The Art and Science of Low Carbohydrate Living: An Expert Guide to Making the Life-Saving Benefits of Carbohydrate Restriction Sustainable and Enjoyable by Stephen Phinney & Jeff Volek

Author:Stephen Phinney & Jeff Volek [Phinney, Stephen]
Language: eng
Format: epub, mobi
ISBN: 9780983490739
Publisher: Beyond Obesity LLC
Published: 2011-07-07T23:00:00+00:00

Complexity of the Genome

The genome consists of over 3 billion base pairs made up of long chains of adenine, guanine, cytosine and thymine on the 23 chromosomes. Humans have about 21,000 genes, each encoding a protein, scattered around the genome. The length of DNA that contains a typical gene extends about 50,000 base pairs, of which only a fraction, say 1,000—actually encode the protein sequence. This means the majority of the genome (~98%) consists of expanses of DNA whose function(s) remain unknown. This is sometimes called ‘junk’ DNA. Interestingly, functions are being found for some of this junk. For example some noncoding DNA sequences are genetic “switches” that regulate when and where genes are expressed. One expert estimates that 5% of the non-coding DNA has a function. So whether a DNA variant is found in a gene or in a non-coding expanse of DNA, it may have meaning for some yet unknown dietary response.

Any one person’s DNA is about 99-99.5% identical to any other person’s DNA. There are two major causes of person-to-person genetic differences. The major cause is named copy number variants. These are many different places in the DNA where the number of copies of a gene can vary from one to many hundreds. For instance, on average, people from cultures that historically have high starch diets (such as Japanese and European Americans) have more copies of the gene salivary amylase than people from cultures that historically eat low starch diets (such as Mbuti and Yakut)[90]. Amylase is involved in digesting carbohydrates. More copies of the salivary amylase gene is correlated with more enzymatic activity. Many but not all genes exhibit copy number variation in humans. Some of these copy variants show evidence of adaptations that took place over a million years ago, while others show evidence for selection by diet only in the last few thousand years.

The second most abundant source of person-to-person variation in DNA consists of single nucleotide polymorphisms (SNPs). A SNP is a place in the DNA where one of the four nucleotides has been replaced by another. These SNPs may change the function or amount of any of our 21,000 proteins. So far, scientists have identified about 20 million SNPs but many more exist. Overall any two people differ for about 3 million SNPs, which is about 0.1% of their total DNA. As an example, SNPs in the Lactase gene are responsible for the persistence of Lactase expression in adults which allows for adults to drink milk – lactose tolerance. Most cultures that kept milk producing animals, such as cows, camels, and goats independently evolved different SNPs in the Lactase gene that allow for lactose tolerance. The task of finding which SNPs and copy number variants are important in respect to nutritional genomics will certainly be challenging. Although the effects of a 0.5-1% difference may seem small, the overall effects on physiology can be large. For example some of the polymorphisms in genes that code for enzymes involved in carbohydrate digestion (i.e., amylase

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