In response to the recent public and media interest regarding dietary choices and eating plans, this document has been compiled by a panel of practitioners and scientists in the field of nutrition and sports science on behalf of the Sports Science Institute of South Africa (SSISA) and the University of Cape Town/South African Medical Research Council Research Unit for Exercise Science and Sports Medicine (ESSM).
As far back as fifty years ago, nutrition scientist, Elsie Widdowson coined the phrase ‘nutritional individuality”, and highlighted the fact that energy and nutrient requirements may differ by as much as two-fold in individuals of similar ages and gender. However, in the past 10 years alone, more than 830,000 scientific publications on diet and health have been published in scholarly journals. This means that in the last decade, there has been a peer-reviewed, scientific study published approximately every 6½ minutes.
To put it mildly, nutrition is a rapidly changing science, with new evidence emerging all the time. In addition, nutrition science recognises the role of genetic and epigenetic phenomena, early life events, and lifestyle choices, which act in concert to modulate the impact of the food we eat on our health status and health outcomes. Moreover, we live in a country where under-nutrition is not uncommon, and in which many South Africans continue to struggle to make ‘ends meet’. These factors will also influence dietary choices, which in turn influence health and the maintenance of a healthy body weight. As a result, making population-based nutritional recommendations for public health is particularly challenging.
There is always lively and robust scientific and public debate around the continually evolving field of nutrition, health and disease. As a result, a number of scientists and practitioners from UCT and SSISA have formulated a statement regarding dietary recommendations, based on their joint interpretation of the current state of evidence (i.e. research published in peer-reviewed scientific journals) as to what constitutes a ‘healthy diet’ in 2012.
The complexity of dietary strategies
The primary aim of any dietary strategy is to provide optimal nutrition for energy, growth and health throughout the life course, while secondary aims include prevention or management of a range of chronic medical conditions, optimising sports performance or weight loss/gain. However, dietary strategies are complex for a number of reasons. There is good evidence to support the concept that individuals have their own unique “metabolic profiles”. That is, one person may ‘handle’ various nutrients, including fat, carbohydrate or protein, or respond to a specific dietary strategy differently to the next person. By ‘handling’ of nutrients, we refer specifically to the body’s use of these nutrients to meet day-to-day requirements, to aid in growth and to contribute to health and well-being.
In addition, a dietary strategy does not act in isolation. Rather, external factors such as a person’s health/disease status, physical activity levels, diet quantity (amounts consumed) and quality (e.g. types of fat and fatty acids, protein and carbohydrate consumed, as well as alcohol intake), stress levels, attitude towards food and eating, and motivation etc. may modify the outcomes or effectiveness of any given strategy. Moreover, individuals may have short-term goals (e.g. weight loss/gain or sporting performance) and/or more long-term goals (e.g. the maintenance of a healthy weight, and reducing the risk of chronic, non-communicable diseases, such as diabetes, heart disease and cancer), which also need to be taken into account. Furthermore, no recommendation for a healthy diet would be complete without an accompanying recommendation to accumulate at least 30 minutes of moderate-to-vigorous physical activity, on most, preferably all days of the week.
There are a number of international standard-setting groups, such as the American Heart Association and the Institute of Medicine, which have considered the evidence, and made recommendations as to what constitutes a ‘healthy diet’. However, the difficulty remains that these recommendations are more general, than specific, and often fail to take into consideration many of the previously mentioned factors that may influence the effectiveness of a dietary strategy. It might be simpler to first consider those dietary recommendations that are unequivocally ‘good’ or healthy for everyone, much like one can confidently say that cigarette smoking is unequivocally ‘bad’, or that regular physical activity is beneficial.
What does the evidence say?
For example, the evidence concerning dietary intake of vegetables and fruit, in particular, cruciferous vegetables or green, leafy vegetables, and health, is emphatically positive. Therefore, the recommendation by the American Heart Association to eat a diet rich in vegetables and fruit, as part of a healthy diet in 2012, is strongly supported. However, while fruit in the diet has generally been shown to be favourably associated with overall lower risk of chronic diseases, the effects, when demonstrated, are much more modest than those for vegetables. Moreover, fruit intake is often combined with vegetable intake in long-term observational studies, making it more difficult to evaluate the stand-alone benefits.
Further, with respect to carbohydrate in the diet, whole grain sources of carbohydrate, and a diet rich in fibre (legumes, vegetables and fruit), are unequivocally healthy. In contrast, the intake of sugar-sweetened beverages and foods with added (indeed often “hidden”) sugars should be discouraged, and have been shown to unequivocally ‘unhealthy’. There is strong evidence that the higher the ‘glycaemic load’ of the diet, or the extent to which the carbohydrate in the foods eaten raises blood sugar and insulin, the greater the association with cardio-metabolic diseases.
The intake of ‘trans fats’ (partially hydrogenated oils which are solid at room temperature) – specifically those in processed foods – has been emphatically associated with increased risk for cardiovascular disease. The evidence concerning saturated fats is not quite as straightforward, with different reviews and meta-analyses coming to differing conclusions. On balance, while not ‘unequivocally bad’, much of the evidence suggests that replacing saturated dietary fats with mono-unsaturated fatty acids, or omega-3 fatty acids, is associated with a lower risk for cardiovascular events.
Diets high in salt have also been linked, in particular, to hypertension, some cardiovascular disease outcomes and certain cancers. Finally, it has been well established that some foods contain allergens to which certain individuals are highly sensitive and for obvious reasons must be avoided. Current evidence suggests that this list includes wheat, nuts, eggs, dairy, soy and fish-containing foods.
Perhaps the most controversial part of the public debate, and where the largest differences of opinion rest, is with respect to the proportion of the diet that should be comprised of carbohydrates and fats. In the past, fats have been ‘demonised’ in much of the ‘layman’s’ interpretation of the scientific evidence, and carbohydrates have been raised up as being unequivocally healthy. The truth probably lies somewhere in the middle, and depends in large measure on the types of carbohydrates and fats that comprise the diet. Either way, it is likely that a more balanced diet, which neither favours fat or carbohydrate, but rather focuses on whole grain and high-fibre sources of carbohydrate and dietary fats that are rich in omega-3 and mono-unsaturated fatty acids, is preferred. While the evidence concerning saturated fats is still open to interpretation, it is likely that relatively low-fat protein sources will prove to be the most beneficial for health in the long-term. This type of diet is also more sustainable, more affordable, and probably has a lower impact on the environment.
Obesity, insulin resistance and weight loss
Factored into this dialogue, is the issue concerning dietary recommendations for obesity, insulin resistance and weight loss. While it has been shown that it is possible for normal-weight, non-obese persons to be insulin resistant or pre-diabetic, it is at least 10-fold more likely if they are carrying those extra kilograms. Therefore, weight loss alone will improve cardio-metabolic risk or decrease the likelihood of cardiovascular disease. To lose weight, energy expenditure, through physical activity and activities of daily living, must exceed energy intake. From a dietary perspective, this can be achieved by an overall reduction in food intake, and/or by manipulating the nutrient content of the diet. There is evidence that certain dietary strategies, including a higher-protein, low-carbohydrate diet, may be associated with greater weight losses, by reducing hunger and also the glycaemic load of the diet. However, there are studies that also show that a low-fat, kilojoule-restricted diet is as effective as the higher-protein, low-carbohydrate intake.
Once weight is lost, the critical issue is the maintenance of a healthy weight. The balance of nutrients that contribute to the overall diet, in terms of carbohydrate, fats and proteins, may differ according to individuals, but overall energy balance needs to be maintained (kilojoules in = kilojoules out). As a result, for some people, this means attention may need to be given to portion sizes, and/or moderating the use of alcohol. In addition, physical activity should be encouraged to maintain lean body mass and improve health.
A comment on research design
In the arena of public debate, there has also been some discussion as to the limitations of longitudinal, observational studies. These studies follow groups of people over years, even decades, and examine the extent to which differences in food intake, such as fruits and vegetables, meat or whole grain cereals, or nutrient intake, such as saturated fats, carbohydrate or protein (as a percent of total energy), are associated with long-term health outcomes. While the results of these studies do not imply causality, they do provide the basis of ‘natural experiments’ that cannot be replicated in the form of clinical trials. The difference is that one tests ‘interventions’ in a randomised, controlled manner, whereas, the longitudinal study tests the association between ‘exposures’ and disease outcomes in free-living individuals, throughout the life-course, in combination with other lifestyle choices. Each of these study designs contributes importantly to our understanding of the diet- disease relationship, and well-designed research that is peer-reviewed should inform dietary recommendations for public health purposes.
In summary, the following dietary strategies for maintaining health and preventing disease are strongly supported by the peer-reviewed scientific literature and include:
• A diet rich in vegetables, especially green leafy vegetables – recommended
• Whole grain sources of carbohydrates – recommended
• A diet high in fibre – recommended
• Sugar-sweetened beverages and foods with added sugar – not recommended
• Highly processed, refined carbohydrates – not recommended
• A diet with a high-glycaemic load – not recommended
• Dietary fats which are rich in omega-3 fatty acids and mono-unsaturated fatty acids – recommended
• Trans-fats or partially-hydrogenated vegetable fats – not recommended
• Maintenance of a healthy weight – recommended
• Regular physical activity, at least 30 minutes of moderate activity on most days – recommended
It is also clear that individuals respond differently to different weight loss interventions, and there will be individuals for whom a particular dietary strategy is more or less effective. In practice, this means that there is no “one size fits all” approach when it comes to finding the best nutritional strategy for any given individual. Instead, a holistic approach that takes into account a person’s history (dietary, medical, age, budget, lifestyle, physical activity, psychological factors, family history of disease, and genetics), will be helpful in determining the most effective strategy to promote long-term adherence and quality of life. It also recommended that individuals with chronic, non-communicable diseases (such as cardiovascular disease, diabetes, or hypertension), those struggling to manage their weight, or those with a history of weight or dietary concerns, should ideally consult with a registered dietitian or an appropriate health professional, to decide on the most effective dietary strategy to promote health.
Our commitment is to continue to monitor the published research pertaining to nutrition science in order to remain current, to be effective in the translation of this research and to engage in relevant research ourselves.