Carbohydrates

Utilization | Requirements | Types | Fiber | Fructose | Athletes | Glycemic Index | Glycemic Load

ExRx.net > Nutrition > Outline

Bread Cereals and Grains Starchy Vegetables Crackers and Snacks Beans, Peas, and Lentils

Utilization

  • Fuel for activity
    • Immediate fuel for activity
    • Sustained energy for aerobic activity
    • Major energy for high intense activity like weight training or sprinting
  • Protein sparer - nervous system uses carbohydrates
    • Lack of carbohydrates causes protein to go through gluconeogenesis (conversion of protein to glucose) and be metabolized
    • Contrary to popular belief, brain can metabolize lactate and ketones as well
    • Lactate (and other nutrients) also shuttle from astrocytes to neurons (Rev Med Suisse Romande, 2000)
  • Carbohydrates are metabolic primers, they are needed to completely burn fat
    • Incomplete combustion of fat will result in ketone bodies
    • Fat burns in the flame of carbohydrates.
  • Carbohydrates are the primary fuel for the CNS.
    • Acute carbohydrate depletion may cause tunnel vision, nausea, irritability
    • Metabolism can adapt to prolonged low carb, high fat/protein diet.

Requirements

  • Too little carbohydrates
    • More fat utilized as fuel source
    • Endurance can be reduced up to 50% until metabolism adapts
    • Glycogen stores become depleted (Costill and Miller, 1980)
    • Ketogenesis
      • Triggered when glycogen store are depleted
        • Fasting, low carbohydrate diets, prolonged exercises
      • Liver produces ketone bodies acetoacetat and ß-hydroxybutyrate
        • via incomplete combustion of fats
        • Ketone bodies are used for energy
      • State of Ketosis
        • Possible symptoms: weakness, dizziness, tunnel vision, fatigue, panting, abnormal EEG, strange breath (acetone)
        • Possible symptoms in diabetics: unconsciousness, coma and even possibly death in rare cases
        • Blood acidosis
          • acid-base homeostasis in the blood is normally maintained (>7.35) through various buffering systems
          • may impair exercise tolerance and performance.
    • Blood glucose levels are initially maintained by converting dietary protein to carbohydrates via gluconeogenesis
      • After about 48 hours, brain utilizes ketones, decreasing reliance on gluconeogenesis thereby decreasing depletion of protein from muscle.
      • If low carbohydrate diet is coupled with insufficient dietary protein or calories
        • protein from lean tissue (muscle) continues to be metabolized by gluconeogenesis
  • Too many carbohydrates
    • Increases triglyceride levels
    • Converted to fat and stored
    • Increases body fat by suppressing fat oxidation

Types of Carbohydrates

  • Simple
    • Monosaccharides
    • Disaccharides
      • Lactose, Sucrose, Maltose
  • Intermediate
    • Oligosaccharides
      • Repeating units of monosaccharides with glycosidic bonds
      • Eg: raffinose and stachyose found in legumes
      • Metabolized in large intensine with with assistance of bacteria
        • Gas producing
  • Complex
    • Polysaccharides
      • Typically monomer consisting of thousands of repeating glucose
      • Straight chain
        • Amylose
      • Branched
        • Amylopectin
        • Glycogen
      • Fiber

Dietary Fiber

  • Undigestible and resistant starches
    • Eg: cellulose, hemicellulose, pectin, gum and mucilage
  • May help in prevention or treatment of certain diseases
    • heart disease, cancer, obesity, diabetes, and hypertension
  • Types
    • Soluble fiber: reduction of serum cholesterol
    • Insoluble fiber: decreases constipation and reduction of colon cancer
    • Resistance starches
  • 20-35 grams recommended
    • Pinto bean and apple supply soluble fiber requirements
  • 40-50 may cause gas; may need to increase fluid intake

Fructose

  • Simple sugar found in many plants
  • Absorbtion and metabolism
    • Lower Glycemic Index as compared to glucose
    • Absorbed from the gastrointestinal tract by a different mechanism that is glucose
    • Metabolized primarily in the liver
    • Also see Fiber's effect on Insulin
  • Refined fructose
    • Chronic consumption of refined fructose linked to metabolic syndrome
    • Diets high in refined fructose increase appitite which may contribute to excessive caloric consumption (Teff, et al. 2004).
      • Fructose can induce leptin resistance

Recommendations

Athletes or Physically Active

  • Traditional / Mainstream Recommendation
    • 50%-60% of calories from carbohydrates have traditionally been recommended
      • 40%-50% from complex carbohydrates
      • 10% from simple sugar
    • Carbohydrate feedings during long duration (> 90 min.) submaximal (<70% VO2) can improve endurance performance (Coyle E & Montain S, 1992; Maughan R, 1991).
      • Carbohydrate ingestions of 30 to 60 grams per hour are required to improve performance
  • Alternative Recommendation
    • Find and consume only the minimum amount of carbohydrate that support optimal performance
      • Recommended by a growing number of experts including but not limited to:
        • Tim Noakes, MD, DSc
        • Dr Peter Attia, MD
        • Dr Mauro Di Pasquale, MD
        • Dr Eric Serrano, MD
    • Also see High Fat Diets Effect on Endurance.

Weight Managment

See Macronutrient Ratio Studies.

Glycemic Index (GI)

  • GI is a measure of the effects of carbohydrates on blood sugar levels
  • High glycemic indexed food should be eaten with other foods
    • Soluble fiber, fat, acidic foods, and protein (particularly meat) significantly blunts insulin spike
    • Keeps blood sugar stable
    • Inhibit hunger shortly after meal or snack
  • GI monitoring may not have applicability in real world
    • Food is typically eaten with other foods
    • Breakfast is only time we eat after fast
    • Studies show those who eat lower GI diet
      • do not have significantly lower blood glucose levels
      • do not lose more weight
    • All values based on 50 grams of Carbohydrates
      • Does not take into account variations of portion size
      • Example: how often does someone eat 50 grams of carbohydrate of carrots?

Glycemic Load (GL)

  • GL takes into account fiber content and portion size of food
  • GL = GI x Carb (grams) / 100

GI (Glycemic Index) is a percentage expressed as decimal relative to white bread
Food Serving Size Carbs (g) GI GL
Potato, baked 1 medium 37 1.21 45
Grape-Nuts (cereal) 1/2 cup 47 0.95 45
Cornflakes (cereal) 1 cup 26 1.19 31
White rice, cooked 1/2 cup 35 0.81 28
Pasta, cooked 1 cup 40 0.71 28
Cheerios (cereal) 1 cup 22 1.06 23
White bread 2 slices 24 1.00 22
Beans, cooked 1/2 cup 27 0.60 16
Corn Chips 1 oz 15 1.05 16
Whole-grain bread 2 slices 24 0.64 15
Wild rice, cooked 1/2 cup 18 0.78 14
All-Bran 1 cup 24 0.60 14
Carrots, cooked 1/2 cup 8 1.31 10
Lentils, cooked 1/2 cup 20 0.41 8
Popcorn, air-popped 1 cup 5 0.79 4

Ornish, D (2007), The Spectrum, Ballantine Books, New York.


Main Menu | Protein | Fat | Substrate Utilization | Glycogen | Food Sources | Nutrition Titles