Like all science, the study of nutrition is ever evolving. Senior nutritionist Rose Carr looks at what’s changed— and what’s emerging — in what we know about eating for optimum health.
The study of nutrition includes research on how we eat, digest, metabolise and store the nutrients from our foods and how this affects our bodies and our health in the short-term and the long-term. Whereas early nutritionists focused on how to prevent deficiency diseases and malnutrition, today there is more focus on promoting long-term health. And of course, how to overcome the problem of obesity and the chronic diseases associated with it.
Nutritional scientists have learnt a lot in recent decades, and a few seemingly very sound theories have been debunked along the way. So where are we now?
The big players
Collectively, carbohydrates, fats and protein are known as the macronutrients. They provide the energy (kilojoules) we need from our food. (While alcohol also provides energy, unlike the macronutrients, we can function without it.) The macronutrients are the big picture of nutrition and with the mountains of research on them, you might expect we would fully understand them by now and be able to give you the final word. Unfortunately, that’s not quite the case.Nutrition is an emerging science. At a few hundred years old, it’s a relative newcomer compared to say, chemistry or astronomy.
Learning about fats
In the 1950s it was observed that people in Naples got around 20 per cent of their kilojoules from dietary fat, compared to people in the US at double that. Along with the remarkably lower cholesterol levels and incidence of heart disease in Naples — which was by contrast a growing problem in the US and other countries — it seemed that high fat intakes were a likely cause for heart disease.
It wasn’t too much later, however, that fats from plant sources (eg. olive oil and vegetable oil), were found to support lower cholesterol levels than a similar diet with fats from animal sources, which contain saturated fats. Part of the ‘solution’ to this problem, unfortunately, created a worse problem: trans fats.
Trans fats are produced when unsaturated fats from plants are partially hydrogenated to make them solid at room temperature; a process typically used to make margarines before it was found trans fats were actually more harmful to us than saturated fats.
Of course, it took some years to realise the potential problem and then confirm it — and in the meantime, people were eating what turned out to be less healthy fats.
Now we know…
- Populations who eat fewer trans fats and fewer saturated fats have lower rates of heart disease.
- There may be differences in how we respond to saturated fats depending on the total food matrix. But we still have a lot to learn about that. For example: saturated fats in hard cheese may not be as harmful as the same saturated fats in milk and butter. In New Zealand, unlike most other countries, artificial trans fats are less of a worry as we get most of our trans fats from natural animal sources such as dairy and meat.
- The World Health Organization recommends we eat no more than one per cent of our daily kilojoules from trans fats and in New Zealand, our consumption is much lower than this at 0.6 per cent.
Are fats that bad?
Over time as Western populations’ girths have expanded, weight-loss (and weight maintenance after weight-loss) has been the subject of a huge amount of research. Research that’s ongoing.
Fats are considerably higher in energy per gram than carbs and protein, and while we do need fats, we don’t appear to need vast amounts. Therefore, the focus of weight-loss has for a long time been on reducing our fat intakes. The theory was that reducing protein and carbs would reduce our food volume too much and would just leave us hungry.
In the 1960s, about 45 per cent of energy in the US diet was from fats, 13 per cent of adults were obese and fewer than one per cent had type 2 diabetes. Now Americans get around 33 per cent of energy from fats, 34 per cent of adults are obese and 11 per cent have diabetes — most with type 2 diabetes. This doesn’t mean, however, that people started eating less fat and got fatter. In fact, many people didn’t actually reduce their fat intakes at all: total energy intakes went up over this period. What happened instead was people started to eat more carbohydrates — often unhealthy refined ones — with the net result being a lot of people eating a high-fat, high-refined carbohydrate diet. On the other hand, others got stuck in an ‘all fat is bad’ time warp.
We always knew that fat was high in energy, providing 37kJ per gram compared to 17kJ per gram for carbs and protein. So adding fat to your diet and reducing carbs by the same weight will add lots of kilojoules. Substituting low-nutrient/high-glycaemic index (high-GI) carbs (such as in doughnuts or chocolate cake) with good fats (such as in nuts or avocado) without adding or reducing kilojoules will provide more nutrients.
Now we know…
- Unsaturated fats are good for us, including monounsaturated fats, polyunsaturated fats and omega-3 fats (both the long-chain omega-3s from fish as well as the omega-3s from plants).
- We expect to learn more about the different saturated fats over time (research suggests some types may be less harmful than others).
- There is currently no evidence to say any saturated fats are actually good for us, so we continue with the advice to limit them.
- When it comes to weight-loss what really matters is total energy, not what proportion of that total energy comes from fat. So when it comes to fat, getting the right types of fats and enough of the good fats is the most important thing for our health.
Having been encouraged to reduce fats in our diets for decades then having that advice revised to just reducing saturated fats, may have led to an unforeseen consequence. Some people reduced their fat intake and increased their consumption of carbohydrate foods. It’s now been shown that if we cut back on saturated fat and replace those kilojoules with poor quality carbs such as sugar-sweetened drinks and white rice instead of good fats, although the ‘bad’ LDL cholesterol is reduced, the ‘good HDL cholesterol is also reduced and triglycerides are increased.
Now we know…
- The overall effect of replacing saturated fat with poor quality carbs is just as bad for heart health as carrying on with the same kilojoules of saturated fat, and could be even worse for people with insulin resistance or type 2 diabetes.
- While lower carbohydrate intakes may be fine, studies have shown benefits from carbohydrate intakes at the higher end as well. But it all comes back to carbohydrate quality. In one of the studies where carbs accounted for 52 per cent of total energy, the diet was based on legumes, vegetables, fruits and whole cereals. Legumes, such as lentils, chickpeas and red kidney beans, are low-GI carbs which are high in fibre and contain protein.
They are a staple in the healthy Mediterranean diet, but in New Zealand our use of legumes is still very low. We would benefit from more.
- Carbohydrate quality is something nutritionists are now starting to talk more about, thinking also about how that can be measured. When we think bread we think carbs, but remember foods don’t come as single nutrients.
Not all carbs are equal
High-GI carbs may actually stimulate excess kilojoule intake as well as leading us down the path towards type 2 diabetes. And while sugar has been popularised as the baddie, it’s not just about sugar. Professor Jim Mann tells us that many starchy carbs such as white rice, mashed potato and low-fibre breads are just as bad for us.
- The GI is a good starting point when evaluating high-carbohydrate foods, but we also need to take into account the other nutrients in food.
- While we expect to see more research around carbohydrate quality, in general most vegetables, legumes, fruits and true whole grain cereals (that haven’t been highly processed) are the good guys in the carb camp.
What does ‘wholegrain’ really mean?
Even choosing so called ‘wholegrain’ foods is not a safe bet any more. In a recent address to the annual conference of the European Association for the Study of Diabetes (EASD) Professor Jim Mann highlighted many so-called wholegrain breads were little better than white bread and “roughly comparable to eating a bag of glucose.” This is because the structure of the grains has been disrupted and they are digested so much more quickly than if the grains were still intact.
Our Food Code permits products to be called ‘wholegrain’ if they contain every part of the grain including the outer layers, bran and germ. The problem is this definition applies even if these parts are separated during processing and regardless of whether the grain is in one piece or milled into fine particles. The processing of grains like this can raise their GI.
|1 slice Bakers Delight Cape Seed loaf||VS||1/3 of a baguette|
|Contains seeds, good fats, vitamins, minerals||Highly refined, higher GI|
What about protein?
Recommendations around the amount of protein in our diets have increased over the last two decades.
Firstly, although in theory we can manage with less protein, it was recognised that around 15 per cent energy from protein was needed to ensure we got enough of the other nutrients that comes with protein foods.
Then it was recognised that as we age our protein needs increase substantially and guidelines for people 70 years and older now advise around 25 per cent higher protein intakes than for younger adults to help maintain muscle mass. It’s long been recommended that we get between 15 to 25 per cent of our kilojoules from protein foods. In fact, the average in New Zealand is at the lower end of this.
The protein leverage theory has gained ground in recent years with increasing research. This theory suggests that if we consume foods low in protein we will keep eating until we have satisfied our protein needs. So we can end up eating way too many kilojoules from fat and carbs. This, along with the knowledge that protein is more satiating than fat and carbs, supports the view that we ought to ensure we are getting some protein throughout the day in each meal and snack.
Now we know…
- The protein leverage theory is relatively new so we’d expect to learn more about that. it has been known for many years, however, that protein is more satiating than carbs or fat.
- Not only does higher protein help with weight-loss, it also helps us keep it off. The large Diogenes study conducted across eight European countries tested different diets and found the best results were achieved with a diet that was both higher in protein (25 per cent energy from protein) and lower in glycaemic load.
What’s the latest on…?
Previously demonised for adding to our cholesterol problems, we now know eggs, while containing cholesterol, are also a nutrient-dense food and saturated fat is the largest cause of high blood cholesterol levels.
It’s recommended that six to seven eggs a week is fine for healthy people. For those at higher risk of heart disease, including anyone with diabetes or high cholesterol, limiting eggs to three eggs a week is still advised. The Heart Foundation is currently reviewing the evidence around egg consumption and cholesterol levels.
As bad as they always were our consumption of trans fats is now pretty low. There is a push by the World Health Organisation to eliminate manufactured trans fats from the global food supply.
Claims that sugar is toxic are catchy but there is no evidence to support them. There is, however, evidence that excess sugar contributes to weight gain, and excess body weight is associated with type 2 diabetes and heart disease. So we really don’t want too much of it. Research has shown that higher intakes of free sugars —such as those found in sugary drinks — are associated with weight gain. Sugar is not solely responsible for our obesity epidemic, but it is part of the equation.
Enter the food technologists!
Just when we think we’re getting a handle on foods, the food technologists come along and change the playing field. To make food higher in fibre, manufacturers have started adding fibres such as inulin and polydextrose. These fibres add sweetness as well, which sounds like a win-win as sugar can be reduced. But we don’t really know what adding these will mean long-term. These fibres may well prove to be completely innocuous for most of us, but it’s a shame that products such as snack bars need fibre added when they could be made from real, fibre-rich foods.
The new rules: Let’s focus on real food
- We are all individuals, so different things will work for us in terms of healthy eating, whether or not our goal is weight-loss.
- There is a range of healthy diets which have different ratios of carbs, protein and fats. Higher and lower amounts of each will work for different people. At Healthy Food Guide, we recommend moderate carbohydrate, moderate fat, and a little more protein than the average Kiwi.
- Focus on the quality of your food. Choose healthy fats, good quality, low-GI carbohydrates and lean protein.
- As well as quality, go for variety. Eat a wide range of whole real foods. This means vegetables, legumes, true wholegrain cereals, trim milk and reduced-fat dairy products, fruit, nuts and seeds, as well as fish and lean meats.
- Don’t forget about the total energy consumed. Keep portions reasonable.
- If your waist measurement is more than half of your height, you’re probably carrying too much weight and consuming too much energy.
- Relax and enjoy your food. If you’re worrying about whether you’re eating too many carbs or not enough protein, you’re probably not enjoying your meal. Food shouldn’t provoke anxiety.