18.5 Nutrition: Energy Balance, Glycemic Index, Fed-Fast Cycle & Free Radicals

18.5 Nutrition: Energy Balance, Glycemic Index, Fed-Fast Cycle & Free Radicals

The biochemistry outline lists nutrition (glycemic index, calories, vitamin deficiencies, fasting and starvation, absorptive state) and free radicals and antioxidants. Vitamin deficiencies are covered in 18.3 and carbohydrate and lipid pathways in 17.2 and 18.1. This section ties them to energy balance, diabetes and wound-healing nutrition.

Calories and Energy Expenditure

Total daily energy expenditure has three parts:

• Resting metabolic rate: about 60–70%, higher with lean mass, fever, burns and sepsis
• Thermic effect of food: about 10%
• Physical activity: the most variable part

Predictive equations estimate needs. The Mifflin-St Jeor equation for men is:

$$\text{REE (kcal/day)} = 10 \times \text{weight (kg)} + 6.25 \times \text{height (cm)} - 5 \times \text{age (y)} + 5$$

For women, subtract 161 instead of adding 5. Worked example: an 80-kg, 180-cm, 50-year-old man has an estimated REE of 800 + 1,125 − 250 + 5 = 1,680 kcal/day, before activity and stress factors.

Body mass index = weight (kg) / height (m)². Overweight is 25–29.9 and obesity is 30 or higher. BMI does not distinguish fat from muscle.

Glycemic Index and Glycemic Load

• Glycemic index (GI): the incremental area under the blood glucose curve after a portion containing 50 g of available carbohydrate, divided by the area after 50 g of reference glucose, multiplied by 100. Low GI is 55 or less, medium 56–69 and high 70 or more.
• What lowers GI: fiber (especially soluble), fat and protein (slower gastric emptying), intact grains, a higher amylose-to-amylopectin ratio, acidity and less processing or cooking.
• Glycemic load (GL) accounts for portion size:

$$\text{GL} = \frac{\text{GI} \times \text{grams of available carbohydrate per serving}}{100}$$

Worked example: watermelon has a high GI of about 72, but a 120-g serving contains only about 6 g of carbohydrate, so GL ≈ 72 × 6 / 100 ≈ 4 (low). A large serving of white rice can have both a high GI and a high GL. For people with diabetes, total carbohydrate and overall diet quality matter more than GI alone, but lower-GI choices blunt postprandial spikes.

The Fed-Fast Cycle

Red blood cells (no mitochondria) and the renal medulla depend on glucose throughout. The liver makes ketones but cannot use them (it lacks succinyl-CoA:3-ketoacid CoA transferase, 18.1).

Malnutrition, Refeeding and Wound Healing

• Marasmus: total calorie deficiency with muscle and fat wasting and no edema.
• Kwashiorkor: protein deficiency relative to calories, with edema (hypoalbuminemia), a fatty liver, skin and hair changes and a distended abdomen.
• Refeeding syndrome: when nutrition restarts after starvation, insulin drives phosphate, potassium and magnesium into cells. Hypophosphatemia can cause muscle weakness, heart failure and arrhythmias. Give thiamine before glucose, start feeding slowly and monitor electrolytes.
• Nutrition and wounds: protein, calories, zinc, vitamin C and vitamin A support collagen synthesis and immune function. Pressure injury guidelines suggest about 1.25–1.5 g/kg/day of protein for adults with wounds who are malnourished or at risk, unless contraindicated (for example, advanced CKD not on dialysis). Albumin and prealbumin fall with inflammation (18.4), so they are imperfect markers of nutritional status.

Free Radicals and Antioxidants

Reactive oxygen species:

• Superoxide (O2•-)
• Hydrogen peroxide (H2O2)
• Hydroxyl radical (•OH), the most damaging, formed from H2O2 and Fe2+ in the Fenton reaction
• Peroxynitrite (from superoxide and nitric oxide)

Sources:

• Electron leak from the mitochondrial electron transport chain
• NADPH oxidase in neutrophils (the respiratory burst, 6.3)
• Xanthine oxidase during reperfusion
• Ionizing radiation, cigarette smoke, iron overload and drug metabolism

Damage:

• Lipid peroxidation of membranes (a chain reaction)
• Protein oxidation and cross-linking
• DNA base damage (8-oxoguanine)

Together these contribute to cell injury (7.1), aging and diabetic complications. Ischemia-reperfusion injury after revascularization or tourniquet release is driven by a burst of ROS when oxygen returns.

Evidence check. Despite the biochemistry, trials of beta-carotene and vitamin E supplements have not prevented cardiovascular disease or cancer. Beta-carotene increased lung cancer risk in smokers, and U.S. preventive guidance recommends against using them for that purpose.