Nutrients, diet and inheritance
You are standing in the canteen queue before a camogie match. Your friend picks crisps and a fizzy drink. You pick a chicken wrap and fruit. Does it matter? And why does your friend's dad say "diabetes runs in our family" while the school nurse says "it depends on lifestyle too"? In this page you will see what food gives your body, how to test and measure it, and how genes and lifestyle work together.
The components of a balanced diet
A balanced diet contains all the nutrients your body needs, in the right amounts. There are six components.
| Component | Main job | Good sources |
|---|---|---|
| Carbohydrates | Main source of energy | Bread, pasta, potatoes, oats |
| Fats (lipids) | Stored energy, insulation, cell membranes | Butter, oils, nuts, oily fish |
| Proteins | Growth and repair of cells; enzymes | Meat, fish, eggs, beans, milk |
| Vitamins | Small amounts for healthy functioning (e.g. vitamin C prevents scurvy) | Fruit and vegetables |
| Minerals | e.g. calcium for bones and teeth; iron for haemoglobin in red blood cells | Dairy products, red meat, spinach |
| Water (and fibre) | Water: reactions, transport, cooling. Fibre: keeps the gut moving | Drinks, fruit; wholegrain bread, vegetables |
Fibre is not digested, but it prevents constipation. Many textbooks list water and fibre separately, so you may see "seven" components.
An energy-rich food is not the same as a healthy food. A balanced diet also depends on age, sex and activity: a teenage hurler needs more energy than a person at a desk.
Name the six components of a balanced diet. For three of them, give one example of a food that is a good source.
Name the six components of a balanced diet. For three of them, give one example of a food that is a good source.
Food tests
Food labels tell you what is in a food, but in the lab you can test for it. Each test uses a reagent that changes colour.
| Nutrient | Test | Positive result |
|---|---|---|
| Starch | Add a few drops of iodine solution | Orange-brown turns blue-black |
| Reducing sugar (e.g. glucose) | Add Benedict's solution and heat in a hot water bath | Blue turns orange-red precipitate (green or yellow for small amounts) |
| Protein | Add Biuret solution (or sodium hydroxide, then dilute copper sulfate) | Blue turns purple/violet |
| Fat | Rub on brown paper or shake with ethanol, then add water | Translucent spot on paper; cloudy white emulsion |
To test a solid food, first crush it and mix it with water. Always wear safety glasses, and use a water bath, not a flame, to heat Benedict's solution.
Name the food test used to detect each of the following and state the positive result: a) starch b) reducing sugar (glucose) c) protein
Name the food test used to detect each of the following and state the positive result:
- a) starch
- b) reducing sugar (glucose)
- c) protein
Energy in food
Nutrients release different amounts of energy. Energy is measured in kilojoules (kJ). These are the values you need:
| Nutrient | Energy per gram |
|---|---|
| Carbohydrate | 17 kJ/g |
| Protein | 17 kJ/g |
| Fat | 37 kJ/g |
Fat has more than twice the energy per gram, which is why fatty foods are so energy-dense.
Energy needs are compared with energy intake. A deficit is need minus intake. To express it as a percentage: $\dfrac{\text{deficit}}{\text{requirement}} \times 100$. If you take in less energy than you use over weeks, the body breaks down stored fat (and eventually muscle), so body mass falls. If you take in more than you use, the extra is stored as fat.
A packet of oat biscuits states that $100\ \text{g}$ contains $62\ \text{g}$ carbohydrate, $20\ \text{g}$ fat and $7\ \text{g}$ protein. Energy values are…
A packet of oat biscuits states that $100\ \text{g}$ contains $62\ \text{g}$ carbohydrate, $20\ \text{g}$ fat and $7\ \text{g}$ protein. Energy values are $17\ \text{kJ/g}$ for carbohydrate and protein and $37\ \text{kJ/g}$ for fat.
- a) Calculate the energy from each nutrient in $100\ \text{g}$.
- b) Calculate the total energy in $100\ \text{g}$.
- c) A student eats a $35\ \text{g}$ portion. Calculate the energy in this portion.
A teenage athlete needs about $12,000\ \text{kJ}$ per day and her diet provides $10,200\ \text{kJ}$ per day. She also reports tiredness and is found to have low iron…
A teenage athlete needs about $12,000\ \text{kJ}$ per day and her diet provides $10,200\ \text{kJ}$ per day. She also reports tiredness and is found to have low iron levels.
- a) Calculate the daily energy deficit and express it as a percentage of her requirement.
- b) Predict the effect on body mass over several weeks if this deficit continues.
- c) Explain why iron is needed in the diet and suggest two good food sources.
- d) Explain why a balanced diet matters, not only the total energy intake.
Inherited factors
Health depends on inherited factors (passed from parents to children) and environmental factors (diet, lifestyle, pollution, infection).
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A gene is a section of DNA that carries the instructions for one characteristic (for example, making one protein).
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An allele is a different version of a gene. You have two alleles of each gene, one from each parent.
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A characteristic is inherited if it is passed on through genes from parents to offspring (for example, blood group or eye colour).
A dominant allele (capital letter, e.g. F) shows up if present. A recessive allele (lower case, f) shows only if both alleles are recessive. A person with one of each (Ff) is a carrier: they do not have the condition but can pass the allele on.
Cystic fibrosis (CF) is an inherited condition, common in Ireland, caused by a recessive allele. Lactose intolerance is also partly inherited: some people lack enough of the enzyme lactase. An enzyme is a protein that speeds up a chemical reaction without being used up, and each enzyme works on one substance. Lactase breaks down lactose into simpler sugars that can be absorbed. Without enough lactase, lactose (milk sugar) is not digested in the small intestine. It passes to the large intestine, where bacteria use it, causing wind, cramps and diarrhoea. People can still get calcium from cheese, yoghurt (low in lactose), lactose-free milk, fortified plant drinks, tinned fish with bones and green vegetables.
Punnett squares
A Punnett square shows the possible combinations of alleles from two parents.
Each child is an independent event. Having one child with CF does not change the chance for the next.
Define the terms "gene", "allele" and "inherited". Give one human characteristic that is inherited.
Define the terms "gene", "allele" and "inherited". Give one human characteristic that is inherited.
Lactose intolerance is more common in some populations than in others and depends partly on inherited genes. A person with lactose intolerance lacks enough of the enzyme…
Lactose intolerance is more common in some populations than in others and depends partly on inherited genes. A person with lactose intolerance lacks enough of the enzyme lactase.
- a) Explain why the lack of lactase causes problems when the person drinks milk.
- b) Explain why the condition can run in families.
- c) Suggest how a person with lactose intolerance can still obtain calcium in the diet.
Cystic fibrosis is an inherited condition caused by a recessive allele. Two parents who are both carriers ($\text{Ff}$) have a child. a) Draw a Punnett square in words…
Cystic fibrosis is an inherited condition caused by a recessive allele. Two parents who are both carriers ($\text{Ff}$) have a child.
- a) Draw a Punnett square in words or table form for this cross.
- b) State the probability that the child has cystic fibrosis.
- c) State the probability that the child is a carrier.
Genes, lifestyle and health
Many conditions are influenced by both genes and environment. In type 2 diabetes, inheriting certain genes raises the risk, but diet and activity strongly affect whether it develops. A person with a family history can lower their risk by:
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eating a balanced diet with less sugar and fat
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being physically active (e.g. 60 minutes a day for teenagers)
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keeping a healthy body mass
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not smoking
Other conditions are almost entirely genetic. Phenylketonuria (PKU) is a recessive condition in which the body cannot break down the amino acid phenylalanine, found in protein. If it builds up it harms the brain. In Ireland, all newborns have a heel-prick test. Early detection is valuable because a special low-protein diet from the first weeks prevents brain damage. PKU is controlled, not cured: the faulty gene is still in every cell, so the diet must continue.
Correlation and causation
A correlation means two things change together, for example high sugar intake and disease. It does not on its own prove that one causes the other. A third factor, such as low physical activity or income, might affect both. To test causation, scientists can:
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compare large groups, changing only one variable (a controlled trial)
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follow people over many years (a long-term study)
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find a mechanism that explains how the cause works
Type 2 diabetes is influenced by both inherited factors and lifestyle. A study in Ireland finds that people with a parent with type 2 diabetes are about $3$ times more…
Type 2 diabetes is influenced by both inherited factors and lifestyle. A study in Ireland finds that people with a parent with type 2 diabetes are about $3$ times more likely to develop it, but that within this group, those with a high-sugar, low-activity lifestyle are far more likely to develop it than those with a balanced lifestyle.
- a) Explain what this shows about the relative roles of genes and environment.
- b) Suggest three lifestyle changes that could lower the risk.
- c) Explain why a correlation between diet and disease does not on its own prove causation, and describe how scientists could strengthen the evidence.
Phenylketonuria (PKU) is a recessive inherited condition in which the body cannot break down the amino acid phenylalanine. It is screened for in newborns in Ireland…
Phenylketonuria (PKU) is a recessive inherited condition in which the body cannot break down the amino acid phenylalanine. It is screened for in newborns in Ireland through the heel-prick test. If untreated, phenylalanine builds up and harms the brain. Treated babies follow a low-protein diet.
- a) Explain why newborn screening is valuable.
- b) Explain why the condition is controlled by diet rather than cured.
- c) A couple, both carriers, have two children. Calculate the probability that both children have PKU.
- d) Discuss one ethical issue related to genetic screening.
Back to the canteen: your friend's crisps and fizzy drink give plenty of energy but few vitamins and minerals, while your wrap and fruit are more balanced. Her family history may raise some risks, but her daily choices help decide the outcome.