Carbohydrates

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

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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 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 stores are depleted
        • Fasting, low carbohydrate diets, prolonged exercises
      • Liver produces ketone bodies acetoacetate 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 intestine 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

  • Indigestible 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
  • Absorption and metabolism
    • Lower Glycemic Index as compared to glucose
    • Absorbed from the gastrointestinal tract by a different mechanism than 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
    • An increase of 150 kcal/day of sugar increases diabetes 1.1%, whereas an increase of the same number of calories from all other foods only increased diabetes 0.1% (Basu 2013)
    • Diets high in refined fructose increase appetite, which may contribute to excessive caloric consumption (Teff, et al. 2004).
      • Fructose can induce leptin resistance

Basu, Yoffe P, Hills N, Lustig RH (2013). The relationship of sugar to population-level diabetes prevalence: an econometric analysis of repeated cross-sectional data. PLoS One. 2013;8(2):e57873.

Teff KL1, Elliott SS, Tschöp M, Kieffer TJ, Rader D, Heiman M, Townsend RR, Keim NL, D'Alessio D, Havel PJ (2004). Dietary fructose reduces circulating insulin and leptin, attenuates postprandial suppression of ghrelin, and increases triglycerides in women. J Clin Endocrinol Metab. 89(6):2963-72.

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 Management

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 from 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.


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