Whey protein can be a fantastic supplement on-plan. It creams up your shakes and smoothies with its slimming powers. Just remember, Trim Healthy Mama is about balance. We don’t want you to sub protein powder at every meal for whole protein sources. Enjoy whey protein daily, if desired, but it should not be breakfast, lunch, and dinner. Gelatin and collagen are also excellent protein supplements.
"Protein Powder - Friend or Foe?" by Serene Allison
What is truth and what is scam when it comes to the popular
(and much fussed about) topic of protein supplements, especially whey protein?
Out of all the myriads of brands, the different processes involved, the slogans and promises made, how do we figure out the good from the bad? Are certain money hungry companies sweet talking us into products with a high price, but a low reward? Directly after Trim Healthy Mama came out we had a lot of questions directed to us about this subject. I knew we had to get to the very bottom of it, especially as it became obvious that one day we’d need to release our own Trim Healthy Mama brand of whey protein. We’d recommended lots of products, in the book, but to our dismay we had no control when companies changed ingredients (not always for the better) and changed the way they processed many of the foods, that we recommended on-plan. We had to come up with our own line of pure stevia and sweeteners and knew it would be the same with whey protein. That day arrived and we now have the Pristine Whey Protein Powder.
Concerning this article, I buried myself deep into the research and I’ve tried to peel back the hype. Now that I’ve come up for air, I want to share my thoughts with you.
First, let’s tackle the subject of whether a protein powder is even necessary.
Can you do the Trim Healthy Mama plan without it? Absolutely – yes! If your budget, or dietary beliefs, prevent you from purchasing whey protein – don’t let that deter you from staying on-plan. Stick around! We know what it is like to struggle financially, to the point where there is literally no room left for supplements, after basic groceries. And, I know what it is like to wrestle with the idea of certain foods not fitting into your own peaceful paradigms. But, lately there is so much confusion on what is natural versus what is processed. It is important to logically decide whether the idea of a powdered supplement can fit into a “whole foods” lifestyle or not. We can all come to different conclusions and still be friends, but I’d like to share mine.
One can argue that we can get all our dietary protein needs from a hunk of real meat, a piece of fish, or a bowl of cultured dairy. True, wonderful whole food forms of protein, like these, are the basis of the Trim Healthy Mama plan and thankfully we get plenty of ’em! If isolated whey protein was the sole form of protein encouraged, we freely admit that would not be a balanced approach. But, we can also get our daily requirement of vitamin C from greens, peppers, and lemons, etc. To obtain our vitamin D, we can choose to sit outside on a sunny afternoon. No, we do not need supplements to fulfill these needs to survive, but in my mind – supplements can help us do something more. They can help us THRIVE.
If I have a bad cold, I will not only reach for a nice hot drink with fresh squeezed lemon, I will also take high doses of pure vitamin C in supplement form. During the winter, I am very diligent about taking a vitamin D3 supplement, as I do not see enough sun. Now I’m probably going to ruffle some feathers, but here goes – I know it might be more of a “Paleo” or “Primal” or “Traditional People Group” approach to sit on some Norwegian Sea-side and suck out the livers of stinky cod fish to get extra vitamin D, if I am lacking, but I’ll skip that – thanks. God has created us in His image, with intelligent design. Therefore, as children of God, we have an incredible capacity for creativity. Processing the cod fish, obtaining the isolated pure oil, and bottling it for concentration and ease for consumers is a smart move. Just sayin’… and just gettin’ feisty about it!
Extracting precious, essential oils via distillation to capture the healing essence of herbs and flowers is using our God-given ingenuity. In the book of Genesis, God told man to have dominion over the earth and the things on the earth. We should not go to the other extreme and rape the earth of its storehouses of abundance or disrespect the balance God has set up in nature, but being wisely creative with the raw materials He has given us is “natural”.
Grok (the Paleo caveman diet hero), is no relation to me. I enjoy many healthy foods and supplements advanced from intelligent science. I am not blind to the fact that there are money-hungry and morally starved companies (and industries) that do not care for consumer health or for preserving the integrity of the foundational ingredients used. Yes, there are fan-dangled products available today that do cause toxic chemical abuse to our bodies. But, just because some music, art,or literature can be marred with dark and evil undertones does not mean that we say all music is out! Let’s not throw the baby out with the bath water.
I say all this to lead back to the question about necessity of whey protein supplementation. No, it doesn’t come out of a cow like that. But, neither does wine pour straight out of a grape or butter just appear from the Jersey’s udder. How about sea salt? You wouldn’t like to drown your nice steak with a cup of ocean water to give it a good salty flavor, would you?
Many natural foods, and even ancient foods, described in the Bible – like simple salt, need to be harvested. Even ancient, artisan staples like sprouted breads and aged cheeses have to undergo procedures. A roasted coffee bean that was once upon a time a little red berry or the steeping and fermenting of tea are nutrient releasing steps. Manufacturing processes are not all degenerative. Some help release the treasure locked inside the food like certain precious gems that are trapped inside the rock. The cashew nut is found inside of a poisonous pod atop an inedible fruit.
So, yes, whey protein powder has undergone a process. But, in my mind, that is okay as long as it is a process with integrity – one that maintains as much nutrition as possible. No, you don’t need whey protein to survive, but it can be a helping hand on your health journey and a great tool for allowing you to become or stay trim. It’s not the cake, but it is the icing. And, the icing just kicks everything up a notch. I will take my slice with the icing on!
The Slimming Effect:
The metabolic pathway for the digestion of protein releases a hormone in your body called glucagon. This hormone is a fat stripper. It literally signals your body to break down your adipose fat tissue. This is why whey protein has been touted as a slimming aid. It amps up glucagon in your body.
Protein also slows the absorption of sugar from the blood stream. As you know, this is also a weight loss plus and a large focus of our Trim Healthy Plan. The body actually burns more calories when digesting protein rich foods and your thermogenic temperature rises in the process. Put simply, that just means your metabolism fires up.
Well, I’ll just eat some meat!
Yes, this can all happen by eating a protein rich hunk of meat (and you get to eat plenty of meat on our plan), but hunks of meat are not always a quick and easy option. Chugging a superfood smoothie enriched with whey protein is far easier during a “busy to do list day”. Whey protein can also be a lighter calorie option for a protein rich snack or dessert. We should not be afraid of calories, but we don’t want to make every meal calorie heavy. We need those all-important caloric change ups and whey protein can help us do that. Whey also contains special goodies for your body that meat alone does not contain.
Aside from its weight loss merits, whey has many talents that can boost your health and strength. It supports muscle growth and recovery, immune function, energy levels, digestive health, mood, and appetite reduction.
Whey protein is among nature’s richest sources of growth-promoting Branched Chain Amino Acids (BCAA), such as lucine. It also contains important sulfur-containing amino acids that are needed for tissue repair and recovery. Beta-lactoglobulin, in whey, increases our absorption of fat-soluble vitamins and prevents the breakdown of muscle tissue. Why is that so important? Because, after the age of 20, without adequate protein intake and strength building exercise, the average person will lose a half a pound of muscle mass per year. That significantly adds up over time and contributes to insulin resistance, which leads to the common “middle-aged spread”.
Whey protein also spares glycogen stores (the energy of your muscles), during exercise, due to its high levels of Branch Chain Amino Acids. These amino acids are essential in that they must be obtained from the body, and are the ones needed by the body, during exercise – to produce energy. Low levels of these amino acids cause fatigue. Supplementing with whey is therefore a wonderful way to pep up!
Whey is very rich in Alpha-lactalbumin. This wonderful protein comprises over 30% of the protein fraction in human breast milk – making it the most important protein fraction for a newborn baby. Alpha- lactalbumin has the highest bio-availability and has the greatest efficiency of any other protein fraction. Studies on Alpha-lactalbumin indicate that it is one of the most effective forms of nutrition that fits the requirements for overall growth and development. It is absorbed easily by the body and provides an anabolic environment for strength and muscle growth.
Alpha-lactalbumin is nature’s highest source of an essential, and often missing amino acid, called Tryptophan. Tryptophan is needed for the manufacture of our brain chemical, serotonin. Proper serotonin levels help keep our appetites in check, regulate our sleep patterns, and improve our mood – especially under stress. Serotonin is known as the happy hormone. Who doesn’t need more of that?
Have you heard of glutathione? It is our body’s most effective immune builder and antioxidant. It is in this power-packed Alpha-lactalbumin that we find the treasure cysteine, which is the direct precursor to glutathione. So, when looking for a whey protein supplement, we must be diligent about searching for one that contains the highest levels of un- denatured Alpha-lactalbumin – to help support our immune systems.
Protein Peptides Control Hunger:
There are still plenty more goodies in whey and we must mention the low molecular weight protein peptides Glycomacropeptides or GMP’s. These wonderful, little peptides stimulate a hormone called cholecystokinin (CCK) that can control our hunger responses. CCK signals to the brain a sense of satiety. A rise in this hormone is always followed by a large reduction of food intake. And, thankfully whey allows that to happen! Whey protein GMPs are shown to elevate CCK production in human studies by 415%, within only 20 minutes post ingestion. Awesome!
Lactoferrin – For Your Gut and to Prevent Disease:
Also in whey’s list of talents is a very small protein called lactoferrin, which has the ability to bind strongly with iron. This is important because free-flowing iron, in the body, causes microbial growth and excess free-radicals. Excess free-radicals lead to disease. Lactoferrin is an excellent anti-microbial scavenging agent. It also inhibits bacteria from attaching from the gut wall, as well as showing ant-viral behavior
Lactoperoxidase is another protein fraction in whey that shows powerful microbial action, but in a different way than lactoferrin. It destroys a wide range of microorganisms and viruses by stimulating a series of enzymatic reactions.
A properly prepared whey protein will also provide a balanced source of the electrolytes calcium, phosphorus, and sodium that will support bone maintenance.
Forms of Whey Protein – the Good, the Bad, and the Ugly:
Not all the powders available provide our body with those health goodies we just discussed. There is rampant confusion over which types of protein powders are best for our health. Some people end up buying the most expensive options, hoping they will be more pure and less processed. That can be a costly trap.
There is a lot of hype pushed on the consumer, from all the different companies, peddling their brands. So, what am I doing here? LOL! Well, I wrote most of this article before we ever decided to launch our own brand of whey protein powder and posted it up on Facebook in pieces. I’m just shining this article up for you now – I was just as passionate then about uncovering the truth as I am now.
Don’t Buy Concentrates (WPC’s):
I think it’s best to start with what to avoid in a whey protein powder. WPC stands for Whey Protein Concentrate. It will state that on the container. These are the protein powders that are cheapest to make. In recent years, they have flooded the market place. Sadly, most of them have been processed using excessive heat. This damages the delicate protein fractions by irreversibly unfolding the amino acid chains and configurations and making them much less biologically active and even harmful.
WPC’s have less protein, per serving, and contain some fat and lactose (milk sugar). Healthy fat is awesome, but not the fat present in this lowest grade of all whey protein supplements. The drying process is usually a harsh process and oxidizes the cholesterol and fat that are still present. Oxidized cholesterol is dangerous to your health. Since lactose is not filtered out either, this sugar (when harshly heated) is not healthy. It is also problematic for people who are lactose intolerant.
These inferior whey products are usually referred to as WPC 80%. This powder is the stuff that is pumped into the chemically sweetened energy bars and cheap tubs of powder you would find at your local grocery store. Make sure to read your labels! If your brand is using a concentrate it is probably so the company can make a cheaper product and a bigger buck for their pocket.
There are very health conscious companies who use concentrates – simply because they want to use the least amount of processing possible for a more bio-active protein. They use the lowest amount of pasteurization, required by law, and use careful drying measures. Some of these companies make their protein powder from milk instead of the whey already pasteurized from cheese making. This means they get to say they only use one process instead of two.
My problem with these brands is that legally there is not one whey protein powder allowed to be sold in America that has not been pasteurized. By law, all commercially sold dairy products must be pasteurized (except by personal hand-to-hand sales, in some states). All whey protein powders have to be either pasteurized using the vat method (145 degrees for 30 minutes) or by the high-temperature short-time method (HTST) (161 degrees for 15 seconds). So, all of these far more expensive and ultra “goody-goody, never put a drop of anything slightly processed into your body… or you’ll die a slow and painful death” companies, selling high cost WPC’s, are promoting a lie. No matter what special wording they use (to convince themselves, or you, about their product being raw and unaltered), the fact remains it has been heated! IN THE USA, no protein powders are raw! End of story.
Some use the term “flash pasteurized”, for the 15 second method, to make it sound better. Others who adopt the lower (not by that much), but much longer method, use terms like “cold processed” or “special low temperature unique pasteurization methods” to try and make you think that their company is doing something a cut above the rest. The only “cold” process I see here is in the cold hard truth that their proprietary process is just one of the normal and ordered legal options of our government. You can do a google search and pull up oodles of protein powders claiming they are raw. Nope! Do we call milk that we buy at the store raw? No, because it is not. It is pasteurized and has undergone either the slow or fast methods that all whey powders have undergone. Look up on the internet how to pasteurize your own milk at home. You’ll be offered the two standard methods on your cook stove –either slow at 145 degrees or fast at 161 degrees. Why are companies trying to pretend these old, standard pasteurization methods are new, special ways to keep whey raw? It’s baloney!
Don’t Fall For Scams!
Whey protein is very fragile and no matter which methods are used there will be an altering of the protein molecules to a very similar extent. Thirty minutes does sound like a long time compared to the faster, higher version of 15 seconds (after which it is immediately cooled to stop the pasteurization process). But, this can be likened to whether I sauté my eggs at medium heat for only a couple of minutes or gently poach them, for a much longer time, in a pan of boiling water that has been removed from its heat source. Either way, I will eventually get firm, opaque egg whites or I wouldn’t want to eat them.
What upsets me is that these companies do have knowledge of basic science, but this is overpowered by their knowledge of how to make a golden profit. Instead of telling the truth, and selling their product for what it is worth, they sell it for exorbitant prices under false disguises. Let me lay down the facts… there is no whey protein powder, or other source protein powder, sold on the American market that has never been subject to the heat I have described. Many months of research have assured me of this.
You can’t tell I’m steamed about this can you? Well, charging $50-$80 for a tub of protein powder, through misleading statements, is daylight robbery – even if the cows were grass-fed! The fat and milk sugar is still in there. It is definitely heated to the point of pasteurization, even if not as harshly as the cheapo brands. Later in this article, I’ll share why oxidized fats, in powder form, are different in your body than cooking a steak and eating it.
A Better Whey:
Do not despair for your lost dream of an effective protein whey supplement. The biological effectiveness of whey, in a carefully processed concentrate (or preferably in the form I am yet to share with you), is still very positive – even under human testing and research data. The microfractions still greatly benefit the body in an anabolic, immune-boosting and weight-reducing way.
Is there another viable option? Yes, Ma’am! Beyond concentrates is the next level of purification – a whey protein in an isolate form, or (WPI). The isolation process will remove the fat, cholesterol, and lactose. It leaves a product that is a more potent form of whey protein. It also removes the bloating and indigestion problem that comes with concentrates that still contain their lactose and newly formed oxidized fats. We’ll also talk about the myth that proteins must be paired with fats, as you read further.
Look For CFM:
Cross-flow micro-filtered (CFM) whey protein isolate is the most undenatured form of whey available on the market, since the denatured particles are actually removed. CFM whey’s native protein structures remain intact, to a large degree, by a cold- processed isolation method using ceramic micro-filters. It costs a lot more for companies to purchase this form of whey than the cheapo isolates, but whey protein selection is when quality really counts.
During this microfiltration process, undesired particles like lactose, cholesterol, and any de-natured fractions are successfully removed. Micro-filtered whey isolate is the protein with the highest biological value of all proteins. It retains high percentages of immunogloblins and minerals and is higher in calcium than other whey protein supplements. If anywhere on your supplement you see CFMr, you hold in your hand the real deal. I was ecstatic when we as a Trim Healthy Mama company were able to source a very pure and wonderful tasting CFM isolate form of whey to pass along to you. Just being transparent here, the whey protein we are providing gives our company the smallest profit of any of our items launched, but we believe in purity and affordability so we have done our best to keep this pristine product as budget friendly as possible. We hope you love it as much as we do.
Beware of Hydrolyzed:
If you ever come across the term “hydrolyzed” when it comes to whey protein – it is best left on the shelf. Hydrolyzed whey is processed using high heat and very acidic conditions. Hydrolyzing whey creates smaller peptides or bonds that link the amino acids. This is done to make an already naturally quick, absorbable protein even quicker to absorb. Many sports nutritionists and dietitians believe that this idea may backfire on our health. They question the wisdom of creating a protein that absorbs faster than your body can healthfully process.
Hydrolyzing also denatures most of whey’s protein molecules and reduces the effectiveness of the protein. This kind of whey processing makes an extremely bitter product. Yet, for all its downsides, it is still one of the most expensive whey forms on the market.
Can You Over-Do Protein?
Another question that comes up – won’t I become overly acidic, and ruin my kidney’s with too much protein?
The rise of the popular low-fat /vegetarian diet era in the 1980’s and 90’s informed us that high amounts of dietary protein (especially animal based protein) is dangerous to our kidneys.
Modern research proves this wrong. Protein does not damage healthy kidneys, even when consumed in amounts higher than what would be considered normal. It is only people with certain kidney diseases, and those with compromised renal function, that cannot handle a lot of protein. This is due to their kidneys not functioning properly, not due to the protein itself. Healthy kidneys filter out the waste products of protein metabolism just like they do slews of other things. It’s what our kidneys were created to do.
Saying that protein is bad for your kidneys is like saying eggs, peanuts, or bread made with healthy, sprouted-grain are bad for every healthy individual. Just those allergic to those foods may have severe reactions to them – not everyone.
How Much Is Too Much?
A recent study revealed that when body builders consumed up to 1.3 grams of protein per pound of body weight, kidney function was not impaired. (Poortmans and Dellalieux, 2000). Dr. Clay Hyght, a chiropractor and health and fitness expert, backs this up with his personal experience. He is a big guy, who works out hard, with lots of muscle. He lives on a diet where he consumes 375 grams of protein a day – an enormous amount! In a transcribed radio interview, he talks about how his kidneys are in perfect condition after many years of eating this way.
In an older study, conducted with female rats, kidney function actually seemed to be improved with a high protein diet. (Sterck et al. 1992). Other studies showed protein intakes, even up to 1.27 grams per pound of body weight, did not cause kidney damage.
Let’s say you are 150 pounds. While our Trim Healthy Mama plan is protein-centered, eating the protein rich meals and snacks we advise will not generally bring your daily protein intake to even 150 grams. This would only be one gram per pound of body weight. Some days, it’s possible you might eat that much protein, but there won’t be harm in it. Shake off your worries about this! I consumed whey protein almost daily, in my last two pregnancies, and these were my healthiest pregnancies ever. Some rat studies have revealed that when rats were given a full 80% protein diet, for long periods of time, none of the rats showed any signs of kidney damage (Zaragoza et al. 1987).
I like the way Dr. Clay Hyght explains protein in one simple sentence. He says, “Protein builds stuff!” It doesn’t just build muscles. It builds hair, skin, nails, tendons, ligaments, blood plasma, organ tissue, and almost 2000 enzymes. Higher protein diets also revealed higher bone mineral content, instead of depletion. (Cooper et al. 1996, Freudenheim et al. 1986).
Together, resistant exercise (like we advise) and ample protein intake, lead to even better increased bone mass. This is so important for us mamas, especially as we get older. Our Trim Healthy plan includes ample protein, not these huge amounts many body builders consume, but enough for many of our mamas to say things like: “My nails are finally growing!”, “My skin is smoothing out!”, or “I have less cellulite on my thighs!”
Studies That Promoted the Scare:
A few original studies, that contributed to the scare about high protein consumption, demonstrated that when dietary protein was raised to 140-225 grams, per day, excess calcium was lost through the urine at a faster rate than normal (Hegsted et al. 1991). This seems like an opposite result than the other research I am citing here. However, in these studies, calcium and phosphorus intake was strictly restricted and not allowed to increase in proportion to the level of protein that was taken in.
Dietary, whole food proteins contain calcium and phosphorus. Protein supplements, that are lacking these minerals, are now usually fortified to correct this imbalance. Thankfully, our CFM whey protein isolate is NATURALLY high in calcium and contains a good amount of phosphorus, so there is no issue for concern.
In a natural scenario of high protein intake, without calcium and phosphorus restrictions, there is actually a positive calcium balance and there are no adverse effects on bone calcium content (Hegsted et al. 1991, Zigler and Filer, Jr 1996 National Academy of Sciences National Research Council 1991).
Of course, I’ m not recommending a diet imbalanced with any macronutrient. We certainly don’t have to consume upwards of 200 grams of protein per day! But, I do want to alleviate the erroneous fears fed to us by health gurus that spout vegetarian “cleaner colons through less animal protein” dogma. Eating plans, like ours, that lean heavily on protein foods also have a strong metabolism-boosting effect, due to the thermic influence of protein and its ability to prevent hunger. Protein-based metabolic increase is double that of carbs or fat! But, thankfully, you also get to eat carbs and lots of fat as a Trim Healthy Mama. We are not about leaving anything out!
Is a Lot of Protein Acid-Forming?
Yes, there are some protein foods that naturally leave an acidic ash in the digestive system, but that is a good thing! It is a part of keeping your body in a perfect PH balance. Being too alkaline is not healthy either. Why? Because God made them for us and called them “good”. Easy answer.
And we Trim Healthy Mamas do not need to worry about being too acidic, even if there was a reason for it. Our diet is rich in glucomannan – konjac root meals and snacks. The konjac root is known as the second most alkaline food on earth. Our diet is also filled with lots of leafy greens and non-starchy veggies, which are extremely alkaline. Perfect balance!
But, Wait… There’s Soy!
Some have written in to us concerned with the addition of soy lecithin in many whey protein powders. And, yes, currently it is in the Trim Healthy Mama Pristine Protein Powder. While I am the purist in the Serene and Pearl duo, and I do not believe in consuming soy as a food or a supplement, I don’t stress out over <1% wt soy lecithin in whey protein powder. It is so minimal. It is not required that it be listed as an ingredient. We are currently working with our whey protein source to provide us with a sunflower lecithin alternative. We will keep pushing, until we make this change, but I believe my health is not adversely affected, in the meantime.
Lecithin itself is a healthy supplement. Studies have shown that lecithin helps in the breakdown of cholesterol, to smaller particles, so they are less likely to build up in vein and arterial walls. It is also thought to prevent a fatty liver. It is rich in choline, which helps metabolize fats. It has been seen to be effective for weight loss, as it helps the body burn fats as a source of energy. But, it is unlikely you will receive many of lecithin’s benefits when consuming the infinitesimal amounts that are used in whey protein powders. We are talking micrograms for amounts. The lecithin, in protein powder, is used to help the powder mix more easily with liquids. It is also used to help facilitate uptake of amino acids, into muscle cells.
Soy has been reported to be a harmful and hormone-disrupting food, specifically soy protein. During the making of lecithin, from soy, almost all the soy protein is removed. Lecithin is actually extracted from soy oil. It would be almost impossible for soy to have an estrogenic effect from the amounts of soy lecithin in protein powder.
I enjoy my food purism. It’s the way I am wired, but I believe in doing all that we can for our health – without making a weird religion out of it. This is when it becomes an idol in our lives and we cannot even live happily in the times God has created us to live in. We need to make wise choices, but we do not need to obsess. I’ve had many years learning that lesson – the hard way. Making wise choices for our health can sometimes mean to give up the constant nit-picking and relaaaaaaaax.
I am only ranting on about this because I have too often fallen into this over-controlling pit of taking complete responsibility for the health of my family. Yes, we have to make choices that foster life and not destruction, but we are never in full control over our health. The buck stops with God. So, yes, I choose to eat organic or grass-fed when I can (sometimes I can’t). I choose to exercise and not forsake healthy sleep patterns, but I am not going to get stuck and distracted from the bigger picture by picking at the little nits all day. I do what I can. And, what I can’t control… I no longer stress over.
So, my advice in short – don’t go and cook with soybean oil or eat soy laden foods, but a bit of soy lecithin (in tinsy..insy…winsy…amounts), I don’t think it effects the big picture. After years of torturing myself with obsessing over small issues, I have realized the best thing for health and sanity is to take a nice, deep breath and give over every obsession to God. Having said all this, if you have an actual allergy to all forms of soy…even soy lecithin, by all means – you can try to seek out a protein powder that uses a sunflower-based lecithin. I have said a lot in one article, devoted to whey protein; however I felt the necessity to go down some “purist” trails that I am too familiar with. Hopefully, it will guide some kindred spirits to a happier and ultimately healthier road.
- Portmans and Dellalieux, 2000
- Sterck et al. 1992
- Zaragoza et al. 1987
- Cooper et al. 1996, Freudenheim et al. 1986
- Hegsted et al. 1991
- Hegsted et al. 1991, Zigler and Filer, Jr 1996 (National Academy of Sciences National Research Council 1991).