Figure 1: Leucine Appearance

400

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0. 60 120 180 240 300 360 420

Time (Minutes)

Adapted from: Boiric, Y. el. al. Slow and fast dietary proteins differently modulate postprandial protein accretion. Prac Natl Acad Sci USA (1997) 94: 14930-14935.

Finally, leucine balance, a measure of how much protein was actually retained in ihe body, was higher for the casein than for the whey. This is an important and often missed point when people examine this research: protein retention, measured in terms of leucine balance, was higher for the casein than whey. This suggests that decreasing protein breakdown, rather than stimulating protein synthesis per sc, might have an overall greater impact on the body's protein stores.

Based on this data, casein become known as a slow/anti-catabolic protein and whey was a fast/anabolic protein. The general recommendation became to consume fast proteins like whey around workouts or first thing in the morning (when there was a need to get blood AA levels up rapidly). Casein was recommended at times when an athlete wanted to stave off catabolism or needed a slow rate of digestion (such as before bedtime).

Related to this idea, some authors have argued that a casein/whey blend would impact on both protein breakdown and synthesis. The idea is that the whey will provide a rapid spike of AAs (stimulating protein synthesis) while the casein would maintain a continuous lower level of AAs (inhibiting protein breakdown); there appears to be some merit to this idea.

Preliminary data presented from the 2005 International Whey Conference suggests that a 50/50 mix of whey and casein might provide optimal results in terms ofgaining lean body mass with training (29). As above, the idea is that whey provides more of a stimulus to protein synthesis while casein limits protein breakdown.

The combination of the two, a stimulation of protein synthesis with a simultaneous inhibition of protein breakdown, would be expected to have the greatest total impact on body protein stores. Emerging data discussed in Chapter 8 supports this idea. It's interesting to note that bodybuilders have long consumed copious amounts ofmilk to gain muscle mass; milk contains roughly 80% casein and 20% whey, providing a mixture of both "slow" and "fast" proteins.

But are all of the above conclusions really warranted based on this single study? Some key factors to keep in mind are that the subjects had fasted overnight (which aflects protein synthesis and breakdown somewhat), were not training, only a single meal was given to measure the impact on protein synthesis and breakdown (no long-term measure of protein gain was made) and no other nutrients were given with the protein. Thus, it has limited relevance to whey or casein taken with other nutrients, in combination with training, or consumed after one or more meals have already been consumed. Further research has since examined some of those confounding variables.

When casein or whey are taken as part of a mixed meal, the difference between the two in terms of either AA appearance or metabolic effects becomes much less pronounced (30). In that study, while the mixed whey meal released AAs into the bloodstream somewhat more quickly, the mixed casein meal still had the edge on overall protein retention. However, as with the original study, the whey group received less total protein than the casein group: perhaps the casein showed greater total protein gain due to the greater amount of protein ingested, rather than any diffcrcnccs in digestion speed.

In a follow-up study, identical amounts of cascin or whey were given with carbohydrate and fat to both young and old individuals (31). The mixed meal containing whey provided a slight benefit for younger individuals compared to casein, and a much greater clTcct in older people. It's important to note that older individuals appear to respond differently to protein intake than younger people so extrapolation of this data is problematic at best.

Another study found that sipping whey protein made it act more like casein and actually had the greatest impact on protein synthesis compared to either whey or casein given all at once (32),| In that study, a total of 30 grams of whey protein was given in 13 discrete drinks (2.3 grams whey protein sipped every 20 minutes) over a period of 4 hours: protein synthesis was higher than in any of the other groups. This type of dosing pattern seems fairly unrealistic for most people. However, it did back up the idea that the big dilTerences between whey and casein were due to differences in digestion speed as mentioned above.

In all of the research cited so far, the researchers were measuring whole body protein synthesis and breakdown, not skeletal muscle synthesis and breakdown specifically. Although popular writers tend to assume that the effect is primarily being seen in skeletal muscle, its just as logical to assume that the proteins being synthesized were in the liver or gut; it turns out that different protein sources do preferentially impact on different tissues in the body.

For example, compared to milk protein, soy lends to be utilized to a greater degree by the gut and liver, providing fewer AAs to peripheral tissues such as muscle (33, 34). This may be due to the speed at which soy is digested (soy is also a fast protein) as well as the overall AA profile; proteins that are low in required AAs may be preferentially utilized by the gut. While maintenance of existing tissues is clearly important, athletes are ultimately concerned with the impact of protein on skeletal muscle in terms of synthesis or breakdown.

Related to this and to the issue of how training might alter the cffccts of fast versus slow proteins, recently either casein or whey were given one hour after resistance training; both had an identical elTect on muscular protein synthesis despite different AA digestion

profiles (35).

Another study found that both skim and whole milk consumed after training impacted positively on protein synthesis (with the whole milk having a slightly greater cffccl) so it may be, as mentioned above, that a mix of casein/whey is superior to either protein individually (36). Related to this, skim milk was recently shown to be superior to soy protein for supporting lean body mass gains with resistance training (37). Further data, discussed in greater detail in Chapter 8 supports the idea that a slow protein, or a mixture offast/slow proteins following training may be superior to a fast protein alone.

Absorption speed of other proteins

What about the digestion rate of other proteins? Unfortunately not much data exists although one researcher has collected data from a variety of studies to make a series of rough estimations on intestinal protein absorption rates (38). Please note that the

numbers below arc quite preliminary as some of the studies used fairly indirect measurements to estimate protein digestion rate (I've noted these with an asterisk). A summary of his results appears in Table 2.

Table 2: Estimated intestinal absorption rates of different protein sources

Protein	Absorption rai
Raw egg protein *	1.4
Cooked egg protein *	2.9
Pea protein	3.5
Milk protein	3.5
Soy protein isolate	3.9
Casein isolate	6.1
Whey isolate	8-10
Tenderloin pork steak *	10.0

• These measurements should be considered as the roughest estimates as the studies used indirect measurements of protein digestion.

Clearly there appear to be fairly significant differences in the speed of digestion of different proteins, but there seems to be somewhat of a problem with the above values in real-world terms. Assuming an average digestion rate of roughly six to seven g/h for protein and 24 hours to potentially digest the day's food intake, this would allow for a maximum protein intake of 168 g/day (38). For a 100 kg (220 pound) individual, this would represent a maximum daily protein intake of 1.68 g/kg (0.75 g/lb).

While this is surprisingly consistent with some some estimates of protein requirements discussed in Chapter 4. there arc clearly athletes who are consuming far more protein than this on a daily basis. Intake recommendations of 2.5-3.0 g/kg (1.1-1.4 g/lb) and often more arc not unusual with some research and researchers supporting that intake level (this is discussed in detail in Chapter 4). Empirically, reports of 300-400 g/day (or more) protein intakes are not uncommon among bodybuilders.

A 100 kg (220 pound) athlete consuming a relatively standard 2.0 g/kg (0.9 g/lb) would still be consuming 200 grams of protein/day, higher than the estimated maximum above. At 3.0 g/kg (1.4 g/lb) or 300 grams of protein per day, he's at nearly double the estimated theoretical maximum based on the digestion rates in Table 2. It scans unlikely that any protein in excess of 168 g/day is simply going undigested.

In a somewhat related vein, studies looking at amino acid infusion suggest that skeletal muscle can handle far more protein than the estimated maximum in terms of stimulating protein synthesis. In one study, protein synthesis rates were examined at different rates of infusion and the maximum response occurred at a rate of 150 mg/kg/h (39). This equates to a dietary intake of 288 g/day or 2.88 g/kg for a 100 kg athlete: this would require a protein digestion speed of roughly 12 g/h over a 24-hour span to achieve. That speed of digestion is higher than any of the values listed in Table 2 above. From a physiological

standpoint, it seems highly unlikely that skeletal muscle would be able to respond to a larger amount of protein than can be physiologically digested per day.

Explaining the contradiction

What the above estimations of protein digestion fail to recognize is that the human digestive system can adapt in terms of the rate of gastric emptying (how fast nutrients empty from the stomach) as well as in maximum transport capacity of those nutrients. The gastrointestinal (GI) tract adapts to changes in diet and this effect tends to be nutrient specific (40). Changes in protein intake only afl'cct protein absorption: changes in carb intake only affect carbohydrate absorption, changes in fat intake only affect fat absorption. While much of the data is in animals, there is human data supporting this effect.

One study examined the rate of nutrient transit through the GI tract in active individuals (including several endurance athletes) with a variety of caloric intakes (41). A liquid meal containing 250 calorics was given to individuals with drastically differing nutrient intakes. As self-reported caloric intake went up from 1250 lo 5300 calories/day, the transit time of the liquid meal dropped from 150-200 minutes to around 50 minutes, a three to four-fold change. The more calories the athletes habitually consumed, the faster they digested their meals.

Studies examining the response to specific nutrients have found similar results. Two weeks of high-fat intake increased the rate of gastric emptying and uptake of dietary fat by about 25% (42). Similarly, three days of high-carbohydrate overfeeding increased the absorption of carbohydrate by about 30% with no impact on the absorption rate of protein (43). No studies specifically looking at protein intake and absorption rates have been done in humans.

However, in rats, a high protein diet fed for 3 weeks increased the rate of gastric emptying of a high protein meal by about 20% (44). This value is at least similar to the changes in humans for carbohydrate and dietary fat so we might expect a similar increase in the rate of protein digestion in humans. Athletes who habitually consume large amounts of protein would be expected to show faster rates of digestion of those proteins than the values shown in Table 2.

The impact of previous meals on digestion speed

Another issue regarding digestion speed that, to my knowledge, has not been examined is the effect of previous meals. As mentioned above, most of the research on this topic (and many others related to protein metabolism) is done in the fasted state. While this serves to minimize the number of variables involved, it raises questions about the real-world applicability of the results.

Meals do not digest immediately and food from a previous meal may be present and still digesting a number of hours later depending on a number of variables such as the size of the meal, form of the meal, macronutrient content, etc. Hxcept for the meal consumed first thing in the morning, all subsequent meals during the day arc likely to overlap with the previous meal in terms of digestion; how this impacts on the topics discussed in this chapter is currently unknown. I'll discuss the issue of meal timing and frequency in Chapter 7.