Breakfast cereals are usually consumed with milk as a breakfast meal. To meet requirements for amino acids (AA), higher quality proteins are needed to complement the protein in cereals to provide a meal that is adequate in all indispensable AA. The digestible indispensable amino acid score (DIAAS) method used to determine protein quality allows for calculation of the protein value of individual ingredients and mixed meals consisting of several proteins. Values for DIAAS are based on values for apparent ileal digestibility (AID) of AA that are corrected for the basal endogenous loss of each individual AA, resulting in values defined as standardized ileal digestibility (SID). Values for SID of AA are additive in mixed meals because these values are independent of basal endogenous losses. Consequently, it is expected that DIAAS obtained for individual food ingredients are additive in a mixed meal, but data to demonstrate this have not been reported. Therefore, the objective of this experiment was to test the hypothesis that AA in milk complement AA in breakfast cereals to provide a balanced meal and that DIAAS in individual foods are additive in a combined meal.
Experimental Procedures
Six ileal cannulated gilts [initial body weight (BW): 55.6 ± 3.7 kg] were allotted to a 6 × 6 Latin square with 6 diets and six 7-d periods. Pigs were housed individually in pens in an environmentally controlled room. The first 5 days of each period were considered the adaptation period to the diet and ileal digesta were collected for 9 h on d 6 and 7 of each period. Three diets contained a breakfast cereal (i.e., corn flakes or quick oats) or dry milk as the sole source of AA. Two additional diets contained a combination of dry milk and corn flakes or quick oats. A nitrogen-free diet was also used to determine basal endogenous losses of AA to enable the calculation of SID of crude protein (CP) and AA. Diets were fed to one pig in each period and no pig received the same diet more than once during the experiment. On the completion of one experimental period, pigs were deprived of feed overnight and the following morning, a new experimental diet was offered. Diet, ingredient, and ileal digesta samples were analyzed for CP, AA and titanium (indigestible marker). The AID and SID of CP and all AA were calculated for corn flakes, quick oats and milk and for the combined cornflakes-milk diet and the quick oats-milk diet. Based on values for the individual ingredients, the SID of the two combined meals were also predicted, and the predicted SID values were compared with the measured SID values. Subsequently, DIAAS was calculated for the breakfast cereals, dry milk, and the two combined meals for 3 separate age groups. For the combined meals, DIAAS was also predicted from the individual ingredient DIAAS values.
Results
The SID for CP, Ile, Met, Thr, Val, Ala, and Glu was greater (P < 0.05) in corn flakes and dry milk than in quick oats, and dry milk had greater (P < 0.05) SID for His, Leu, Lys, Phe, Asp, Cys, and Tyr than both cereals. Quick oats had less (P < 0.05) SID of Trp than dry milk and also less (P < 0.05) SID of Ser than corn flakes (Table 1). Differences between measured and predicted SID in the corn flakes-dry milk diet differed (P < 0.05) from zero for Ile, Glu, and Ser, and tended (0.05 < P ≤ 0.10) to differ from zero for Leu, Val, and Cys. However, for all other AA, no differences between measured and predicted values for SID were observed. For the quick oats-dry milk diet, the SID of Ser tended (0.05 < P ≤ 0.10) to be different between measured and predicted values, but for CP and all other AA, no differences between measured and predicted values for SID of AA were observed (Table 2).
For all age groups, dry milk had the greatest (P < 0.05) DIAAS, whereas corn flakes had the least (P < 0.05) DIAAS. The first limiting AA for infants was Lys in corn flakes and quick oats, whereas Trp was first limiting in dry milk. For children from 6 mo to 3 yr and for older children, adolescents, and adults, the first limiting AA in corn flakes and quick oats was also Lys, but the sulfur AA (SAA) were first limiting in dry milk for these groups (Table 3). Regardless of age, there were no differences between the measured and predicted DIAAS for the combined meals (Table 4).
Key points
- Cereals can be low in Lys due heat damaged caused by overheating.
- Breakfast cereals are low in protein quality and therefore have low DIAAS, and milk has high DIAAS, which reflects grater protein quality for all age groups.
- The combination of cereals and milk provides a balanced ratio of AA, and if consumed in sufficient quantities, this combination will meet dietary requirements for AA.
- DIAAS is additive in mixed meals and that allows for calculation of the protein quality of mixed meals from the quality of individual ingredients. Therefore, if a database for DIAAS in individual food ingredients can be established, it is possible to predict DIAAS in combinations of different foods.
Table 1. Standardized ileal digestibility (SID) of crude protein (CP) and amino acids (AA) in ingredients1
1Values are means and pooled SEMs. Labeled means in a row without a common superscript letter differ, P < 0.05.
2Standardised ileal digestibility values were calculated by correcting values for apparent ileal digestibility for the basal ileal endogenous losses. Endogenous losses of amino acids were calculated from pigs fed the N-free diet as follows (g/kg DM intake): CP, 16.83; Arg, 0.76; His, 0.18; Ile, 0.32; Leu, 0.49; Lys, 0.38; Met, 0.09; Phe, 0.29; Thr, 0.49; Trp, 0.08; Val, 0.52; Ala, 0.61; Asp, 0.75; Cys, 0.22; Glu, 0.89; Ser, 0.43; Tyr, 0.25.
3n indicates the number of replicates for each item within each treatment.
Table 2. Measured and predicted values for standardized ileal digestibility (SID) of crude protein (CP) and amino acids (AA) in the combined meals
*Measured vs. predicted, P ≤ 0.05; **Measured vs. predicted, P ≤ 0.01; +Measured vs. predicted, 0.05 < P ≤ 0.10.
1n indicates the number of replicates for each item within each treatment.
Table 3. Digestible indispensable amino acid (DIAA) reference ratio and digestible indispensable amino acid scores (DIAAS) in ingredients1, 2
1Values are means and pooled SEMs. Labeled means in a row without a common superscript letter differ, P < 0.05.
2First-limiting AA is in parenthesis. AAA, aromatic AA; SAA, sulfur AA.
3n indicates the number of replicates for each item within each treatment.
4DIAA reference ratios and DIAAS were calculated using the recommended AA scoring pattern for an infant (birth to 6 months). The indispensable AA reference patterns are expressed as mg amino acid/g protein: His, 21; Ile, 55; Leu, 96; Lys, 69; SAA, 33; AAA, 94; Thr, 44; Trp, 17; Val, 55 (4).
5DIAA reference ratios and DIAAS were calculated using the recommended AA scoring pattern for a child (6 months to 3 years). The indispensable AA reference patterns are expressed as mg amino acid/g protein: His, 20; Ile, 32; Leu, 66; Lys, 57; SAA, 27; AAA, 52; Thr, 31; Trp, 8.5; Val, 43 (4).
6DIAA reference ratios and DIAAS were calculated using the recommended AA scoring pattern for an older child, adolescent and adult. The indispensable AA reference patterns are expressed as mg amino acid/g protein: His, 16; Ile, 30; Leu, 61; Lys, 48; SAA, 23; AAA, 41; Thr, 25; Trp, 6.6; Val, 40 (4).
Table 4. Measured and predicted values for digestible indispensable amino acid scores (DIAAS) in combined meals1
*Measured vs. predicted, P ≤ 0.05; **Measured vs. predicted, P ≤ 0.01; +Measured vs. predicted, 0.05 < P ≤ 0.10.
1First-limiting AA is in parenthesis. AAA, aromatic AA; SAA, sulfur AA.
2n indicates the number of replicates for each item within each treatment.
3DIAA reference ratios and DIAAS were calculated using the recommended AA scoring pattern for an infant (birth to 6 months). The indispensable AA reference patterns are expressed as mg amino acid/g protein: His, 21; Ile, 55; Leu, 96; Lys, 69; SAA, 33; AAA, 94; Thr, 44; Trp, 17; Val, 55 (4).
4DIAA reference ratios and DIAAS were calculated using the recommended AA scoring pattern for a child (6 months to 3 years). The indispensable AA reference patterns are expressed as mg amino acid/g protein: His, 20; Ile, 32; Leu, 66; Lys, 57; SAA, 27; AAA, 52; Thr, 31; Trp, 8.5; Val, 43 (4).
5DIAA reference ratios and DIAAS were calculated using the recommended AA scoring pattern for an older child, adolescent and adult. The indispensable AA reference patterns are expressed as mg amino acid/g protein: His, 16; Ile, 30; Leu, 61; Lys, 48; SAA, 23; AAA, 41; Thr, 25; Trp, 6.6; Val, 40 (4).
