(3) Dietary fats & dietary proteins

Dietary fats

Sources of Cholesterol

• Derived from:

  • Diet (animal products only)

  • Endogenous synthesis (main source). Hypercholesterolemia is mainly due to endogenous cholesterol synthesis, not dietary intake, and is associated with increased CHD risk.

Plasma cholesterol is transported as lipoproteins LDL, HDL & VLDL.

• LDL=atherogenic

• HDL=anti-atherogenic

Fatty acid composition of triacyclglycerols (TAGs)

TAGs are the most important of dietary fats. Their effect on blood lipids depends on the type of fatty acids.

Effects of Different Fatty Acids in TAGs:

• Myristic (C14) & Palmitic (C16) (saturated FAs) → ↑ Plasma cholesterol

• Stearic acid (C18) → Minimal effect on cholesterol

Monounsaturated Fatty Acids (MUFA)

Usually plant-based oils. Ex, olive oil, nuts, etc. They help to ↓ total cholesterol, ↓ LDL and maintains or ↑ HDL.

Mediterranean populations (with high olive oil intake) show lower CHD incidence.

Polyunsaturated FA (PUFA)

Omega-6 (ω-6 PUFA)

Mainly the linoleic acid (18:2 = 18 carbons, 2 double bonds). Sources: vegetable, sunflower, or corn oil, nuts, avocadoes.

Effects

• ↓ LDL cholesterol

• ↓ HDL cholesterol

Because of the possible peroxidation of PUFA, and subsequent release of free radicals, MUFA are often more recommended/preferred.

Omega-3 (ω-3 PUFA)

Includes α-Linolenic acid (ALA)(18:3), found mainly in plant oil & nuts. EPA (Eicosapentaenoic acid 20:5) and DHA (Docosahexaenoic acid 22:6) mainly in fish oil.

Although it has little effect on LDL & HDL, the benefits of omega-3 PUFA in diet include:

• Anti-inflammatory

• ↓ Plasma TAG

• ↓ Blood pressure

• ↓ Thrombosis risk

• ↓ Cardiac arrhythmias

• ↓ Cardiovascular mortality

Essential FAs (EFA)

Include linoleic & α-linolenic acid, which are imp. for membrane fluidity and eicosanoid synthesis. Also, DHA in infant formula is imp. for brain development.

As such, 2 fatty fish meals/week are recommended (e.g., salmon, sardines, mackerel).

EFA deficiency, caused mostly by fat malabsorption, results in scaly dermatitis due to skin ceramides depletion.

Trans fatty acids

Unsaturated fats that behave like saturated fats metabolically.

They’re formed by hydrogenation of vegetable oils (never naturally from plants) and are found in margarine, fried foods and commercial baked goods.

Health Effects

• ↑ LDL

• ↓ HDL

• ↑ CHD risk

Mediterranean diet

It is high in MUFA (olive oil: main fat source, nuts), moderate PUFA (fish, plant oils), low saturated fat, high plant and fresh foods, and low red meat.

Compared to Western diet, the health effects of this diet are:

• ↓ Total cholesterol

• ↓ LDL

• ↓ TAG

• ↑ HDL

Dietary proteins

They provide amino acids and are the major source of nitrogen. Protein requirements can be determined by measuring nitrogen balance, which is the difference between nitrogen intake and nitrogenous compounds’ output (urea in urine, feces, sweat).

Protein (g) = Nitrogen (g) × 6.25

(Protein ≈ 16% nitrogen)

Calculation of nitrogen balance:

N balance= Protein(g)/6.25 - (UUN+4)

Types of Nitrogen Balance

Average Daily Protein Requirement

Adults require 0.66 g/kg/day

High protein intake does NOT create a positive nitrogen balance. It increases both protein synthesis and breakdown (protein turnover). The protein catabolism is ATP costly and increases diet-induced thermogenesis.

Consumption of excess protein

Shows no physiologic advantage. The metabolic fate of excess protein consumed is:

Deamination of excess amino acids—→ Carbon skeleton metabolized into→ energy or acetyl coA for fat synthesis.

Health Risks:

Loss of urinary nitrogen—→ ↑ Urinary calcium loss—→ ↑ Kidney stones (nephrolithiasis)—→ ↑ Osteoporosis risk

Protein-sparing effect of carbohydrates

Low Carbohydrate Intake (<130 g/day) —→ Gluconeogenesis from amino acids—→ Muscle protein breakdown.

Thus, adequate carbohydrate intake is protein sparing.

Protein Quality: Animal vs Plant

Protein quality refers to its ability to provide essential amino acids (EAAs).

Protein-Energy Malnutrition (PEM)

In developed countries: most commonly seen in patients with medical conditions that cause malabsorption or cases of injury, trauma, infections. The patients become highly catabolic.

In developing countries: inadequate intake of protein and/or calories is the primary cause of PEM. It’s where the two severe PEM forms occur.

Effects

• Weak immunity

• Increased infections

• Growth failure

Severe forms of PEM include:

Balanced Diet & Meal Composition

A balanced diet can be defined as one that contains a variety of foods in quantities and proportions which adequately fulfil an individual’s needs.

The body requires >40 different nutrients

• Needs vary by:

  • Age

  • Gender

  • Lifestyle

  • Health condition

The United States Department of Agriculture (USDA) has designed a food

guide called MyPlate for providing nutritional advice. It’s a food-group based guidance (not nutrient-based like RDA). Region and culture specific. Focused on balanced food portions.