Most cholesterol is made in the liver and then sent to the cells that need it, bound to a lipoprotein called LDL (low density lipoprotein). LDL is often called "bad cholesterol" because high levels are associated with an increased risk of heart disease. HDL (high density lipoprotein) is known as "good cholesterol" because it carries cholesterol away from the arteries to the liver. Current recommendations are that levels of "bad" LDL-cholesterol should be less than 3mmol/l (millimoles per litre) and "good" HDL-cholesterol more than 1mmol/l.
Before the study, volunteers had their blood cholesterol levels sampled. They were then randomly allocated into three groups and followed one of three diets for four weeks.
- The first group was asked to switch from animal fats (full fat milk, full fat cheese, butter) to vegetable-based or low fat options. They were asked to cut out eggs, bacon and sausages, and stick to skinless chicken.
- The second group wasn't asked to give up any of the foods but were asked to eat 75g of oats a day, equivalent to three servings. Oats are full of fibre and any form of fibre - whether it is from grains, legumes (beans and lentils) or vegetables is likely to lower cholesterol by binding with fat and cholesterol in the gut and stopping it being absorbed.
- The third group was asked to eat normally but to add into their diet 60g of almonds a day (two handfuls). In recent years tree nuts such as almonds, walnuts and hazelnuts have become hugely popular thanks to studies which suggest they can lower cholesterol. Tree nuts are rich in fibre and plant sterols which may delay fat and cholesterol absorption.
The full Portfolio diet includes not only nuts and oats but plant sterols and soya. Plant sterols are found in fruit, vegetables and nuts, but in low amounts and they are in fortified margarines and yoghurts. Soya products have a reputation as a healthy alternative to dairy, but most randomised controlled studies don't show an effect. Soy seems to work by inhibiting cholesterol synthesis in the liver. Amounts of 15-25g have been recommended in order to get the maximum effect. A study published in 2011 found that people who tried the diet for six months saw an average reduction of about 13% in LDL-cholesterol. The best results were obtained by those who stuck closest to it.
So how did the programme volunteers get on? The results were not quite as expected.
- Almonds: Half the group had a positive response and one person had an 18% reduction in their total cholesterol. However, some people had an adverse response. Their cholesterol actually went up, in some cases significantly. The raised levels of cholesterol in some almond eaters balanced the falls in others. On average there was no change.
- The porridge eaters and the low-animal-fat group did rather better, with an average fall in LDL-cholesterol of 10% and 13% respectively.
- Michael Mosley on the combination approach: His LDL cholesterol fell by 42%. This is in line with what most people experience who go on statins. Why did he do so well? Either the combination approach works better than isolated approaches, or his body responds more dramatically to the combination than most people.
The 'Portfolio' Diet feature on Trust Me, I'm a Doctor, BBC2, July 22 2015
