Carbohydrates and sugars

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Carbohydrates and sugars


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USMLE® Step 1 questions

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High Yield Notes

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Carbohydrates and sugars

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USMLE® Step 1 style questions USMLE

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A study is conducted to determine the physiology of the digestion and absorption of carbohydrates. The digestion of carbohydrates begins in the mouth and finishes in the small intestine where it is absorbed. Which of the following is true about the absorption of carbohydrates?  


Carbohydrates include both simple sugars which are little ring-shaped molecules made of carbon, hydrogen and oxygen - either alone or in pairs, as well as more complex carbohydrates, which are formed when these the rings link up together to make long chains.

Carbohydrates provide us with calories or energy, and simple sugars in particular play many roles in our diet - they sweeten lemonade, balance out an acidic miso soup, fuel yeast in rising dough and alcohol, and help preserve jams and jellies.

We have an innate liking for sweetness, which simple sugars provide.

Historically, simple sugars were available in more modest quantities than they are today, and eating too many calories from sugar can become a problem.

Unhealthy diets, including those with too many calories from simple sugars, are associated with an increased risk for diseases like obesity, diabetes mellitus, and cardiovascular disease, but the good news is that a healthy diet can reduce that risk as well.

Sugars are found naturally in plants like fruits, vegetables, and grains, as well as animal products like milk and cheese.

Added sugars are the sugars that get added to foods like cereals, ketchup, energy bars, and even salad dressings.

To be clear, even if the sugar being added comes from a natural source like sugar cane or honey, it’s still considered an added sugar. In fact, a variety of ingredients listed on food labels may be sources of added sugars, some of which you’re likely familiar with.

Sugar actually refers to a family of molecules called saccharides - monosaccharides where “mono” means one, so one sugar molecule, disaccharides where “di” means two, so two sugar molecules linked together, oligosaccharides where “oligo” means a few, so it’s three to nine sugar molecules linked together, and polysaccharides where “poly” means many, so it’s ten or more sugar molecules linked together.

Glucose is the most important member of the sugar family and it’s a monosaccharide. It’s the main source of calories for the body, and is able to cross the blood brain barrier and nourish the brain.

Another monosaccharide is fructose which is commonly found in honey, fruits, and root vegetables.

Finally, there’s the monosaccharide, galactose, known as milk sugar.

It’s known as milk sugar because it’s only found in nature when it links with glucose to form lactose, a disaccharide found in the milk of mammals, including cow and human breast milk.

Sucrose, or table sugar, is another disaccharide and it’s formed when fructose links up with glucose.


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  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "The definition of dietary fiber – discussions at the Ninth Vahouny Fiber Symposium: building scientific agreement" Food & Nutrition Research (2010)
  6. "Dietary sugars and body weight: systematic review and meta-analyses of randomised controlled trials and cohort studies" BMJ (2012)

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