Perjalanan Makanan di Dalam Tubuh (Mungkin 2024)
Protein
Protein, seperti karbohidrat dan lemak, mengandungi karbon, hidrogen, dan oksigen, tetapi juga mengandungi nitrogen, komponen kumpulan kimia amino (NH 2), dan dalam beberapa kes sulfur. Protein berfungsi sebagai bahan struktur asas tubuh serta menjadi pemangkin biokimia dan pengatur gen. Selain air, protein merupakan bahagian utama otot, tulang, organ dalaman, dan kulit, kuku, dan rambut. Protein juga merupakan bahagian penting dalam membran sel dan darah (contohnya, hemoglobin). Enzim, yang memangkin reaksi kimia dalam tubuh, juga protein, seperti antibodi, kolagen dalam tisu penghubung, dan banyak hormon, seperti insulin.
Tisu di seluruh badan memerlukan pembaikan dan penggantian yang berterusan, dan dengan itu protein tubuh terus berubah, dipecah dan kemudian disintesis semula mengikut keperluan. Protein tisu berada dalam keseimbangan dinamik dengan protein dalam darah, dengan input dari protein dalam diet dan kehilangan melalui air kencing, tinja, dan kulit. Pada orang dewasa yang sihat, penyesuaian dilakukan agar jumlah protein yang hilang seimbang dengan jumlah protein yang dimakan. Namun, dalam tempoh pertumbuhan yang cepat, kehamilan dan menyusui, atau penyembuhan setelah sakit atau penipisan, tubuh berada dalam keseimbangan nitrogen positif, kerana lebih banyak protein dipertahankan daripada diekskresikan. Kebalikannya berlaku semasa sakit atau membuang, apabila terdapat keseimbangan nitrogen negatif kerana lebih banyak tisu dipecah daripada disintesis.
Asid amino
The proteins in food—such as albumin in egg white, casein in dairy products, and gluten in wheat—are broken down during digestion into constituent amino acids, which, when absorbed, contribute to the body’s metabolic pool. Amino acids are then joined via peptide linkages to assemble specific proteins, as directed by the genetic material and in response to the body’s needs at the time. Each gene makes one or more proteins, each with a unique sequence of amino acids and precise three-dimensional configuration. Amino acids are also required for the synthesis of other important nonprotein compounds, such as peptide hormones, some neurotransmitters, and creatine.
Food contains approximately 20 common amino acids, 9 of which are considered essential, or indispensable, for humans; i.e., they cannot be synthesized by the body or cannot be synthesized in sufficient quantities and therefore must be taken in the diet. The essential amino acids for humans are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Conditionally indispensable amino acids include arginine, cysteine, and tyrosine, which may need to be provided under special circumstances, such as in premature infants or in people with liver disease, because of impaired conversion from precursors.
The relative proportions of different amino acids vary from food to food (see table). Foods of animal origin—meat, fish, eggs, and dairy products—are sources of good quality, or complete, protein; i.e., their essential amino acid patterns are similar to human needs for protein. (Gelatin, which lacks the amino acid tryptophan, is an exception.) Individual foods of plant origin, with the exception of soybeans, are lower quality, or incomplete, protein sources. Lysine, methionine, and tryptophan are the primary limiting amino acids; i.e., they are in smallest supply and therefore limit the amount of protein that can be synthesized. However, a varied vegetarian diet can readily fulfill human protein requirements if the protein-containing foods are balanced such that their essential amino acids complement each other. For example, legumes such as beans are high in lysine and low in methionine, while grains have complementary strengths and weaknesses. Thus, if beans and rice are eaten over the course of a day, their joint amino acid patterns will supplement each other and provide a higher quality protein than would either food alone. Traditional food patterns in native cultures have made good use of protein complementarity. However, careful balancing of plant proteins is necessary only for those whose protein intake is marginal or inadequate. In affluent populations, where protein intake is greatly in excess of needs, obtaining sufficient good quality protein is usually only a concern for young children who are not provided with animal proteins.
Asid amino penting * dalam beberapa makanan biasa **
| telur ayam, rebus, 1 besar (50 g) | daging lembu, tanpa lemak, panggang, 100 g (3.5 oz) | susu lembu, rendah lemak, 2%, 246 g (8 fl oz) | kacang soya, panggang kering, 86 g (1/2 cawan) | tepung jagung, bijirin penuh, 122 g (1 cawan) | tepung, gandum utuh, 120 g (1 cawan) | kacang pinto, rebus, 171 g (1 cawan) | biji bunga matahari, panggang kering, 28 g (1 oz) | |
|---|---|---|---|---|---|---|---|---|
| * Dalam miligram. | ||||||||
| ** Angka garis bawah menunjukkan mengehadkan asid amino. | ||||||||
| Sumber: Jean AT Pennington, Bowes dan Nilai Makanan Bahagian Gereja yang Biasa Digunakan, edisi ke-17. (1998). | ||||||||
| triptofan | 77 | 277 | 115 | 494 | 70 | 254 | 166 | 84 |
| threonin | 302 | 1,080 | 366 | 1,478 | 372 | 474 | 592 | 223 |
| isoleusin | 343 | 1,111 | 490 | 1,651 | 355 | 610 | 621 | 274 |
| leucine | 538 | 1,954 | 795 | 2,772 | 1,215 | 1,111 | 1,122 | 399 |
| lisin | 452 | 2,057 | 644 | 2,265 | 278 | 454 | 964 | 225 |
| metionin | 196 | 633 | 205 | 459 | 207 | 254 | 212 | 119 |
| fenilalanin | 334 | 965 | 393 | 1,777 | 487 | 775 | 759 | 281 |
| valin | 384 | 1,202 | 544 | 1,699 | 501 | 742 | 735 | 316 |
| histidin | 149 | 846 | 220 | 918 | 303 | 380 | 392 | 152 |
Pengambilan protein
The World Health Organization recommends a daily intake of 0.75 gram of good quality protein per kilogram of body weight for adults of both sexes. Thus, a 70-kg (154-pound) man would need 52.5 grams of protein, and a 55-kg (121-pound) woman would need about 41 grams of protein. This recommendation, based on nitrogen balance studies, assumes an adequate energy intake. Infants, children, and pregnant and lactating women have additional protein needs to support synthesis of new tissue or milk production. Protein requirements of endurance athletes and bodybuilders may be slightly higher than those of sedentary individuals, but this has no practical significance because athletes typically consume much more protein than they need.
Protein consumed in excess of the body’s needs is degraded; the nitrogen is excreted as urea, and the remaining keto acids are used for energy, providing 4 kilocalories per gram, or are converted to carbohydrate or fat. During conditions of fasting, starvation, or insufficient dietary intake of protein, lean tissue is broken down to supply amino acids for vital body functions. Persistent protein inadequacy results in suboptimal metabolic function with increased risk of infection and disease.
