DEAR DR. BLONZ: I want an explanation for the post-meal sensation of tiredness long associated with Thanksgiving. Is it the turkey, or isn’t it? Last year, we had a hearty mealtime discussion about this, with a humorous footnote of all of us sitting around exhausted after the meal. But a number of us only had vegetarian dishes. This year, I wanted to go armed with some facts. What do you say? -- M.Q., Tucson, Arizona

DEAR M.Q.: Turkey does not contain any sleep-inducing chemicals; the connection between Thanksgiving and tiredness relates more to the nature of the feast. It all comes down to the size and makeup of the meal: The greater the amount of protein, fat and carbohydrate consumed, the more pronounced the soporific effect.

Survival equates with sustenance. So as foods are eaten, the body’s priorities shift, and blood flow gets shunted toward the organs that play a role in digestion and absorption. During mealtime, the brain makes shifts in its neurotransmitter controller substances. The body has its own home-brewed relaxation cocktail that encourages us to sit back, relax, and facilitate our ability to take it all in.

And assuming the Thanksgiving feast is your evening meal, all this takes place near the end of the day, when our energy is already on the wane. It’s a perfect scene for post-meal tiredness.

By the time the feast has finished, some of the component parts of the appetizers and the first of the other digested foods will have begun to enter the bloodstream: the great highway by which newly digested nutrients find their way to where they are needed. Carbohydrates break down to simple sugars and cause a release of insulin, the “I am fed” hormone that signals the availability of energy resources in the blood while keeping the blood sugar level from rising too high. Another of insulin’s effects is to encourage the amino acids liberated from the meal’s protein to enter cells to help with repairs or to build new tissue where needed.

Our bloodstream does not flow into the central nervous system, where the brain resides. Our brain is protected by a blood-brain barrier that controls the nature and amount of substances allowed in. Amino acids compete with each other to gain access. Tryptophan, one of the essential amino acids, was thought to be a key player in the turkey-tiredness connection. This makes intuitive sense because, once in the brain, tryptophan can be converted to serotonin -- a neurotransmitter that can elicit relaxation and calmness. Tryptophan is also associated with the production of melatonin, a hormone connected with sleep.

Turkey is a complete protein, and it does indeed contain tryptophan (though not significantly more than other complete proteins). But tryptophan must get to the brain to make you tired, and it is at a disadvantage because it’s found in smaller amounts than other amino acids in most foods. Carbohydrates tilt the odds through their stimulation of insulin, which causes many amino acids to enter other tissue. Tryptophan is less affected by insulin, and, its competition reduced, is better able to enter the brain and have its effect.

The tiredness you feel is due more to the overall volume and types of foods eaten at Thanksgiving. In particular, it’s the carbohydrate foods such as stuffing, candied yams, cranberries or dessert that lead to the fatigued feeling. If you were to have some other meat as the centerpiece, or even have a vegan Thanksgiving feast, the effects would be the same.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: I keep reading and seeing videos about the benefits of colon cleansing, speaking of it in the most glowing, essential-for-good-health terms. I cannot imagine this is something that you would advocate, but want your opinion on the topic. Is it beneficial? Is it safe? -- F.S., Las Vegas

DEAR F.S.: The goal of advertising is to motivate an individual to buy. Objectivity and accuracy can be spun, or bypassed outright when there is no evidence to support the claims.

There are many names for “cleansing” procedures designed to purge the contents of the large intestine, otherwise known as the colon. Aside from “colon cleansing,” these terms include: high colonics, colonic irrigations, coffee enemas, detoxification therapies, and hydro-colon therapy. Some work from the top down, using laxative products and dietary supplements, while others go from the bottom up, relying on the insertion of a rubber tube inside the rectum to flush out the colon with water or other liquids.

What kind of message could make such a procedure attractive?

First, advertisers must convince audiences that there is a problem to be solved, before offering their product or procedure as the solution. Consider that we cannot see what is going on inside our colons. This leaves us open to the suggestion that all sorts of toxins, parasites and other undigested waste are trapped inside. “Impacted” is often an operative word. We get told that an unclean colon prevents needed nutrients from being absorbed, and can divert the attention of the immune system, leaving us vulnerable to a host of ailments. A list of common conditions usually follows, including: constipation, excess gas, irritable bowel syndrome, muscle aches and pains, headaches, depression and fatigue. And of course, colon cancer is often mentioned.

The cleansing procedure gets portrayed as an essential adjunct to the intoxications wrought by our polluted environment and the standard American diet. Add a few pictures, a bunch of glowing testimonials and you have your recipe for a sale.

The first recorded mention of colon cleansing comes from ancient Egypt, and the procedure has had periodic revivals throughout history. It became popular in the United States in the 1920s and 1930s, but as medical science gained a better understanding of the functions of the digestive system, there was no scientific evidence that the routine use of such procedures could help against disease or contribute to long-term health. Today, however, the internet is “impacted” with dubious commercial sites delivering the above scenario and offering products for sale with little in the way of content oversight. You can even find these products at local food stores, next to the dietary supplements.

As to the facts, we all need to appreciate that the cells that comprise the lining of the intestines are shed periodically, which means that the lining of your intestine changes on its own from month to month. This lends doubt to the rather gross concept that caked-on toxins hang around our colon to wreak havoc with our health. Consider also that, except for water, the nutrients in our food get absorbed before they reach the colon.

There is no question that regular bowel movements are an important part of good health, but “regular” can vary from person to person. The whole foods and fiber we eat determine how well our digestive and elimination systems work, and should therefore be viewed as an essential part of our daily diet.

It is our foods and lifestyle that determine our state of health. All this makes more sense than relying on unproven, archaic “flushing” procedures. Read more about colonics at tinyurl.com/j4k7xdm. And here is a link to FTC dealings with a purveyor of these products: tinyurl.com/y9pm4nyj.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: You have written about different methods of cooking and how they affect a food’s nutrients, but can you talk about how boiling, roasting, microwaving and frying compare? -- K.B., Las Vegas

DEAR K.B.: As a general rule, the longer the time from field to plate, the more nutrients fresh foods will lose. Cold can slow down this process, but it’s heat that serves as the prime nutrient destroyer. The greater the heat and the longer the time of exposure, the greater the nutrient destruction.

Minerals are the only nutrients that are unaffected by heat. Proteins, carbohydrates and fats have a reasonable degree of heat stability, but they, too, will succumb if the heat is high enough. Vitamins tend to be the most vulnerable nutrients, with water-soluble vitamins being the most affected. Vitamin C, thiamin and folic acid, for example, can be destroyed by heat; fat-soluble vitamins can also be destroyed by extended cooking times. There is less data on other plant chemicals (phytochemicals), but it is probable that they would be vulnerable to heat destruction as well.

A list of cooking methods from quickest to slowest might look something like this: microwaving, sauteing, stir-frying, pressure-cooking, steaming, deep-fat frying, roasting, broiling, boiling and baking. Cooking times, of course, can vary from food to food.

Look at potatoes, for example. Because it is cooked rapidly, a microwaved potato will have the greatest amount of vitamin C, whereas a baked potato will have the least. A boiled potato will fall somewhere in the middle.

Nutrients can be lost in ways other than outright destruction. The use of water with boiling can pose a problem, in that some nutrients will dissolve in water. If the water is then discarded, these nutrients are then lost in addition to those destroyed by the heat. Steaming and pressure-cooking also use water, but it’s less of an issue because less water is used with these methods.

DEAR DR. BLONZ: In a recent column, a reader remarked, when comparing lemon juice with vinegar, that the latter took their breath away. This is because the acetic acid in vinegar is volatile, and the citric acid in lemon juice is not. Vinegar can be “tasted” by both the nose and the tongue, whereas the citric acid in lemon juice is only tasted by the tongue. A useful feature of vinegar’s volatility is that it can be reduced in a recipe if it’s too strong simply by heating with the lid off.

This is from a chemist who has smelled a lot of things in his career. -- M.M., via email

DEAR M.M.: Thanks for your helpful comment. As you correctly point out, it is important to consider all the senses when considering a flavor impression.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.