Sunday, May 20, 2012

The Nutrition Debate #53: On the Digestion and Absorption of Food

Food digestion physiology varies between individuals and with the composition and size of a meal. There are other factors, but a primer on what is most common and universal will be useful to understanding the various processes and their effect on our biological systems. This digest (note the pun) should provide some enlightenment.
Digestion, the hormonal process, has three phases. The cephalic phase begins when receptors in the head are stimulated by an emotional state (thought of food), sight, smell, taste and chewing. The cerebral cortex, the medulla oblongata and the hypothalamus are all involved. The vegus nerve is the messenger, triggering various enzymatic and hormonal activities. The gastric phase starts with distension, acidity, and the presence of amino acids and peptides in the stomach. The intestinal phase is initiated by distension, acidity, and osmolarity of digestive products in the intestine (we’ll explain).
Digestion, the mechanical and chemical process, begins with chewing. Chewing breaks food down to smaller units that are in turn broken down by enzymes to the smallest units to permit them to be absorbed, mostly in the small intestine, into the blood. Saliva containing mucus and the enzyme amylase is secreted from three pairs of salivary glands located in the mouth cavity. Mucus moistens the food and amylase partially digests polysaccharides (starches) into a disaccharide called maltose. About 30% of starch is broken down in the mouth (optimum pH: 6.8 or weakly acidic).
In addition, papillae on the surface of the tongue secrete the enzyme lingual lipase which begins the process of breaking down some fats from triglycerides into diglycerides. Long chain triglycerides cannot be absorbed unless completely broken down to monoglycerides, so the process starts here and continues in the stomach. As much as 30% of fat is broken down within 1 to 20 minutes of ingestion by lingual lipase alone, according to Wikipedia.
Food then passes through the pharynx and descends the esophagus to the stomach, a sac that stores and mixes and processes the food into a milky solution called chyme. Glands lining the stomach secrete hydrochloric acid, which is necessary for protein digestion by the enzyme pepsin. The stomach’s high acidity (optimum pH 1.8) inhibits the breakdown of carbohydrates within it, but it does produce a small amount of the enzyme lipase to continue the digestion of fats. So far, although the breakdown of food particles by enzymatic and mechanical action is continuing apace, virtually no absorption of nutrients into the bloodstream has occurred. The exceptions are water, some simple sugars, and other small molecules like alcohol that are absorbed in the stomach, passing through the membrane of the stomach and entering the circulatory system directly.
The final stages of digestion and most of the nutrient absorption occur in the next portion of the tract: the small intestine. The small intestine is divided into three segments – duodenum, jejunum and ileum. The duodenum in turn is connected to the hepato-pancreatic duct which connects to the liver and the gall bladder (to store and supply bile) and to the pancreas. These organs and glands provide digestive enzymes and an alkaline fluid (pH 8.5) to neutralize the acid being emptied from the stomach.
In the duodenum, dipeptides from protein in the stomach are broken down to amino acids by enzymes from the pancreas. The enzymes break down the disaccharides maltose, lactose and sucrose into the monosaccharides glucose, fructose and galactose.  The major portion of fat digestion, the breakdown of diglycerides by lipase into free fatty acids and glycerol, also happens here.
Absorption of nutrients occurs mostly in the jejunum and ileum. Amino acids (from protein) and the monosaccharides (glucose, fructose and galactose, all from carbohydrates) are water soluble nutrients and enter the blood directly through the villi and microvilli in the endothelial layer of the small intestine wall. The surface area of these many small structures (imagine a shag carpet) increases the absorption interface by 600 times, roughly equivalent to the surface of a tennis court. However, the products of fat digestion, fatty acids and glycerol, are not water soluble and therefore enter the circulation through the lymph system by a process called passive diffusion in which no energy is required.
Passive or simple diffusion requires no energy input from the body because it is driven by concentration. Fats, water and some minerals simply cross the membrane barrier because the concentration of the substance (fat, water, etc.) is lower on the other side. It is an equalization process. The small intestine is full of nutrients and the blood is not, so they cross over. This equalization process is also called osmosis, ergo ‘osmolarity of digestive products’ referred to above. 
Fructose is transported across the membrane barrier by a process called ‘facilitated diffusion,’ which means it may need a little help depending on the concentration gradient. Glucose and galactose, the other monosaccharides, as well as amino acids, also require energy for active transport, which occurs against a concentration gradient. See The Nutrition Debate #52, “The Thermic Effect of Foods,” for the energy requirements of protein digestion as compared to carbohydrates, especially of processed foods.
Finally, gastric emptying, which is the rate that food leaves the stomach to enter the small intestine, is tightly controlled. Liquids are emptied much more quickly than solids, and carbohydrates are emptied first, followed by protein, fat and fiber. Gastric emptying has attracted medical interest as “rapid gastric emptying is related to obesity and delayed gastric emptying syndrome is associated with Type 2 diabetes, aging and gastroesophageal (acid) reflux or GERD.” But that’s another subject. Next week: Paleo book reviews. 
© Dan Brown 5/20/12

Monday, May 14, 2012

The Nutrition Debate #52: The Thermic Effect of Food

Thermogenesis is the process of heat production in organisms, and heat is a form of expended energy. The unit of measurement is a calorie. Human metabolism is comprised of three components of energy expenditure: 1) the energy expended by the basal metabolic rate (to keep the resting organism “going,” i.e. alive), 2) the energy expended through exercise (motion), and 3) the energy expended due to the “cost” of processing food for storage and use. This last component is known as the thermic effect of food.  It is also sometimes called Diet-Induced Thermogenesis (DIT).
It is estimated that the thermic effect of food is about 10% of the caloric intake of any given meal, “though the effect varies substantially for different food components,” according to Wikipedia. Of the three food components (called macronutrients), fat has minimal thermic effect. Carbohydrates, especially simple sugars and carbs in highly processed, manufactured foods, are very easy to process and have very little thermic effect. Proteins, on the other hand (and whole food, complex carbohydrates to a lesser extent) are harder to process and have a much larger thermic effect. The ratio of protein to carb energy expenditure is generally between 2:1 and 3:1, depending on how “simple” the carb is.
So, for total daily energy expenditure there's a resting metabolic rate component (60-70% of total), the physical activity component (15-30% of total), and the thermic effect of food component (~10%). While 10% is a small percentage, if the energy expended to digest, absorb and eliminate protein is two or more times the amount required for carbs and fat, wouldn’t replacing carbs with protein in an isocaloric diet thus increase the metabolic rate and help to burn more calories including stored fat? And if the difference in the burn rate was even greater for the simple sugars and processed carbs made easier to digest by manufacturing, couldn’t that account for why our metabolisms burn less on today’s modern processed-food diets? Thus, to naturally increase our metabolic rate, we should 1) increase the amount of protein in our diet and 2) replace simple sugars and processed-foods in the diet with whole foods, that is, unprocessed, complex carbohydrates. Note that 2) above assumes that you don’t already have insulin resistance (IR), pre-diabetes, or full-blown Type 2 diabetes, or are obese. It you are, you should severely restrict/limit the carbohydrates in your diet.
The typical American (or for my international readers, Western) diet currently gets about 15% of calories from protein. The Standard American Diet (SAD for short), espoused by the FDA and promoted on food packages in the Nutrition Facts Panel, is 10% protein. (On a 2,000 calorie diet, 50g/day x 4 cal/g = 200 cal.) I get about 25% of my calories from protein.
So, who’s looking at this? Barr SB and Wright JC, that’s who, in their paper, “Postprandial energy expenditure in whole-food and processed-food meals: implications for daily energy expenditure,” published online July 2010 in The Journal of Food and Nutrition Research. The abstract on PubMed explains:Empirical evidence has shown that rising obesity rates closely parallel the increased consumption of processed foods (PF)… in the USA. Differences in postprandial thermogenic responses to a whole-food (WF) meal vs. a PF meal may be a key factor in explaining obesity trends, but currently there is limited research exploring this potential link.” And their conclusion: “Ingestion of the particular PF meal tested in this study decreases postprandial energy expenditure by nearly 50% compared with the isoenergetic WF meal. This reduction in daily energy expenditure has potential implications for diets comprised heavily of PFs and their associations with obesity” (Italics added by me).
The processed food meal decreased energy expenditure by nearly 50%. Wow! That’s a big difference in thermic effect. And the meals they were comparing were “either ‘whole’ or ‘processed’ cheese sandwiches; multi-grain bread and cheddar cheese were deemed whole, while white bread and processed cheese product were considered processed.” Imagine if they had chosen a real whole food instead of ‘multigrain bread’ which is a far, far cry from a real whole food as readers here know. And what if it had been a protein food instead? The authors explained, “A more strict WF would be one devoid of any processing, such as a specific fruit, vegetable, or meat. However, for the present study, we sought to compare two meals that were familiar to the Western diet, and could be easily interchangeable.”And while the much higher thermic effect of protein is probably too small to have a noticeable effect on weight loss in the short term, over a period of months or years this difference becomes significant. Have I discovered another cause for the obesity epidemic? Just joking, folks. I am just your humble commentator bringing you the insights of the cognoscenti.                   
© Dan Brown 5/13/12

Sunday, May 6, 2012

The Nutrition Debate #51: Dietary Cholesterol

Pay attention - dietary cholesterol is the cholesterol you eat. Serum cholesterol is the cholesterol in your blood. They are not only distinct; they are largely independent of each other. The following excerpt from Wikipedia gets “into the weeds” a bit, but is necessary to provide a framework.
“While the absolute production quantities vary with the individual, group averages for total human body content of cholesterol, with the U.S population, commonly run about 35,000 mg (assuming lean build; varies with body weight and build) and about 1,000 mg/day ongoing production. Dietary intake plays a smaller role, 200-300 mg/day being common values; for pure vegetarians, essentially 0 mg/day, but this typically does not change the situation very much because internal production increases to largely compensate for the reduced intake.”
Assuming a “lean (body) build” for the U.S. population is a dubious proposition, as must be patently obvious to even the casual observer. My body is certainly not lean. Neither do I eat just 200-300mg/day of dietary cholesterol. I typically average 600-650 mg/day. I know this from the days (years, actually) when I tracked carefully. But, the HHS/USDA Dietary Guidelines for Americans age 2 and older urge us to eat no more than 300mg daily (200mg with CVD risk factors). Does this make any sense when we consider that most societies wean at 3 or 4, and breast milk is 55% fat, mostly saturated, and loaded with cholesterol? “Mother's milk is especially rich in cholesterol and contains a special enzyme that helps the baby utilize this nutrient. Babies and children need cholesterol-rich foods throughout their growing years to ensure proper development of the brain and nervous system,” says Dr. Mary Enig, the doyenne of lipid chemistry and the author of the definitive biochemistry guide, “Dietary Fats.”
“The brain is the most cholesterol-rich organ in the body, most of which comes from in situ synthesis,” begins the abstract of a paper, “Diabetes and insulin in regulation of brain cholesterol metabolism,” from the Joslin Diabetes Center and Harvard Medical School published in Cell Metabolism (2010 Dec 1; 12(6):567-79).Yet, many people have deprived themselves for decades of cholesterol-laden foods (shrimp, eggs, butter, cream, liver, even a marbled steak) in an effort to comply with the government’s public health guidelines. What a pity (for them)!
Perhaps the most telling statement in the quoted paragraph, however, is that vegetarians have serum cholesterol levels similar to omnivores “…because internal production increases to largely compensate for the reduced intake.” Vegetarians have avoided eating cholesterol-loaded foods altogether “but this typically does not change the situation very much.” Admittedly, they completely gave up eating animal-based foods – meaning they ate absolutely no cholesterol – for a different reason, but they did not thereby lower their serum cholesterol “very much.”
The reason, as the numbers In the Wiki-quoted paragraph above note, is that most cholesterol, typically 80-90% within the body, is created and controlled by internal production by all the cells in the body, with typically slightly greater relative production by hepatic (liver) cells. As described in The Nutrition Debate #24 cell structure relies on fat membranes to separate and organize intracellular water, proteins and nucleic acids, and cholesterol is one of the components of all animal cell membranes. More on cholesterol within this blog can be found at The Nutrition Debate #25 , “Understanding Your Lipid Panel,” and The Nutrition Debate #45 , “Do You Need to Lower Your Cholesterol?”
But get this: “For many,” the Wikipedia entry on atherosclerosis continues, “especially those with greater than optimal body mass and increased glucose levels, reducing carbohydrate intake (especially simple forms), not fat and cholesterol, is often more effective for improving lipoprotein expression patterns (i.e. cholesterol), weight and blood glucose values. For these reasons, medical authorities much less frequently promote the low dietary fat concept than was common prior to about the year 2005.” That’s certainly been true for me, as I have reported here with my own example (n = 1). Since switching to a high fat/high cholesterol, moderate protein and very low carbohydrate diet about 10 years ago, my HDL has doubled and my triglycerides have dropped by two-thirds, while my LDL has been constant and my T. Chol. has risen only slightly. Remember the common laboratory formula for serum cholesterol: T. Chol. = LDL + HDL + TG/5.
Finally, Ancel Keys, the father of the (infamous) Lipid (saturated fat/heart disease) Hypothesis, is equally famous in some circles for having said later in life, according to Malcolm Kendrick, author of “The Great Cholesterol Myth,” “There’s no connection whatsoever between cholesterol in food and cholesterol in blood. And we’ve known that all along.”
Given that we were born to drink mother’s milk, and our brain is mostly cholesterol, it strikes me as pretty natural and healthy to eat cholesterol rich foods. As the ‘70s margarine commercial said, “It’s not nice to fool Mother Nature.”
© Dan Brown 5/6/12