Carbohydrates

Chapter 5

What are Dietary Carbohydrates?

• Organic compounds containing– Carbon– Oxygen– Hydrogen

– CH2O

• Formed naturally in nature

• Synthesized

• ~4 kcal/gram

Types of Carbohydrates

• Simple CHO– Monosaccharides

• Glucose (dextrose, grape sugar)• Fructose (levulose, fruit sugar)• Galactose (milk sugar)

– Disaccharide• Maltose (malt sugar, glucose & glucose)• Lactose (milk sugar, glucose & galactose)• Sucrose (cane or table sugar, glucose & fructose)

CHO Types

• Complex CHO– 3 or more glucose molecules– Polysaccharide (>2 molecules)

• Plant starches • Animal starch (glycogen)• glucose polymer (10-20 molecules)

– Maltodextrin– polycose

– Fiber

Plant Starches

• Contains many (up to several thousand) monosaccharides

• Storage form of CHO in plants• Types

– Amylopectin – branched-chain molecule that is rapidly digested and absorbed

– Amylose – long chain molecule that is more slowly digested and absorbed

• Most starches contain combinations of both

Concentration Units

• Mole = gram molecular weight• A mole is the weight in grams of a

particular substance, like glucose• Example

– Glucose is C6H12O6

– Atomic weight of C is 12, H is 1, O is 16– Multiply the atomic weight X the number of

that element in the molecule and sum it up.- 1 mole glucose is 180 grams

Muscle Glycogen Units

• Muscle glycogen/glucose concentration are expressed per kg of dry weight (d.w.) or per kg of wet weight (w.w.)

• In muscle, w.w. x 4.5 = d.w.

• Normal Muscle Concentration– 12-16 g/kg w.w or 1.7 g/100 g muscle– 65-90 mmole/kg w.w– Can be ~ doubled by carbo loading

Liver Glycogen Units

• Normal liver glycogen is 50-75 g/kg w.w.• Liver glycogen reduced by 50% after 1

hour of exercise at 75% VO2max. • At rest, glucose output is 150 mg/min

– 60% from liver glycogen– 40% from gluconeogenesis

• At >75% VO2max– Glucose output is 1 g/min– 90% from glycogen

TABLE 4.7 Major hormones involved in regulation of blood glucose levels

Hormone Gland Stimulus Action

Insulin Pancreas Increase in blood glucose

Helps transport glucose into cells; decreases blood glucose levels.

Glucagon Pancreas Decrease in blood glucose; Exercise stress

Promotes gluconeogenesis in liver; helps increase blood glucose levels.

Epinephrine Adrenal Exercise stress; decrease in blood glucose

Promotes glycogen breakdown and glucose release from the liver: helps increase blood glucose levels

Cortisol Adrenal Exercise stress; decrease in blood glucose

Promotes breakdown of protein and resultant gluconeogenesis; helps increase blood glucose levels

Causes of Muscular Fatigue Related to CHO Use

• Muscle Glycogen Depletion– Fatigue begins to occur at approx 30-40

mmole/kg ww.– Short duration high intensity (<60 sec) not

affected until glycogen drops below 20 mmole/kg

– Normal glycogen levels – 60-90 mM/kg– Max levels – 200 mM/kg

Causes of Fatigue Continued

• Liver Glycogen Depletion– Normal glycogen range: 250-300 mM/Kg– At rest, glucose from liver is 150 mg/min

• 60% from liver glycogen• 40% from gluconeogenesis

– During exercise at 75% VO2max, output is 1 gm/min with 90% from glycogen.

– Normoglycemia: 60-100 mg/dl– Hyperglycemia: >140 mg/dl– Hypoglycemia: <45 mg/dl

• Reactive Hypoglycemia

Glycemic Index (GI)

The GI reflects the rate of digestion and absorption of CHO

GI =

Blood glucose area after test food

Blood glucose area after reference food

X 100

Glycemic Load

• Glycemic index relative to the serving size

• Some CHO have high GI but are consumed in small quantities per serving

• GL = (GI x CHO/serving)/100

• Ratings of glycemic loads– High GL = >20– Medium GL = 11-19– Low GL = <11

Use Of GI In Sports Nutrition

• Before Exercise: A low-GL CHO should be eaten, particularly before prolonged exercise, to promote sustained CHO availability

• During Exercise: Moderate to High-GL CHO foods or drinks are most appropriate

• After Exercise: High-GL CHO for glycogen resynthesis

Major Factors Influencing Skeletal Muscle CHO Metabolism During Exercise

• Exercise Intensity

• Exercise Duration

• Training

• Diet

Effect of Intensity of Exercise on CHO Utilization

Exogenous CHO Oxidation & Intensity of Exercise

• Exo CHO oxidation increases with increasing intensity up to 60% VO2max.

• Peaks at 1.0-1.1 g/min• Above 60% VO2max, increases in CHO

oxidation are due to increases in muscle glycogen oxidation up to 4 g/min

• Limitation is due to rate of digestion, absorption and transport of glucose into systemic circulation

Effect of Exercise Duration on CHO Utilization

Effect of Training on CHO Utilization

Effect of Diet on CHO Utilization

Mixed Diet

Low CHO

High CHO

Daily CHO Reqirements

• Minimum of 100 g/day for nervous system

• Moderate duration/low intensity training of 1-3 hours – 5-7 g/kg

• Moderate to heavy endurance training of 1-2 hours – 7-12 g/kg

• Extreme exercise of 4-6 hours – 10-12 g/kg

General Considerations

• Must get adequate overall nutrition in addition to the CHO– Vitamins and minerals– Adequate protein– Total calories

• Glycemic Index needs to be considered

Consumption 3-4 hours Before Exercise (Pre-Competition Meal)

• Up to 2-3 g/kg of CHO (200-300 gm)

• Moderate to high glycemic index CHO

• Minimal fat and protein

• Increases muscle and liver glycogen

Less Than 1 Hour Before Exercise

• Individuals prone to reactive hypoglycemia should avoid CHO, especially high glycemic CHO– May increase glycogen use

• Up to 100 g low to moderate GI if no feeding during exercise

• If feeding during exercise, then nothing up to 1 hour before.

During Exercise

• Maximal use of exogenous CHO is ~ 70 grams per hour

• Timing of feedings seem unimportant• 5-10% solution of 15-20 grams every 15-20 min

– 8 oz of Gatorade contains approx. 15 grams of CHO– High Fructose may cause stomach upset

• Multiple CHO types may increase use• Always test feeding prior to competition

After Exercise

• Glycogen resynthesis rate is about 5-7% per hour

• 2 hour window following exercise for maximal rate of resynthesis

• High glycemic foods• Combination of CHO and protein is best in

a 3:1 ratio of CHO to protein. However, if total CHO intake is sufficient, PRO doesn’t matter.

When Recovery Time is <8 hrs

• 1.2 gm/kg immediately and each hour for 4-6 hours

• Can consume in one bolus each hour or smaller quantities every 30 minutes

• If lesser amounts of CHO are available, consuming protein with CHO in 3:1 ratio is best (may also increase protein anabolism)

• CHO in fluid solution will also replace fluids

When Recovery Time is >8 hours

• Most important factor is meeting total CHO requirement if the intake is spread throughout the next 24 hours.

• However, you can’t wait until the last minute as total resynthesis takes approximately 20 hours

Factors Affecting Glycogen Resynthesis

• Trained have higher rates than untrained

• A lower starting concentration will increase the rate of synthesis

• No difference in fiber types???

• Eccentric exercise has lower rates than concentric exercise after 18-72 hours, but not up to 6 hours

• No difference in males and females

Table 4.8 CHO LoadingOriginal Classic Method

Day 1 Depletion exercise

Day 2 High-protein/fat, 15-20% CHO

Day 3 High-protein/fat, 15-20% CHO

Day 4 High-protein/fat, 15-20% CHO

Day 5 High CHO (70-80%)

Day 6 High CHO

Day 7 High CHO

Day 8 Competition

Table 4.8 CHO LoadingContemporary Recommended Method

Day 1 Depletion exercise (optional)

Day 2 Mixed diet (50-55% CHO)

Day 3 Mixed diet

Day 4 Mixed diet

Day 5 High CHO (70-80%)

Day 6 High CHO

Day 7 High CHO

Day 8 Competition

Most Recent Loading Technique

• 3 min of supramaximal exercise

• 10-12 gm/kg CHO for 24 hrs.

Alcohol As An Energy Substrate

• 7 kcal/gram• By-products of alcohol metabolism

released in blood appear to be of little importance to exercising muscle

• Alcohol consumed prior to exercise may contribute 5% of energy over 90 min of exercise

• Alcohol requires more O2 for metabolism than CHO or fat

• Alcohol may interfere with glucose metabolism

• Reduced aerobic endurance at 80-85% of VO2max

• May reduce rate of gluconeogenesis

• Typically represents non-nutritive excess calories contributing to fat storage