Starch and lactose are both carbohydrates, but they're not chemically identical. You find them in different places in nature, and your body digests and processes them differently. Of the two, lactose is more likely to cause digestive difficulty. If you're lactose intolerant, you aren't able to digest lactose, but can still digest and process starch without difficulty.
Starch Chemistry
Starch is the common name for a chemical called amylose, which is made up of long chains of glucose molecules chemically bonded together. Glucose is a sugar; it tastes sweet, and your cells use it as a source of energy. When you eat starch -- which you do any time you consume grain-based food or starchy vegetables like potatoes -- your digestive tract breaks the amylose down into its constituent glucose units, which you absorb into your bloodstream.
Lactose Chemistry
Lactose is the chemical name for milk sugar. Unlike starch, lactose tastes sweet -- though not as sweet as other, more familiar sugars like table sugar. Also like starch, lactose contains glucose -- it's made up of a molecule of glucose chemically bonded to a molecule of the related sugar galactose, explain Dr. Mary Campbell and Dr. Shawn Farrell in their book "Biochemistry." When you consume lactose, your digestive tract breaks it down into glucose and galactose, which you absorb separately into your bloodstream.
Starch Concerns
An inability to digest or process starch is exceedingly rare. While some individuals have difficulty with the fiber often found alongside starch -- in whole grain, for instance -- and others have trouble with the gluten protein in starchy foods, almost no one lacks the ability to digest and process starch. The main concern with starch is that your body digests and absorbs it so effectively that sources of pure starch -- like refined flour -- can elevate your blood sugar very rapidly. This, repeated frequently over time, increases your likelihood of obesity and type 2 diabetes.
Lactose Concerns
Unlike starch sensitivity, lactose intolerance is relatively common among adults, particularly those of non-European descent. If you have lactose intolerance, you don't produce lactase, which is the enzyme your body uses to digest lactose. When you consume dairy, the lactose passes into your lower intestine, where bacteria break it down and produce large quantities of gas, explain Dr. Mary Campbell and Dr. Shawn Farrell in their book "Biochemistry." Lactose intolerant individuals need to avoid dairy, or use lactose-free dairy substitutes instead.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
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