased? The answer to that question remains an area of controversy. It is generally accepted that endurance athletes should consume 1.2 to 1.4 grams of protein per kilogram of body weight per day. Strength and power athletes are recommended to consume 1.4 to 1.8 grams of protein per kilogram of body weight per day. So, I guess the answer is a qualified yes. There is little evidence to support consuming greater amounts of protein because research has shown that even at 1 gram of protein per kilogram of body weight, strength trainers build muscle as well as those on twice the amount of protein.

Can you get too much protein? This is another area of contention. The emergence of high protein weight loss diets and the high protein intake of athletes have made nutritional scientists take a serious look into the issue. Here is what they have learned: Consumption of large quantities of processed meats such as hot dogs, sausages and deli meats has been linked to increased risk of type 2 diabetes, cardiovascular disease and colorectal cancer. These types of protein products should be avoided.

There is a controversy regarding high protein’s effect on bone health. Digesting proteins releases acids the body usually neutralizes with calcium and other buffering agents in the blood. Eating lots of protein takes a lot of calcium, some of which may be pulled from bone. A high protein diet for a few weeks won’t have much effect on bone strength. Doing it for a prolonged time could weaken bone. It is also clear that low protein diets are harmful to bones. We know protein is an important structural component of bone. It is incorporated into the organic matrix of bone for collagen formation, upon which mineralization occurs. By weight, bone tissue is 70% mineral (primarily calcium and phosphorous) 22% protein and 8% water. Dairy products are key to maintaining good bone health in high protein diets. They are unique sources of protein because their calcium content is high relative to their protein content, and they contain other supporting nutrients such as phosphorous, magnesium, zinc and vitamins A, D and K. High protein may increase urinary calcium excretion, but whether or not calcium balance is adversely affected is uncertain largely in part to the presence of these other dietary nutrients. Individuals who want to take higher levels of protein for extended periods of time should make sure their dietary calcium intake is 1200 mg per day and vitamin D intake of 1000 IU per day - at a minimum. The use of a calcium supplement can help facilitate this.

There is no good evidence that correlates the amount of protein intake to an increase in cancer risk provided processed meats are not a main protein dietary source.

To date, only one large, prospective study – the Nurse’s Health Study – has investigated the association between dietary protein and heart disease risk or stroke. In this study, women who ate the most protein (about 110 grams per day) were 25% less likely to have had a heart attack or to have died of heart disease than the women who ate the least protein (about 68 grams per day). Whether the protein came from animal or plants or whether it was part of a low-fat or higher-fat diet didn’t seem to matter. These results offer reassurance that eating a lot of protein doesn’t harm the heart. In fact, it is possible that eating more protein while cutting back on simple carbohydrates may benefit the heart. We still recommend choosing lean meats and low fat dairy products whenever possible and avoiding processed meats.

One surprising finding from nutritional research is that people who regularly eat nuts (an excellent source of protein) are less likely to have heart attacks or die from heart disease than those who rarely eat them. Several studies have shown a consistent 30-50% lower risk of myocardial infarction, sudden cardiac death or cardiovascular disease associated with eating almonds or walnuts several times a week. At 185 calories per ounce, eating nuts as a snack in addition to regular meals is not a good idea. But adding a few walnuts to a salad is a great idea.

Ongoing research has yielded inconsistent results on any role of protein and the development of diabetes. The amount of lean protein in the diet doesn’t seem to adversely affect the development of adult-onset diabetes.

Protein digestion releases nitrogen waste. This nitrogen waste from protein metabolism arrives at your kidneys in the form of “urea” and must be filtered out or it builds up in the body and is toxic. There is no evidence that high protein diets cause kidney damage in healthy individuals. However, if you have kidney damage or kidney failure, you will be advised to restrict your protein intake.

When is the best time to eat proteins? There is evidence that a high protein meal makes you feel full and satisfies your appetite longer. Since nutritional experts recommend a blend of protein sources, you will likely get your protein in more than one meal per day. There are no studies that indicate any specific benefits to how daily protein requirements are met. You chose what is best. It is more important that you get high quality proteins: lean meat, fish, legumes, nuts, poultry, soy and grains.

The bottom line is that for healthy individuals, there is no apparent harm in high protein diets provided you drink plenty of water and have adequate calcium intake. There is evidence that processed meats are the exception. They represent the only documented protein associated risk. There is very little science, however, that athletes require protein intake greater than 1.8 grams per kilogram of body weight per day under any circumstances.

References:

  1. Campbell,W.W., et al. 1995, Effects of resistance training and dietary protein intake on protein metabolism in older adults. Amer. J. of Physiology 268: E1143-53.
  2. Consolazio,G.F., Johnson, H.L., Nelson, R.A., Dramise,J.G., and Skala, J.H., (1975) Protein metabolism during intensive physical training in the young adult. Am. J. Clin. Nutr. 28:29.
  3. Dohm, G.L. (1985) Protein nutrition for the athlete. Clin. Sports Med. 3: 595.
  4. Feskanich, D., Willet,W.C., Stampter,M.J. and Colditz< G.A., Protein consumption and bone fracture in women. Am. J. Epidemiol. 1996; 143:472-9.
  5. Hu, F.B., Stampter,M.J., Manson,J.E., et al. Dietary protein and risk of ischemic heart disease in women. Am. J. Clin. Nutr. 1999;70:221-7.